Re: [Qemu-devel] TCG flow vs dyngen

Re: [Qemu-devel] TCG flow vs dyngen

[Raphael Lefevre]

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On Wed, Dec 15, 2010 at 4:17 AM, Stefano Bonifazi <stefboombastic@gmail.com> wrote:

> On 12/11/2010 03:44 PM, Blue Swirl wrote:

> 

> Hi!

> Thank you very much! Knowing exactly where I should check, in a so big

> project helped me very much!!

> Anyway after having spent more than 2 days on that code I still can't

> understand how it works the real execution:

> 

> in cpu-exec.c : cpu_exec_nocache i find:

> 

>> /* execute the generated code */

>>    next_tb = tcg_qemu_tb_exec(tb->tc_ptr);

> 

> and in cpu-exec.c : cpu_exec

> 

>> /* execute the generated code */

>> 

>>                    next_tb = tcg_qemu_tb_exec(tc_ptr);

> 

> so I thought tcg_qemu_tb_exec "function" should do the work of executing the

> translated binary in the host.

> But then I found out it is just a define in tcg.h:

> 

>> #define tcg_qemu_tb_exec(tb_ptr) ((long REGPARM (*)(void

>> *))code_gen_prologue)(tb_ptr)

> 

> and again in exec.c

> 

>> uint8_t code_gen_prologue[1024] code_gen_section;

> 

> Maybe I have some problems with that C syntax, but I really don't understand

> what happens there.. how the execution happens!

> 

> Here instead  with QEMU/TCG I understood that at runtime the target binary

> is translated into host binary (somehow) .. but then.. how can this new host

> binary be run? Shall the host code at runtime do some sort of (assembly

> speaking) branch jump to an area of memory with new host binary instructions

> .. and then jump back to the old process binary code?

 

1. As I know, the host codes translated from the target instructions exist by the format of object file, that’s why they can be executed directly.

2. I think you catch the right concept in some point of view, one part of the internal of QEMU does such jump & back works certainly.

 

> If so, can you explain me how this happens in those lines of code?

 

I only can give a rough profile, the code you listed do a simple thing:

Modify the pointer of the host code execution to point the next address that the host processor should continue to execute.

 

> I am just a student.. unluckily at university they just tell you that a cpu

> follows some sort of "fetch ->decode->execute" flow .. but then you open

> QEMU.. and wow there is a huge gap for understanding it, and no books where

> to study it! ;)

 

The QEMU is not used to simulate the every details of the processor should behave, it just try to approximate the necessary operations what a machine should be!

“fetch->decode->execute” flow only need to be concerned when you involve into the hardware design.

 

Raphaël Lefèvre


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Re: [Qemu-devel] TCG flow vs dyngen

[Stefano Bonifazi]

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On 01/16/2011 03:46 PM, Raphael Lefevre wrote:
>
> On Wed, Dec 15, 2010 at 4:17 AM, Stefano Bonifazi 
> <stefboombastic@gmail.com> wrote:
>
> > On 12/11/2010 03:44 PM, Blue Swirl wrote:
>
> >
>
> > Hi!
>
> > Thank you very much! Knowing exactly where I should check, in a so big
>
> > project helped me very much!!
>
> > Anyway after having spent more than 2 days on that code I still can't
>
> > understand how it works the real execution:
>
> >
>
> > in cpu-exec.c : cpu_exec_nocache i find:
>
> >
>
> >> /* execute the generated code */
>
> >>    next_tb = tcg_qemu_tb_exec(tb->tc_ptr);
>
> >
>
> > and in cpu-exec.c : cpu_exec
>
> >
>
> >> /* execute the generated code */
>
> >>
>
> >>                    next_tb = tcg_qemu_tb_exec(tc_ptr);
>
> >
>
> > so I thought tcg_qemu_tb_exec "function" should do the work of 
> executing the
>
> > translated binary in the host.
>
> > But then I found out it is just a define in tcg.h:
>
> >
>
> >> #define tcg_qemu_tb_exec(tb_ptr) ((long REGPARM (*)(void
>
> >> *))code_gen_prologue)(tb_ptr)
>
> >
>
> > and again in exec.c
>
> >
>
> >> uint8_t code_gen_prologue[1024] code_gen_section;
>
> >
>
> > Maybe I have some problems with that C syntax, but I really don't 
> understand
>
> > what happens there.. how the execution happens!
>
> >
>
> > Here instead  with QEMU/TCG I understood that at runtime the target 
> binary
>
> > is translated into host binary (somehow) .. but then.. how can this 
> new host
>
> > binary be run? Shall the host code at runtime do some sort of (assembly
>
> > speaking) branch jump to an area of memory with new host binary 
> instructions
>
> > .. and then jump back to the old process binary code?
>
> 1. As I know, the host codes translated from the target instructions 
> exist by the format of object file, that’s why they can be executed 
> directly.
>
> 2. I think you catch the right concept in some point of view, one part 
> of the internal of QEMU does such jump & back works certainly.
>
> > If so, can you explain me how this happens in those lines of code?
>
> I only can give a rough profile, the code you listed do a simple thing:
>
> Modify the pointer of the host code execution to point the next 
> address that the host processor should continue to execute.
>
> > I am just a student.. unluckily at university they just tell you that 
> a cpu
>
> > follows some sort of "fetch ->decode->execute" flow .. but then you open
>
> > QEMU.. and wow there is a huge gap for understanding it, and no books 
> where
>
> > to study it! ;)
>
> The QEMU is not used to simulate the every details of the processor 
> should behave, it just try to approximate the necessary operations 
> what a machine should be!
>
> “fetch->decode->execute” flow only need to be concerned when you 
> involve into the hardware design.
>
> Raphaël Lefèvre
>
Thank you very much!
I've already solved this problem.. Right now I am fighting with the 
possibility of changing qemu-user code for making it run several 
binaries in succession .. But it seems to remember the first translated 
code.. Nobody answered to my post about it, do you have any idea?


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Re: [Qemu-devel] TCG flow vs dyngen

[Raphaël Lefèvre]

On Sun, Jan 16, 2011 at 11:21 PM, Stefano Bonifazi
<stefboombastic@gmail.com> wrote:
>
> Thank you very much!
> I've already solved this problem.. Right now I am fighting with the possibility of changing qemu-user code for making it run several binaries in succession .. But it seems to remember the first translated code.. Nobody answered to my post about it, do you have any idea?
>

Sorry for my belated on this discussion, after I searched for the
topics you posted, it seems two main problems are unsolved? (Am I
right?? I'm not sure...)

1. "I edited QEMU user, more exactly qemu-ppc launching the main function
(inside main.c) from another c function I created, passing it the
appropriate parameters. ...balabala" at Jan, 2011

2. "how can I check the number of target cpu cycles or target
instructions executed inside qemu-user (i.e. qemu-ppc)?
Is there any variable I can inspect for such informations?" at Dec, 2010

If I'm not correct, please let me know where the problem is.

Raphaël Lefèvre

Re: [Qemu-devel] TCG flow vs dyngen

[Stefano Bonifazi]

> Sorry for my belated on this discussion, after I searched for the
> topics you posted, it seems two main problems are unsolved? (Am I
> right?? I'm not sure...)
>
> 1. "I edited QEMU user, more exactly qemu-ppc launching the main function
> (inside main.c) from another c function I created, passing it the
> appropriate parameters. ...balabala" at Jan, 2011
>
> 2. "how can I check the number of target cpu cycles or target
> instructions executed inside qemu-user (i.e. qemu-ppc)?
> Is there any variable I can inspect for such informations?" at Dec, 2010
>
> If I'm not correct, please let me know where the problem is.
>
> Raphaël Lefèvre
Hi!
Thank you very much for Your concern!
Honestly I had lost hope in any help, I even contacted directly some 
developers in this mailing list without luck!
I am a student who needs to use qemu for a project where it will be used 
for its capabilities of running PowerPC code.
As you can imagine qemu goes far beyond the knowledge in electronics and 
computer science of a student. Nevertheless I have to do that!
I have been studying all the possible technical documents available in 
the internet, but it is really not much at all , not sufficient for 
getting the code and being able of understanding it .. It is in C, even 
not modular C++
Anyway with some help from this mailing list, and a lot of studying 
about assembly, loaders, compilers.. I am going on, though there are 
still big problems due of the nature of the QEMU code..
First of all, I am starting from qemu-user, more specifically, qemu-ppc 
as I don't need the full system capabilities, and it is easier for me to 
control the binary target memory with qemu-user.
Originally I started with a lot of work on libqemu .. until some 
developer here told me it was deprecated (though still in the source) 
and not working fine.
I edited the code of qemu-ppc so that another function of mine calls 
qemu-user main, with the appropriate parameters.. The pursued goal was 
to launch it several times with different target binaries in succession..
For some reason, I still can't find out, qemu code remembers the old 
code, running it instead of the new loaded binary.. and if I flush the 
cache of translated code before loading a new binary it stops and can't 
go on!
My workaround to this problem was compiling qemu-ppc as a dynamic 
library and load it at runtime.. I also managed to load multiple copies 
of it (with dlmopen each at a different address space) ..in fact I need 
to run more than one qemu-ppc at the same time but a new big problem 
popped up now: the target binary is loaded always at a fixed address.. 
no matter if another qemu-ppc already loaded code there.. it is like the 
internal elf loader can't understand those addresses are not available, 
and then relocate them ..
I tried to link (ld) the binary target elf as position independent code, 
but then qemu-ppc complains it can't find  /usr/lib/libc.so.1 and  
/usr/lib/ld.so.1

To sum up the problems are (in order of importance):
  - making the elf loader relocate the target code into other addresses 
when the default ones (I guess those embedded into the target binary 
when it is not compiled as position independent code) are taken
  - making qemu-user able of running more than one target binary in 
succession
  - counting qemu-user executed instructions

My university is a public one, so my project will be open to the 
community, I will also upload the documentation I am writing about qemu 
coming from the knowledge I am acquiring working on it, so that, I hope, 
other people will find less frustrating the first steps into developing 
qemu!

Any help will be more than welcome!

Thank you in advance!
Stefano B.

Re: [Qemu-devel] TCG flow vs dyngen

[Peter Maydell]

2011/1/16 Stefano Bonifazi <stefboombastic@gmail.com>:
> My workaround to this problem was compiling qemu-ppc as a dynamic library
> and load it at runtime.. I also managed to load multiple copies of it (with
> dlmopen each at a different address space) ..in fact I need to run more than
> one qemu-ppc at the same time

This approach seems very unlikely to work -- in general qemu in
both system and user mode assumes that there is only one
instance running in the host process address space, and things
are bound to clash. (Linux doesn't seem to have dlmopen but
google suggests that it puts the library in its own namespace
but not its own address space.) Running each qemu as its own
process and using interprocess communication for whatever
coordination you need between the various instances seems
more likely to be workable to me. This will also fix your "can't run
more than one binary in succession" problem, because you can
just have the first qemu run and exit as normal and launch a
second qemu to run the second binary.

-- PMM

Re: [Qemu-devel] TCG flow vs dyngen

[Stefano Bonifazi]

Thank you very much for Your fast reply!


On 01/16/2011 07:29 PM, Peter Maydell wrote:
> Linux doesn't seem to have dlmopen
http://www.unix.com/man-page/All/3c/dlmopen/

#define __USE_GNU
#include <dlfcn.h>

lib_handle1 = dlmopen(LM_ID_NEWLM,"./libqemu-ppc.so", RTLD_NOW);

I am developing that on a clean ubuntu 10.10
> but google suggests that it puts the library in its own namespace
> but not its own address space.
I need to make the different instances of qemu-user exchange data .. 
obviously keeping all of them in the same address space would be the 
easiest way (unless I have to change all qemu code ;) ) Running each 
qemu as its own
> process and using interprocess communication for whatever
> coordination you need between the various instances seems
> more likely to be workable to me. This will also fix your "can't run
> more than one binary in succession" problem, because you can
> just have the first qemu run and exit as normal and launch a
> second qemu to run the second binary.
>
> -- PMM
Exactly, it was the easiest way also for me.. and I've already done it, 
works smoothly .. the only big problem is that it is not good for my 
teacher.. he says it should work the dynamic library way o.O
Working with libraries even solved the problem of consecutive runs, 
though according to me it is not good a software when you must reboot it 
for making it run again fine.. sounds more Windows style :D
Clearly it makes memory "dirty" and do not clean after the target 
process completes its execution.. leaving the OS care about it.
I tried zeroing all global variables before starting a new execution 
without results (other than making it stall) .. After very long time 
spent trying to find a solution I think the problem should be with the 
mmap' ings stuff in the loader .. the same reason why 2 different 
libraries with their own namespaces clash according to me.. the elf 
loaders work globally within the unique address space .. I think for a 
guru of loaders-linkers should not be so difficult to patch it.. but not 
for a student who almost heard about them for the first time  ;)
Any help is very appreciated :)
Thank you again!
Stefano B.

Re: [Qemu-devel] TCG flow vs dyngen

[Raphaël Lefèvre]

2011/1/17 Stefano Bonifazi <stefboombastic@gmail.com>:
>
> Hi!
> Thank you very much for Your concern!
> Honestly I had lost hope in any help, I even contacted directly some
> developers in this mailing list without luck!

I guess many good developers in mailing list are still try their best
to solve your problems, such as Blue Swirl, Paolo Bonzini, Stefan
Weil, Peter Maydell, Mulyadi Santosa, Andreas Färber and Alexander
Graf (hope I won't lost anyone that had helped you, and the order of
name list without any meaning) ...etc., every developer has his
expertises, and it is hard to recognize all of the activities of qemu.
Please trust one thing: you are not alone:).

> I am a student who needs to use qemu for a project where it will be used for
> its capabilities of running PowerPC code.
> As you can imagine qemu goes far beyond the knowledge in electronics and
> computer science of a student. Nevertheless I have to do that!
> I have been studying all the possible technical documents available in the
> internet, but it is really not much at all , not sufficient for getting the
> code and being able of understanding it .. It is in C, even not modular C++

Due to the lack of tehnical document of qemu and you are a student
(maybe study for master/phd degree?), some literatures that published
on IEEE/ACM may give you some inspiration and help (suppose that your
university have bought the authority for download). As I know, though
the issue of qemu is relative new for the academia, there still are
literatures have been discussed. Maybe you can find which research
domain categorized that is most approximative to your works. If any
literature has inspired you or related to your research, don't
hasitate to discuss.

> Anyway with some help from this mailing list, and a lot of studying about
> assembly, loaders, compilers.. I am going on, though there are still big
> problems due of the nature of the QEMU code..
> First of all, I am starting from qemu-user, more specifically, qemu-ppc as I
> don't need the full system capabilities, and it is easier for me to control
> the binary target memory with qemu-user.

Is there any reason why should you use the user mode of qemu, not the
system mode? Sometime, the system mode of qemu will release you from
the nightmare for managing the memory hierarchy. Maybe you can start
from talking about what is the original goal of the project instead of
falling into the hell of code tracing.

> Originally I started with a lot of work on libqemu .. until some developer
> here told me it was deprecated (though still in the source) and not working
> fine.
> I edited the code of qemu-ppc so that another function of mine calls
> qemu-user main, with the appropriate parameters.. The pursued goal was to
> launch it several times with different target binaries in succession..
> For some reason, I still can't find out, qemu code remembers the old code,
> running it instead of the new loaded binary.. and if I flush the cache of
> translated code before loading a new binary it stops and can't go on!
> My workaround to this problem was compiling qemu-ppc as a dynamic library
> and load it at runtime.. I also managed to load multiple copies of it (with
> dlmopen each at a different address space) ..in fact I need to run more than
> one qemu-ppc at the same time but a new big problem popped up now: the

I need to thanks the Peter Maydell explained the principle that I'm
not familiar with. And from your description, would you want to invoke
multi-cores? Because I cannot imagine which application need to run
multiple qemu-ppc at the same time.

> target binary is loaded always at a fixed address.. no matter if another
> qemu-ppc already loaded code there.. it is like the internal elf loader
> can't understand those addresses are not available, and then relocate them
> ..
> I tried to link (ld) the binary target elf as position independent code, but
> then qemu-ppc complains it can't find  /usr/lib/libc.so.1 and
>  /usr/lib/ld.so.1
>

The above description seems to be out of my scope to answer, because I
only studied on system mode of qemu.

> To sum up the problems are (in order of importance):
>  - making the elf loader relocate the target code into other addresses when
> the default ones (I guess those embedded into the target binary when it is
> not compiled as position independent code) are taken

Maybe the problem only can be solved by re-write the loader if you
insist to use user mode. (just as your response to Peter)

>  - making qemu-user able of running more than one target binary in
> succession

Will m"ore than one target binary in succession (assume A then B then
C)" be achieved by "compile ABC into one binary in sequence"?

>  - counting qemu-user executed instructions

I guess all the works before this are for the goal: "counting
qemu-user executed instructions", am I right? If so, the paper
published in IEEE 2010 maybe give some help (I guess)
http://ieeexplore.ieee.org/xpl/freeabs_all.jsp?arnumber=5475901 (Make
sure that your university can access it)

Raphaël Lefèvre

Re: [Qemu-devel] TCG flow vs dyngen

[Peter Maydell]

2011/1/16 Stefano Bonifazi <stefboombastic@gmail.com>:
> I need to make the different instances of qemu-user exchange data ..
> obviously keeping all of them in the same address space would be the easiest
> way (unless I have to change all qemu code ;) )

The problem is that you're trying to break a fundamental
assumption made by a lot of qemu code. That's a large
job which involves understanding, checking and possibly
changing lots of already written code. In contrast, the
code you need to exchange data between the instances is
going to be fairly small and self contained and you'll already
understand it because you've written it/will write it. I think
it's pretty clear which one is going to be easier.

>> Running each qemu as its own
>> process and using interprocess communication for whatever
>> coordination you need between the various instances seems
>> more likely to be workable to me.

> Exactly, it was the easiest way also for me.. and I've already done it,
> works smoothly .. the only big problem is that it is not good for my
> teacher.. he says it should work the dynamic library way o.O

I think he's wrong. (You might like to think about what happens
if the program being emulated in qemu user-mode does a fork()).

Basically you're trying to do things the hard way; maybe
you can get something that sort of works in the subset of
cases you care about, but why on earth put in that much
time and effort on something irrelevant to the actual problem
you're trying to work on?

-- PMM

Re: [Qemu-devel] TCG flow vs dyngen

[Stefano Bonifazi]

Hi!
  In case you are interested in helping me, I'll give you a big piece of 
news I've just got (even my teacher is not informed yet! :) )
I've just managed to make more than one instance of qemu-user run at the 
same time linking the target code with a specified address for the code 
section (-Ttext address of ld).
It works fine and this proves my idea that the problem is within the elf 
loader..
Making it relocate the target code properly would fix the problem ;)
Now let's work on it :)
Regards,
Stefano B.

On 01/16/2011 08:02 PM, Stefano Bonifazi wrote:
> Thank you very much for Your fast reply!
>
>
> On 01/16/2011 07:29 PM, Peter Maydell wrote:
>> Linux doesn't seem to have dlmopen
> http://www.unix.com/man-page/All/3c/dlmopen/
>
> #define __USE_GNU
> #include <dlfcn.h>
>
> lib_handle1 = dlmopen(LM_ID_NEWLM,"./libqemu-ppc.so", RTLD_NOW);
>
> I am developing that on a clean ubuntu 10.10
>> but google suggests that it puts the library in its own namespace
>> but not its own address space.
> I need to make the different instances of qemu-user exchange data .. 
> obviously keeping all of them in the same address space would be the 
> easiest way (unless I have to change all qemu code ;) ) Running each 
> qemu as its own
>> process and using interprocess communication for whatever
>> coordination you need between the various instances seems
>> more likely to be workable to me. This will also fix your "can't run
>> more than one binary in succession" problem, because you can
>> just have the first qemu run and exit as normal and launch a
>> second qemu to run the second binary.
>>
>> -- PMM
> Exactly, it was the easiest way also for me.. and I've already done 
> it, works smoothly .. the only big problem is that it is not good for 
> my teacher.. he says it should work the dynamic library way o.O
> Working with libraries even solved the problem of consecutive runs, 
> though according to me it is not good a software when you must reboot 
> it for making it run again fine.. sounds more Windows style :D
> Clearly it makes memory "dirty" and do not clean after the target 
> process completes its execution.. leaving the OS care about it.
> I tried zeroing all global variables before starting a new execution 
> without results (other than making it stall) .. After very long time 
> spent trying to find a solution I think the problem should be with the 
> mmap' ings stuff in the loader .. the same reason why 2 different 
> libraries with their own namespaces clash according to me.. the elf 
> loaders work globally within the unique address space .. I think for a 
> guru of loaders-linkers should not be so difficult to patch it.. but 
> not for a student who almost heard about them for the first time  ;)
> Any help is very appreciated :)
> Thank you again!
> Stefano B.
>
>
>

Re: [Qemu-devel] TCG flow vs dyngen

[Raphaël Lefèvre]

2011/1/17 Stefano Bonifazi <stefboombastic@gmail.com>:
> Hi!
>  In case you are interested in helping me, I'll give you a big piece of news
> I've just got (even my teacher is not informed yet! :) )
> I've just managed to make more than one instance of qemu-user run at the
> same time linking the target code with a specified address for the code
> section (-Ttext address of ld).
> It works fine and this proves my idea that the problem is within the elf
> loader..
> Making it relocate the target code properly would fix the problem ;)
> Now let's work on it :)
> Regards,
> Stefano B.
>

Congratulation~ just keep going on~!

Raphaël Lefèvre

Re: [Qemu-devel] TCG flow vs dyngen

[Lluís]

Stefano Bonifazi writes:

> Hi!
>  In case you are interested in helping me, I'll give you a big piece of news
> I've just got (even my teacher is not informed yet! :) )

I still don't understand what is your high-level objective...


Lluis

-- 
 "And it's much the same thing with knowledge, for whenever you learn
 something new, the whole world becomes that much richer."
 -- The Princess of Pure Reason, as told by Norton Juster in The Phantom
 Tollbooth

Re: [Qemu-devel] TCG flow vs dyngen

[Rob Landley]

On 01/16/2011 10:01 AM, Raphaël Lefèvre wrote:
> On Sun, Jan 16, 2011 at 11:21 PM, Stefano Bonifazi
> <stefboombastic@gmail.com> wrote:
> 2. "how can I check the number of target cpu cycles or target
> instructions executed inside qemu-user (i.e. qemu-ppc)?
> Is there any variable I can inspect for such informations?" at Dec, 2010

Keep in mind I'm a bit rusty and not an expert, but I'll give a stab at
answering:

You can't, because QEMU doesn't work that way.  QEMU isn't an
instruction level emulator, it's closer to a Java JIT.  It doesn't
translate one instruction at a time but instead translates large blocks
of code all at once, and keeps a cache of translated blocks around.
Execution jumps into each block and either waits for it to exit again
(meaning it jumped out of that page and QEMU's main execution loop has
to look up what page to execute next, possibly translating it first if
it's not in the cache yet), or else QEMU interrupts it after while to
fake an IRQ of some kind (such as a timer interrupt).

You may want to read Fabrice Bellard's original paper on the QEMU design:

http://www.usenix.org/event/usenix05/tech/freenix/full_papers/bellard/bellard.pdf

Since that was written, dyngen was replaced with tcg, but that does the
same thing in a slightly different way.

Building a QEMU with dyngen support used to use the host compiler to
compile chunks of code corresponding to the target operations it would
see at runtime, and then strip the machine language out of the resulting
.o files and save them in a table.  Then at runtime dyngen could
generate translated pages by gluing together the resulting saved machine
language snippets the host compiler had produced when qemu was built.
The problem was, beating the right kind of machine language snippets out
of the .o files the compiler produced from the example code turned out
to be VERY COMPILER DEPENDENT.  This is why you couldn't build qemu with
gcc 4.x for the longest time, gcc's code generator and the layout of the
.o files changed in a bunch of subtle ways which broke dyngen's ability
to extract usable machine code snippets to put 'em into the table so it
could translate pages at runtime.

TCG stands for "Tiny Code Generator".  It just hardwires a code
generator into QEMU.  They wrote a mini-compiler in C, which knows what
instructions to output for each host qemu supports.  If QEMU understands
target instructions well enough to _read_ them, it's not a big stretch
to be able to _write_ them when running on that kind of host.  (It's
more or less the same operation in reverse.)  This means that QEMU can
no longer run on a type of host it can't execute target code for, but
the solution is to just add support for all the interesting machines out
there, on both sides.

So, when QEMU executes code, the virtual MMU faults a new page into the
virtual TLB, and goes "I can't execute this, fix it up!"  And the fixup
handler looks for a translation of the page in the cache of translated
pages, and if it can't find it it calls the translator to convert the
target code into a page of corresponding host code.  Which may involve
discarding an existing entry out of the cache, but this is how
instruction caches work on real hardware anyway so the delays in QEMU
are where they'd be on real hardware anyway, and optimizing for one is
pretty close to optimizing for the other, so life is good.

The chunk you found earlier is a function pointer typecast:

#define tcg_qemu_tb_exec(tb_ptr) \
  ((long REGPARM (*)(void *))code_gen_prologue)(tb_ptr)

Which looks like it's calling code_gen_prologue() with tp_ptr as its
argument (typecast to a void *), and it returns a long.  That calls a
translated page, and when the function returns that means the page of
code needs to jump to code somewhere outside of that page, and we go
back to the main loop to figure out where to go next.

The reason QEMU is as fast as it is is because once it has a page of
translated code, actually _running_ it is entirely native.  It jumps
into the page, and executes natively until it leaves the page.   Control
only goes back to QEMU to switch pages or to handle I/O and interrupts
and such.  So when you ask "how many clock cycles did that instruction
take", the answer is "it doesn't work that way".  QEMU emulates at
memory page level (generally 4k of target code), not at individual
instruction level.

(Oh, and the worst thing you can do to QEMU from a performance
perspective is self-modifying code.  Because the virtual MMU has to
strip the executable bit off the TLB entry and re-translate the entire
page next time something tries to execute it.  It _works_, it's just
slow.  But again, real hardware can hiccup a bit on this too.)

Does that answer your question?

Rob

Re: [Qemu-devel] TCG flow vs dyngen

[Stefano Bonifazi]

On 01/23/2011 10:50 PM, Rob Landley wrote:
> On 01/16/2011 10:01 AM, Raphaël Lefèvre wrote:
>> On Sun, Jan 16, 2011 at 11:21 PM, Stefano Bonifazi
>> <stefboombastic@gmail.com>  wrote:
>> 2. "how can I check the number of target cpu cycles or target
>> instructions executed inside qemu-user (i.e. qemu-ppc)?
>> Is there any variable I can inspect for such informations?" at Dec, 2010
> Keep in mind I'm a bit rusty and not an expert, but I'll give a stab at
> answering:
>
> You can't, because QEMU doesn't work that way.  QEMU isn't an
> instruction level emulator, it's closer to a Java JIT.  It doesn't
> translate one instruction at a time but instead translates large blocks
> of code all at once, and keeps a cache of translated blocks around.
> Execution jumps into each block and either waits for it to exit again
> (meaning it jumped out of that page and QEMU's main execution loop has
> to look up what page to execute next, possibly translating it first if
> it's not in the cache yet), or else QEMU interrupts it after while to
> fake an IRQ of some kind (such as a timer interrupt).
>
> You may want to read Fabrice Bellard's original paper on the QEMU design:
>
> http://www.usenix.org/event/usenix05/tech/freenix/full_papers/bellard/bellard.pdf
>
> Since that was written, dyngen was replaced with tcg, but that does the
> same thing in a slightly different way.
>
> Building a QEMU with dyngen support used to use the host compiler to
> compile chunks of code corresponding to the target operations it would
> see at runtime, and then strip the machine language out of the resulting
> .o files and save them in a table.  Then at runtime dyngen could
> generate translated pages by gluing together the resulting saved machine
> language snippets the host compiler had produced when qemu was built.
> The problem was, beating the right kind of machine language snippets out
> of the .o files the compiler produced from the example code turned out
> to be VERY COMPILER DEPENDENT.  This is why you couldn't build qemu with
> gcc 4.x for the longest time, gcc's code generator and the layout of the
> .o files changed in a bunch of subtle ways which broke dyngen's ability
> to extract usable machine code snippets to put 'em into the table so it
> could translate pages at runtime.
>
> TCG stands for "Tiny Code Generator".  It just hardwires a code
> generator into QEMU.  They wrote a mini-compiler in C, which knows what
> instructions to output for each host qemu supports.  If QEMU understands
> target instructions well enough to _read_ them, it's not a big stretch
> to be able to _write_ them when running on that kind of host.  (It's
> more or less the same operation in reverse.)  This means that QEMU can
> no longer run on a type of host it can't execute target code for, but
> the solution is to just add support for all the interesting machines out
> there, on both sides.
>
> So, when QEMU executes code, the virtual MMU faults a new page into the
> virtual TLB, and goes "I can't execute this, fix it up!"  And the fixup
> handler looks for a translation of the page in the cache of translated
> pages, and if it can't find it it calls the translator to convert the
> target code into a page of corresponding host code.  Which may involve
> discarding an existing entry out of the cache, but this is how
> instruction caches work on real hardware anyway so the delays in QEMU
> are where they'd be on real hardware anyway, and optimizing for one is
> pretty close to optimizing for the other, so life is good.
>
> The chunk you found earlier is a function pointer typecast:
>
> #define tcg_qemu_tb_exec(tb_ptr) \
>    ((long REGPARM (*)(void *))code_gen_prologue)(tb_ptr)
>
> Which looks like it's calling code_gen_prologue() with tp_ptr as its
> argument (typecast to a void *), and it returns a long.  That calls a
> translated page, and when the function returns that means the page of
> code needs to jump to code somewhere outside of that page, and we go
> back to the main loop to figure out where to go next.
>
> The reason QEMU is as fast as it is is because once it has a page of
> translated code, actually _running_ it is entirely native.  It jumps
> into the page, and executes natively until it leaves the page.   Control
> only goes back to QEMU to switch pages or to handle I/O and interrupts
> and such.  So when you ask "how many clock cycles did that instruction
> take", the answer is "it doesn't work that way".  QEMU emulates at
> memory page level (generally 4k of target code), not at individual
> instruction level.
>
> (Oh, and the worst thing you can do to QEMU from a performance
> perspective is self-modifying code.  Because the virtual MMU has to
> strip the executable bit off the TLB entry and re-translate the entire
> page next time something tries to execute it.  It _works_, it's just
> slow.  But again, real hardware can hiccup a bit on this too.)
>
> Does that answer your question?
>
> Rob
Wow! Thank you! That's an ANSWER!
Gold for who's studying all of that! Though at the stage of my work I 
had to "understand" almost all of it, your perfect summary make 
everything much clearer..
About counting instructions I found that counting the instructions of 
each executed TB was a very good approximation, sure the cache represent 
a major problem, already translated TB can't be counted that way.. I'd 
like to disable the cache, but the parameter singlestep doesn't seem to 
work for qemu-user.
  Right now I am stuck with another problem .. maybe with your 
experience you can tell me whether it is possible at all..
I am trying to shift in memory the target executable .. now the code is 
"supposed" to be loaded by the elfloader at the exact start address set 
at link time ..
Inside elfloader there is even a check for verifying whether that 
address range is busy.. but no action is taken in that case o.O
Maybe I'll post a new thread about this problem (bug?) .. anyway if you 
think you can help me anyway I'll give you further details..
Thank you really very much again for your great explanation!
Best Regards!
Stefano B.

Re: [Qemu-devel] TCG flow vs dyngen

[Rob Landley]

On 01/23/2011 04:25 PM, Stefano Bonifazi wrote:
> I am trying to shift in memory the target executable .. now the code is
> "supposed" to be loaded by the elfloader at the exact start address set
> at link time ..

Ah, elf loading.  That's a whole 'nother bag of worms.

Oddly enough, I was deling with this last year trying to debug the
uClibc dynamic linker.  I blogged a bit about it at the time:

  http://landley.net/notes-2010.html#12-07-2010

(And the next few days.  Sigh, I never did go back and fill in the
holes, did I?)

> Inside elfloader there is even a check for verifying whether that
> address range is busy.. but no action is taken in that case o.O
> Maybe I'll post a new thread about this problem (bug?) .. anyway if you
> think you can help me anyway I'll give you further details..

Tired right now, but if you post a clearer question (what are you trying
to _do_) and cc: me on it I'll try to respond.

Maybe I can find some decent documentation to point you at, or maybe
I'll write some...

Rob

Re: [Qemu-devel] TCG flow vs dyngen

[Stefano Bonifazi]

On 01/24/2011 12:40 AM, Rob Landley wrote:
> On 01/23/2011 04:25 PM, Stefano Bonifazi wrote:
>> I am trying to shift in memory the target executable .. now the code is
>> "supposed" to be loaded by the elfloader at the exact start address set
>> at link time ..
> Ah, elf loading.  That's a whole 'nother bag of worms.
>
> Oddly enough, I was deling with this last year trying to debug the
> uClibc dynamic linker.  I blogged a bit about it at the time:
>
>    http://landley.net/notes-2010.html#12-07-2010
>
> (And the next few days.  Sigh, I never did go back and fill in the
> holes, did I?)
>
>> Inside elfloader there is even a check for verifying whether that
>> address range is busy.. but no action is taken in that case o.O
>> Maybe I'll post a new thread about this problem (bug?) .. anyway if you
>> think you can help me anyway I'll give you further details..
> Tired right now, but if you post a clearer question (what are you trying
> to _do_) and cc: me on it I'll try to respond.
>
> Maybe I can find some decent documentation to point you at, or maybe
> I'll write some...
>
> Rob
Thank you!
  I read your post, and yup you also noticed the weird of load_bias.. 
and wondered how it can work on x86..
But I think your work was on qemu-system.. I am working on qemu-user..
Yup better to post a new thread, I'll cc: you there!
Thank you very much!
Stefano B

[Qemu-devel] Re: TCG flow vs dyngen

[Stefano Bonifazi]

On 01/17/2011 12:59 PM, Lluís wrote:
> Stefano Bonifazi writes:
>
>> Hi!
>>   In case you are interested in helping me, I'll give you a big piece of news
>> I've just got (even my teacher is not informed yet! :) )
>
> I still don't understand what is your high-level objective...
>
>
> Lluis
>
Hi!
Sorry I've noticed your reply only know (dunno why I was not notified by 
email!)
Do you mean what is my final goal?

[Qemu-devel] Re: TCG flow vs dyngen

[Stefano Bonifazi]

On 01/16/2011 10:08 PM, Raphaël Lefèvre wrote:
> 2011/1/17 Stefano Bonifazi<stefboombastic@gmail.com>:
>> Hi!
>>   In case you are interested in helping me, I'll give you a big piece of news
>> I've just got (even my teacher is not informed yet! :) )
>> I've just managed to make more than one instance of qemu-user run at the
>> same time linking the target code with a specified address for the code
>> section (-Ttext address of ld).
>> It works fine and this proves my idea that the problem is within the elf
>> loader..
>> Making it relocate the target code properly would fix the problem ;)
>> Now let's work on it :)
>> Regards,
>> Stefano B.
>>
>
> Congratulation~ just keep going on~!
>
> Raphaël Lefèvre
>
>
Thank you!
  Working on the elf loader I found out many problems on that code..
If you are interested you can have a look to my last post!
Best regards!
Stefano B.

[Qemu-devel] Re: TCG flow vs dyngen

[Stefano Bonifazi]

On 01/16/2011 08:24 PM, Peter Maydell wrote:
> 2011/1/16 Stefano Bonifazi<stefboombastic@gmail.com>:
>> I need to make the different instances of qemu-user exchange data ..
>> obviously keeping all of them in the same address space would be the easiest
>> way (unless I have to change all qemu code ;) )
>
> The problem is that you're trying to break a fundamental
> assumption made by a lot of qemu code. That's a large
> job which involves understanding, checking and possibly
> changing lots of already written code. In contrast, the
> code you need to exchange data between the instances is
> going to be fairly small and self contained and you'll already
> understand it because you've written it/will write it. I think
> it's pretty clear which one is going to be easier.
>
>>> Running each qemu as its own
>>> process and using interprocess communication for whatever
>>> coordination you need between the various instances seems
>>> more likely to be workable to me.
>
>> Exactly, it was the easiest way also for me.. and I've already done it,
>> works smoothly .. the only big problem is that it is not good for my
>> teacher.. he says it should work the dynamic library way o.O
>
> I think he's wrong. (You might like to think about what happens
> if the program being emulated in qemu user-mode does a fork()).
>
> Basically you're trying to do things the hard way; maybe
> you can get something that sort of works in the subset of
> cases you care about, but why on earth put in that much
> time and effort on something irrelevant to the actual problem
> you're trying to work on?
>
> -- PMM
>
>
Well my teacher's answer was that it is useless doing that, as there are 
already plenty of solutions based on IPC .. they are interested in this 
other approach, testing it .. They are not interested on how difficult 
it can be for a student, how long it can take.. :(
Best regards,
Stefano B.

Re: [Qemu-devel] Re: TCG flow vs dyngen

[Lluís]

Stefano Bonifazi writes:

> Do you mean what is my final goal?

Exactly. A higher level perspective of what is our ultimate goal might
help others figure out better ways to do it.

Right now I don't remember what you posted your where technically trying
to do, but I do remember it looked convoluted to me.


Lluis

-- 
 "And it's much the same thing with knowledge, for whenever you learn
 something new, the whole world becomes that much richer."
 -- The Princess of Pure Reason, as told by Norton Juster in The Phantom
 Tollbooth

Re: [Qemu-devel] Re: TCG flow vs dyngen

[Stefano Bonifazi]

On 01/24/2011 02:36 PM, Lluís wrote:
> Stefano Bonifazi writes:
>
>> Do you mean what is my final goal?
> Exactly. A higher level perspective of what is our ultimate goal might
> help others figure out better ways to do it.
>
> Right now I don't remember what you posted your where technically trying
> to do, but I do remember it looked convoluted to me.
>
>
> Lluis
>
Sorry if I could not explain it better before, but it was not totally 
clear for me too since the beginning, as I get new specs from my teacher 
on the way, according what I manage to do, and where I find big obstacles!
Now, the final goal is to get multiple instances of qemu-ppc driven by a 
systemc project executing on a x86 machine, with the different qemu-ppc 
instances used as emulators for power-pc binaries.. I would get the 
results of the run of the various ppc binaries back to the systemc 
project and work with the results then.
I've already managed to integrate systemc with qemu-ppc, and I managed 
to load multiple instances of qemu together, by loading it as a dynamic 
library.
I think much confusion about my goals was originated by the fact that 
the first attempt (failed) was to use qemu-user for loading many target 
binaries one after the other.. Then I changed for having many instances 
of qemu-user at the same time inside the same process..
The actual problem is letting qemu-user able of loading target code at a 
different address than the one chosen by the link editor when creating 
the binary..
If you are interested in that I've just created a new post about it:
   http://lists.nongnu.org/archive/html/qemu-devel/2011-01/msg02361.html
Best regards,
Stefano B.

Re: [Qemu-devel] TCG flow vs dyngen

[Peter Maydell]

2011/1/23 Rob Landley <rob@landley.net>:
> Keep in mind I'm a bit rusty and not an expert, but I'll give a stab at
> answering:

...here's a couple of clarifications:

>> 2. "how can I check the number of target cpu cycles or target
>> instructions executed inside qemu-user (i.e. qemu-ppc)?

> You can't, because QEMU doesn't work that way. QEMU isn't an
> instruction level emulator, it's closer to a Java JIT.

Being a JIT doesn't prohibit counting target instructions executed.
It just means that counting them generally requires generating
code to do the counting at runtime, so it's a more complicated
change to make than it would be in a non-JIT emulator.

The major reason for not counting cycles is that for an emulation
of a modern CPU this is pretty nearly impossible: the number
of cycles an instruction takes can depend on whether it causes
a cache miss, which CPU internal pipeline it uses, whether it
needs to stall waiting for a result from an earlier insn, whether
the CPU correctly predicted the branch leading up to it or not,
and on and on. You would need to precisely model all the
internals of each variant of each CPU, which would be a
mammoth undertaking requiring probably unpublished internal
data, and if you ever managed to finish it then it would run
incredibly slowly and would probably contain enough bugs you
couldn't trust the data it gave you anyway.

> This means that QEMU can
> no longer run on a type of host it can't execute target code for

This isn't correct; for instance there's hppa support in TCG for hppa
hosts but no hppa target support, and there's sh4 target support
but no TCG backend for it. The two ends are cleanly separated in
qemu and don't generally depend on each other.

-- PMM

Re: [Qemu-devel] TCG flow vs dyngen

[Stefano Bonifazi]

On 01/24/2011 03:32 PM, Peter Maydell wrote:
>
> Being a JIT doesn't prohibit counting target instructions executed.
> It just means that counting them generally requires generating
> code to do the counting at runtime, so it's a more complicated
> change to make than it would be in a non-JIT emulator.
>
What do you mean? Should I change the code of qemu-user for counting the 
instructions, or should I add code into the target binaries?
> The major reason for not counting cycles is that for an emulation
> of a modern CPU this is pretty nearly impossible: the number
> of cycles an instruction takes can depend on whether it causes
> a cache miss, which CPU internal pipeline it uses, whether it
> needs to stall waiting for a result from an earlier insn, whether
> the CPU correctly predicted the branch leading up to it or not,
> and on and on. You would need to precisely model all the
> internals of each variant of each CPU, which would be a
> mammoth undertaking requiring probably unpublished internal
> data, and if you ever managed to finish it then it would run
> incredibly slowly and would probably contain enough bugs you
> couldn't trust the data it gave you anyway.
>
Yup, I think it was just a silly mistake of mine when in the first post 
I wrote cycles.. that was because for me anything that can estimate how 
long it takes to do the work would be fine.. I can't simply check the 
time because that is host machine dependent... Number of executed 
instructions would be fine..
>> This means that QEMU can
>> no longer run on a type of host it can't execute target code for
> This isn't correct; for instance there's hppa support in TCG for hppa
> hosts but no hppa target support, and there's sh4 target support
> but no TCG backend for it. The two ends are cleanly separated in
> qemu and don't generally depend on each other.
>
Well I experienced a strange behavior some time ago that initially made 
me think mr Rob was right on that though I knew host support and target 
support were separated in qemu: I tried to make directly qemu-ppc on a 
x86_64 machine from inside ppc-linux-user folder (i can do fine onto x86 
machine) and it failed because there was no tgc/x86_64/tcg_target.h, 
whereas doing the make from within the main folder worked.
So I do not understand very well.. is there some required headers fix 
when using the main make file?
  Best regards!
Stefano B.

Re: [Qemu-devel] Re: TCG flow vs dyngen

[Lluís]

Stefano Bonifazi writes:

> Now, the final goal is to get multiple instances of qemu-ppc driven by a systemc
> project executing on a x86 machine, with the different qemu-ppc instances used
> as emulators for power-pc binaries.. I would get the results of the run of the
> various ppc binaries back to the systemc project and work with the results then.

If I understand this correctly, the execution of one of your PPC cores
is oblivious of the others (they share no guest physical memory).

If that's true, it would be extremely more simple to make systemc launch
a separate process with a modified version of linux-user. This modified
version would just change the main loop in a way that it can communicate
with systemc, e.g., using shared memory.

Of course, depending on the granularity of synchronization that you
want, this might not prove useful: if systemc and the linux-user
processes all execute in the finest possible synchronization (e.g.,
comunicate after each TB or even after each instruction), the
communication overhead might prove too high.

Still, if you want a somewhat looser synchronization, you could take a
look at how COTSon [1] does it.


[1] http://sites.google.com/site/hplabscotson/

Lluis

--
 "And it's much the same thing with knowledge, for whenever you learn
 something new, the whole world becomes that much richer."
 -- The Princess of Pure Reason, as told by Norton Juster in The Phantom
 Tollbooth

Re: [Qemu-devel] TCG flow vs dyngen

[Lluís]

Stefano Bonifazi writes:

> On 01/24/2011 03:32 PM, Peter Maydell wrote:
>>
>> Being a JIT doesn't prohibit counting target instructions executed.
>> It just means that counting them generally requires generating
>> code to do the counting at runtime, so it's a more complicated
>> change to make than it would be in a non-JIT emulator.
>>
> What do you mean? Should I change the code of qemu-user for counting the
> instructions, or should I add code into the target binaries?


If I recall this correctly, target-i386 has a generic function (whose
name I don't remember) called whenever the rdtsc instruction is
executed.

This function rebuilds the counter that contains the number of executed
instructions (more or less, this number can be tuned from a variety of
sources).


Lluis

--
 "And it's much the same thing with knowledge, for whenever you learn
 something new, the whole world becomes that much richer."
 -- The Princess of Pure Reason, as told by Norton Juster in The Phantom
 Tollbooth

Re: [Qemu-devel] Re: TCG flow vs dyngen

[Stefano Bonifazi]

Hi!

Thank you for answering me!

> If I understand this correctly, the execution of one of your PPC cores
> is oblivious of the others (they share no guest physical memory).
>
No! They do share the same address space.. the way I am loading the 
different qemu-ppc instances divides their namespaces allowing them to 
coexist, but they share the same address space anyway (that is the same 
of the caller process too), that is what I want for a communication.
The problem is that they are at the same time oblivious of the others 
and each of them wants to map its target binary at the same unique 
virtual address (again see my last post about relocating target code)..
I tried successfully the way of IPC (interprocess communication) having 
a different qemu-ppc spawned by systemc as a process, then using shared 
memory and signals for communicating.. pretty easy and well working, but 
the specs of my project (university) do not let me using IPC..

> [1] http://sites.google.com/site/hplabscotson/
Thank you, I am a student of digital electronics, with not big knowledge 
about developing in linux but this project is very interesting for my 
field.. some sort of alternative to systemc if I understand fine! Thanks 
surely I'll have a look at that!
Best regards!
Stefano B.

Re: [Qemu-devel] TCG flow vs dyngen

[Dushyant Bansal]

[-- Attachment #1: Type: text/plain, Size: 650 bytes --]

On Monday 24 January 2011 08:26 PM, Stefano Bonifazi wrote:
> On 01/24/2011 03:32 PM, Peter Maydell wrote:
>>
>> Being a JIT doesn't prohibit counting target instructions executed.
>> It just means that counting them generally requires generating
>> code to do the counting at runtime, so it's a more complicated
>> change to make than it would be in a non-JIT emulator.
>>
> What do you mean? Should I change the code of qemu-user for counting 
> the instructions, or should I add code into the target binaries?
You should see this pdf 
(www.ecs.syr.edu/faculty/yin/Teaching/TC2010/Proj4.pdf). It talks about 
tracing the instructions.

--
Dushyant

[-- Attachment #2: Type: text/html, Size: 1218 bytes --]

Re: [Qemu-devel] Re: TCG flow vs dyngen

[Lluís]

Stefano Bonifazi writes:

> Hi!
> Thank you for answering me!

>> If I understand this correctly, the execution of one of your PPC cores
>> is oblivious of the others (they share no guest physical memory).
>>
> No! They do share the same address space.. the way I am loading the different
> qemu-ppc instances divides their namespaces allowing them to coexist, but they
> share the same address space anyway (that is the same of the caller process
> too), that is what I want for a communication.

Sure, but each core runs a separate (guest) process. It's not that they
are (guest) threads of the same (guest) application.


> The problem is that they are at the same time oblivious of the others and each
> of them wants to map its target binary at the same unique virtual address (again
> see my last post about relocating target code)..

Right. The important point here is that each qemu core runs a separate
(guest) process.


> I tried successfully the way of IPC (interprocess communication) having a
> different qemu-ppc spawned by systemc as a process, then using shared memory and
> signals for communicating.. pretty easy and well working, but the specs of my
> project (university) do not let me using IPC..

That sounds silly, but there's no way out if this is in the rules of the
project.


>> [1] http://sites.google.com/site/hplabscotson/
> Thank you, I am a student of digital electronics, with not big knowledge about
> developing in linux but this project is very interesting for my field.. some
> sort of alternative to systemc if I understand fine! Thanks surely I'll have a
> look at that!

Well, if you don't know about avilable simulators, there's a plethora of
them. From the top of my head, I can give you these other names:

* SESC
  http://sesc.sourceforge.net/
* M5
  http://www.m5sim.org/wiki/index.php/Main_Page
* GEMS
  http://www.cs.wisc.edu/gems/
* SimpleScalar
  http://www.simplescalar.com/
* Graphite
  http://groups.csail.mit.edu/carbon/?page_id=111


Lluis

--
 "And it's much the same thing with knowledge, for whenever you learn
 something new, the whole world becomes that much richer."
 -- The Princess of Pure Reason, as told by Norton Juster in The Phantom
 Tollbooth

Re: [Qemu-devel] TCG flow vs dyngen

[Rob Landley]

On 01/24/2011 04:17 AM, Stefano Bonifazi wrote:
>  I read your post, and yup you also noticed the weird of load_bias.. and
> wondered how it can work on x86..
> But I think your work was on qemu-system.. I am working on qemu-user..

My post wasn't on qemu-anything, it was while I was trying to debug the
uClibc dynamic loader on a new platform (the Qualcomm Hexagon) that
Linux support still hasn't gone upstream for yet.

The thing is, the kernel currently _does_ work, so studying the relevant
kernel code (and possibly the dynamic loader code) is one way to learn
how it currently works.

Rob

Re: [Qemu-devel] TCG flow vs dyngen

[Stefano Bonifazi]

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On 01/24/2011 07:02 PM, Dushyant Bansal wrote:
> On Monday 24 January 2011 08:26 PM, Stefano Bonifazi wrote:
>> On 01/24/2011 03:32 PM, Peter Maydell wrote:
>>>
>>> Being a JIT doesn't prohibit counting target instructions executed.
>>> It just means that counting them generally requires generating
>>> code to do the counting at runtime, so it's a more complicated
>>> change to make than it would be in a non-JIT emulator.
>>>
>> What do you mean? Should I change the code of qemu-user for counting 
>> the instructions, or should I add code into the target binaries?
> You should see this pdf 
> (www.ecs.syr.edu/faculty/yin/Teaching/TC2010/Proj4.pdf). It talks 
> about tracing the instructions.
>
> --
> Dushyant
Wow thank you! It sounds incredibly interesting!!
> What we really need is to insert a function call into the
> translated code, so when each instruction is executed at runtime, our 
> inserted function will be
> executed.
Again wow!! Is that really possible? Some sort of callback triggered at 
every instruction execution?
Do you have any another document explaining that?
This pdf just gives instructions on how to do it on an old version of 
qemu (disas_insn doesn't exist at all on my code now), and does not 
explain what it is, what's behind that suggested code ..
Also the code for single step would be of great help to me! I really 
needed that.. but when I tried it on qemu-user didn't work at all..
Thank you very much!
Best regards,
Stefano B.




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Re: [Qemu-devel] TCG flow vs dyngen

[Stefano Bonifazi]

Hi! Thanks for replying me!
> The thing is, the kernel currently _does_ work, so studying the relevant
> kernel code (and possibly the dynamic loader code) is one way to learn
> how it currently works.
Sorry what kernel? Qemu's? Linux's?

Re: [Qemu-devel] TCG flow vs dyngen

[Rob Landley]

On 01/24/2011 03:16 PM, Stefano Bonifazi wrote:
> Hi! Thanks for replying me!
>> The thing is, the kernel currently _does_ work, so studying the relevant
>> kernel code (and possibly the dynamic loader code) is one way to learn
>> how it currently works.
> Sorry what kernel? Qemu's? Linux's?

QEMU isn't a kernel, it's an emulator.  Linux is a kernel.

I meant Linux loads and runs Linux ELF executables.  That's pretty much
the definition of "how to do it".  So if there's ever a conflict between
"how qemu does it" and "how the Linux kernel does it", the Linux kernel
is going to win.  (And yes, this has come up before, for me it was
http://www.mail-archive.com/qemu-devel@nongnu.org/msg25336.html )

That said, QEMU's currently working fairly well on this front too, so
studying either should work pretty well...

One advantage of the kernel is "cat /proc/$PID/maps" which lets you know
what the mappings are, and then you can look up the appropriate chunks
of the executable and read the elf spec:

  http://refspecs.freestandards.org/elf/elf.pdf

And to be honest, the best way to get up to speed on this is to read this:

  http://www.muppetlabs.com/~breadbox/software/tiny/teensy.html

Where some guy asked "ok, what do we actually NEED" and then set out to
prove it.

This book is pretty good too, although so dry it's almost unreadable.
You might have better luck getting a paper copy out of the library:

  http://www.iecc.com/linker/

Rob

Re: [Qemu-devel] TCG flow vs dyngen

[Dushyant Bansal]

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>> You should see this pdf 
>> (www.ecs.syr.edu/faculty/yin/Teaching/TC2010/Proj4.pdf). It talks 
>> about tracing the instructions.
>>
>> --
>> Dushyant
> Wow thank you! It sounds incredibly interesting!!
>> What we really need is to insert a function call into the
>> translated code, so when each instruction is executed at runtime, our 
>> inserted function will be
>> executed.
> Again wow!! Is that really possible? Some sort of callback triggered 
> at every instruction execution?
Yes, this mechanism works. I have written a code to count different 
kinds of instructions.
> Do you have any another document explaining that?
No. But maybe you can try to understand this through qemu source code. 
Here are some resources for that 
http://stackoverflow.com/questions/4501173/a-call-to-those-who-have-worked-with-qemu
> This pdf just gives instructions on how to do it on an old version of 
> qemu (disas_insn doesn't exist at all on my code now), and does not 
> explain what it is, what's behind that suggested code ..
> Also the code for single step would be of great help to me! I really 
> needed that.. but when I tried it on qemu-user didn't work at all..
It exists in file qemu/target-i386/translate.c You are also talking 
about qemu source code privided here http://wiki.qemu.org/Download, right?
If you need, I can give the source code of counting implementation with 
some documentation.
Hope this helps.

--
Dushyant

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Re: [Qemu-devel] TCG flow vs dyngen

[Stefano Bonifazi]

> That said, QEMU's currently working fairly well on this front too, so
> studying either should work pretty well...
>
Mr Richard Henderson's patch on elfload.c says I was right.. at least 
the version I am working on (qemu-0.13.0) had some bugs and weaknesses 
though it worked smoothly for most cases..

> And to be honest, the best way to get up to speed on this is to read this:
>
>    http://www.muppetlabs.com/~breadbox/software/tiny/teensy.html
uhmm seems a good piece.. maybe one of the last I still didn't have :)
Thank you!!
Stefano B.

Re: [Qemu-devel] TCG flow vs dyngen

[Stefano Bonifazi]

Again wow!! Is that really possible? Some sort of callback triggered at 
every instruction execution?
> Yes, this mechanism works. I have written a code to count different 
> kinds of instructions. 
Great! that opens a lot of possibilities!.
> It exists in file qemu/target-i386/translate.c 
Ops right! I checked target-ppc/translate.c as I need Power-PC as 
target.. I wonder what function replaces it there..
> You are also talking about qemu source code privided here 
> http://wiki.qemu.org/Download, right?
Yes I am using this http://wiki.qemu.org/download/qemu-0.13.0.tar.gz
> If you need, I can give the source code of counting implementation 
> with some documentation.
> Hope this helps.
>
Wow that would be awesome! I'd really appreciate it very much! Thank you! :)
You are free of sending it to my address! :)

Best regards!!
Stefano B.

Re: [Qemu-devel] TCG flow vs dyngen

[Edgar E. Iglesias]

On Tue, Jan 25, 2011 at 10:04:39AM +0100, Stefano Bonifazi wrote:
> Again wow!! Is that really possible? Some sort of callback triggered at 
> every instruction execution?
> > Yes, this mechanism works. I have written a code to count different 
> > kinds of instructions. 
> Great! that opens a lot of possibilities!.
> > It exists in file qemu/target-i386/translate.c 
> Ops right! I checked target-ppc/translate.c as I need Power-PC as 
> target.. I wonder what function replaces it there..
> > You are also talking about qemu source code privided here 
> > http://wiki.qemu.org/Download, right?
> Yes I am using this http://wiki.qemu.org/download/qemu-0.13.0.tar.gz
> > If you need, I can give the source code of counting implementation 
> > with some documentation.
> > Hope this helps.
> >
> Wow that would be awesome! I'd really appreciate it very much! Thank you! :)
> You are free of sending it to my address! :)

Hi,

If you are interested in instruction counting maybe you should take
a look at the -icount option as well.

Cheers

Re: [Qemu-devel] TCG flow vs dyngen

[Stefano Bonifazi]

On 01/25/2011 10:05 AM, Edgar E. Iglesias wrote:
> On Tue, Jan 25, 2011 at 10:04:39AM +0100, Stefano Bonifazi wrote:
>> Again wow!! Is that really possible? Some sort of callback triggered at
>> every instruction execution?
>>> Yes, this mechanism works. I have written a code to count different
>>> kinds of instructions.
>> Great! that opens a lot of possibilities!.
>>> It exists in file qemu/target-i386/translate.c
>> Ops right! I checked target-ppc/translate.c as I need Power-PC as
>> target.. I wonder what function replaces it there..
>>> You are also talking about qemu source code privided here
>>> http://wiki.qemu.org/Download, right?
>> Yes I am using this http://wiki.qemu.org/download/qemu-0.13.0.tar.gz
>>> If you need, I can give the source code of counting implementation
>>> with some documentation.
>>> Hope this helps.
>>>
>> Wow that would be awesome! I'd really appreciate it very much! Thank you! :)
>> You are free of sending it to my address! :)
> Hi,
>
> If you are interested in instruction counting maybe you should take
> a look at the -icount option as well.
>
> Cheers
Thank you!
Already tried long ago, it doesn't work with qemu-user..If I remember 
fine its core was in files not used in qemu-user :(
Regards,
Stefano B.