BOOT_CS

BOOT_CS

[H. Peter Anvin]

Anyone happen to know of any legitimate reason not to reload %cs in
head.S?  I think the following would be a lot cleaner, as well as a
lot safer (the jump and indirect branch aren't guaranteed to have the
proper effects, although technically neither should be required due to
the %cr0 write):

@@ -117,10 +147,7 @@
        movl %cr0,%eax
        orl $0x80000000,%eax
        movl %eax,%cr0          /* ..and set paging (PG) bit */
-       jmp 1f                  /* flush the prefetch-queue */
-1:
-       movl $1f,%eax
-       jmp *%eax               /* make sure eip is relocated */
+       ljmp $__BOOT_CS,$1f     /* Clear prefetch and normalize %eip
*/
 1:
        /* Set up the stack pointer */
        lss stack_start,%esp


I've been doing some cleanups in head.S after making the early page
tables dynamic.

	-hpa

Re: BOOT_CS

[Coywolf Qi Hunt]

H. Peter Anvin wrote:
> Anyone happen to know of any legitimate reason not to reload %cs in
> head.S?  I think the following would be a lot cleaner, as well as a
> lot safer (the jump and indirect branch aren't guaranteed to have the
> proper effects, although technically neither should be required due to
> the %cr0 write):
> 
> @@ -117,10 +147,7 @@
>         movl %cr0,%eax
>         orl $0x80000000,%eax
>         movl %eax,%cr0          /* ..and set paging (PG) bit */
> -       jmp 1f                  /* flush the prefetch-queue */
> -1:
> -       movl $1f,%eax
> -       jmp *%eax               /* make sure eip is relocated */
> +       ljmp $__BOOT_CS,$1f     /* Clear prefetch and normalize %eip
> */
>  1:
>         /* Set up the stack pointer */
>         lss stack_start,%esp
> 
> 
> I've been doing some cleanups in head.S after making the early page
> tables dynamic.
> 
> 	-hpa
> -

IMHO, why bother to re-reload %cs again?

In setup.S, %cs is reloaded already. The enable paging code maps the
address identically, so %cs already contains the proper selector.


Coywolf


-- 
Coywolf Qi Hunt
Admin of http://GreatCN.org and http://LoveCN.org

Re: BOOT_CS

[Jamie Lokier]

Coywolf Qi Hunt wrote:
> >(the jump and indirect branch aren't guaranteed to have the
> >proper effects, although technically neither should be required due to
> >the %cr0 write):
>
> 
> IMHO, why bother to re-reload %cs again?
> 
> In setup.S, %cs is reloaded already. The enable paging code maps the
> address identically, so %cs already contains the proper selector.

It's to flush the instruction prefetch queue: that's one of the side
effects of ljmp.

I recall an Intel manual that said ljmp is required when switching
between real and protected modes, to flush the prefetch queue.

Unfortunately I don't remember what that manual said about just setting PG.

I'd guess that current generation CPUs don't care about ljmp when
changing modes, but older ones certainly do.

-- Jamie

Re: BOOT_CS

[Eric W. Biederman]

hpa@zytor.com (H. Peter Anvin) writes:

> Anyone happen to know of any legitimate reason not to reload %cs in
> head.S?  

Other than the fact it is strongly rude and error prone to depend on
the contents of a global descriptor table you did not setup?

If we did the lgdt boot_gdt before hand I don't see any problems
though.  

But at the point we could as easily do lgdt cpu_gdt_descr, and use
__KERNEL_CS which is better anyway.

> I think the following would be a lot cleaner, as well as a
> lot safer (the jump and indirect branch aren't guaranteed to have the
> proper effects, although technically neither should be required due to
> the %cr0 write):
> 
> @@ -117,10 +147,7 @@
>         movl %cr0,%eax
>         orl $0x80000000,%eax
>         movl %eax,%cr0          /* ..and set paging (PG) bit */
> -       jmp 1f                  /* flush the prefetch-queue */
> -1:
> -       movl $1f,%eax
> -       jmp *%eax               /* make sure eip is relocated */
> +       ljmp $__BOOT_CS,$1f     /* Clear prefetch and normalize %eip
> */
>  1:
>         /* Set up the stack pointer */
>         lss stack_start,%esp
> 
> 
> I've been doing some cleanups in head.S after making the early page
> tables dynamic.

That is almost nice.  Care to export where the bottom of the page
tables or even better where the bottom of the kernel is for those
folks who want to place their ramdisk as low in memory as possible?

Eric

Re: BOOT_CS

[H. Peter Anvin]

Eric W. Biederman wrote:
> hpa@zytor.com (H. Peter Anvin) writes:
> 
>>Anyone happen to know of any legitimate reason not to reload %cs in
>>head.S?  
> 
> Other than the fact it is strongly rude and error prone to depend on
> the contents of a global descriptor table you did not setup?
> 

We already do that, as you might have noticed (we set all the data 
registers to __BOOT_DS; CS is the only that is changed.)

> 
> That is almost nice.  Care to export where the bottom of the page
> tables or even better where the bottom of the kernel is for those
> folks who want to place their ramdisk as low in memory as possible?
> 

The problem is that you don't know until it's too late, since it can 
depend on dynamic factors.  This is part of why your insistence of 
putting the ramdisk in the "most incorrect" position is simply wrong.

	-hpa

Re: BOOT_CS

[Eric W. Biederman]

"H. Peter Anvin" <hpa@zytor.com> writes:

> Eric W. Biederman wrote:
> > hpa@zytor.com (H. Peter Anvin) writes:
> >
> >>Anyone happen to know of any legitimate reason not to reload %cs in
> >> head.S?
> > Other than the fact it is strongly rude and error prone to depend on
> > the contents of a global descriptor table you did not setup?
> >
> 
> We already do that, as you might have noticed (we set all the data registers to
> __BOOT_DS; CS is the only that is changed.)

Right and it would be a cleanup not to touch __BOOT_DS.  We have
already reloaded it in  arch/i386/boot/compressed/head.S anyway.

> > That is almost nice.  Care to export where the bottom of the page
> > tables or even better where the bottom of the kernel is for those
> > folks who want to place their ramdisk as low in memory as possible?
> >
> 
> The problem is that you don't know until it's too late, since it can depend on
> dynamic factors.  This is part of why your insistence of putting the ramdisk in
> the "most incorrect" position is simply wrong.

Nope.  On other architectures where the bootloader has access to
vmlinux this works just fine, all dynamic factors can be contained in
the bss.  We don't go very long before we reserve memory after all.
It is only on x86 where there is not enough information that it is
problematic. 

Putting the ramdisk right after the kernel (text + data + bss) is the
"most correct" position.  Anything else is likely to break when the
firmware changes.  This has already happened 2 or 3 times on x86.

When putting the ramdisk right after the kernel if anything breaks you
will notice it immediately, and the kernel will be fixed.

If I truly put an initrd at the top of memory the kernel would not
even be able to read the ramdisk.

Eric

[PATCH] Re: BOOT_CS

[Coywolf Qi Hunt]

[-- Attachment #1: Type: text/plain, Size: 1260 bytes --]

Jamie Lokier wrote:

> Coywolf Qi Hunt wrote:
> 
>>>(the jump and indirect branch aren't guaranteed to have the
>>>proper effects, although technically neither should be required due to
>>>the %cr0 write):

???

>>
>>
>>IMHO, why bother to re-reload %cs again?
>>
>>In setup.S, %cs is reloaded already. The enable paging code maps the
>>address identically, so %cs already contains the proper selector.
> 
> 
> It's to flush the instruction prefetch queue: that's one of the side
> effects of ljmp.

Re-loading %cs and flushing prefetch queue are two different things.

> 
> I recall an Intel manual that said ljmp is required when switching
> between real and protected modes, to flush the prefetch queue.

Not necessarily ljmp, imho

> 
> Unfortunately I don't remember what that manual said about just setting PG.
> 
> I'd guess that current generation CPUs don't care about ljmp when
> changing modes, but older ones certainly do.
> 
> -- Jamie
> 

FYI, intel's example code located in STARTUP.ASM Listing arround line
180, chapter 9, IA-32 Intel Architecture Software Developer's Manual,
Volume 3: System Programming Guide



Please consider my patch for this issue.


	Coywolf



-- 
Coywolf Qi Hunt
Admin of http://GreatCN.org and http://LoveCN.org





[-- Attachment #2: patch-cy0402230 --]
[-- Type: text/plain, Size: 456 bytes --]

--- head.S.orig	2004-02-18 11:57:16.000000000 +0800
+++ head.S	2004-02-23 12:35:24.000000000 +0800
@@ -115,10 +115,8 @@
 	movl %cr0,%eax
 	orl $0x80000000,%eax
 	movl %eax,%cr0		/* ..and set paging (PG) bit */
-	jmp 1f			/* flush the prefetch-queue */
-1:
-	movl $1f,%eax
-	jmp *%eax		/* make sure eip is relocated */
+	pushl $1f		/* flush the prefetch-queue */
+	ret			/* and normalize $eip */
 1:
 	/* Set up the stack pointer */
 	lss stack_start,%esp

Does Flushing the Queue after PG REALLY a Necessity?

[Coywolf Qi Hunt]

[-- Attachment #1: Type: text/plain, Size: 1198 bytes --]

H. Peter Anvin wrote:

> Anyone happen to know of any legitimate reason not to reload %cs in
> head.S?  I think the following would be a lot cleaner, as well as a
> lot safer (the jump and indirect branch aren't guaranteed to have the
> proper effects, although technically neither should be required due to
> the %cr0 write):

Anyone happen to know of any legitimate reason to flush the prefetch
queue after enabling paging?

I've read the intel manual volume 3 thoroughly. It only says that after
entering protected mode, flushing is required, but never says
specifically about whether to do flushing after enabling paging.

Furthermore the intel example code enables protected mode and paging at
the same time. So does FreeBSD. There's really no more references to check.

  From the cpu's internal view, flushing for PE is to flush the prefetch
queue as well as re-load the %cs, since the protected mode is just about
to begin. But no reason to flushing for PG, since linux maps the
addresses *identically*.

If no any reason, please remove the after paging flushing queue code,
two near jump.


	Coywolf

(patch enclosed)
-- 
Coywolf Qi Hunt
Admin of http://GreatCN.org and http://LoveCN.org


[-- Attachment #2: patch-cy0402232 --]
[-- Type: text/plain, Size: 381 bytes --]

--- head.S	2004-02-18 11:57:16.000000000 +0800
+++ head-cy.S	2004-02-23 17:19:02.000000000 +0800
@@ -115,11 +115,7 @@
 	movl %cr0,%eax
 	orl $0x80000000,%eax
 	movl %eax,%cr0		/* ..and set paging (PG) bit */
-	jmp 1f			/* flush the prefetch-queue */
-1:
-	movl $1f,%eax
-	jmp *%eax		/* make sure eip is relocated */
-1:
+
 	/* Set up the stack pointer */
 	lss stack_start,%esp
 

Re: [PATCH] Re: BOOT_CS

[Jamie Lokier]

Coywolf Qi Hunt wrote:
> >It's to flush the instruction prefetch queue: that's one of the side
> >effects of ljmp.
> 
> Re-loading %cs and flushing prefetch queue are two different things.

Yes, they are.

> FYI, intel's example code located in STARTUP.ASM Listing arround line
> 180, chapter 9, IA-32 Intel Architecture Software Developer's Manual,
> Volume 3: System Programming Guide

Thanks.

> Please consider my patch for this issue.

I don't have the ability to look at that manual right now.

Your patch uses two instructions to flush the queue (push+ret) instead
of one (jmp or ljmp).  Is that documented as reliable?  I can easily
imagine an implementation which decodes one instruction after a mode
change predictably, but not two.

I doubt that it makes a difference - we're setting PG, not changing
the instruction format - but I'd like us to be sure it cannot fail on
things like 386s and 486s, and similar non-Intel chips.

-- Jamie

Re: Does Flushing the Queue after PG REALLY a Necessity?

[Philippe Elie]

On Mon, 23 Feb 2004 at 18:27 +0000, Coywolf Qi Hunt wrote:

> H. Peter Anvin wrote:
> 
> >Anyone happen to know of any legitimate reason not to reload %cs in
> >head.S?  I think the following would be a lot cleaner, as well as a
> >lot safer (the jump and indirect branch aren't guaranteed to have the
> >proper effects, although technically neither should be required due to
> >the %cr0 write):

jump is sufficent when setting PG and required with cpu where cr0 write
does not serialize.

> Anyone happen to know of any legitimate reason to flush the prefetch
> queue after enabling paging?
> 
> I've read the intel manual volume 3 thoroughly. It only says that after
> entering protected mode, flushing is required, but never says
> specifically about whether to do flushing after enabling paging.
> 
> Furthermore the intel example code enables protected mode and paging at
> the same time. So does FreeBSD. There's really no more references to check.
> 
>  From the cpu's internal view, flushing for PE is to flush the prefetch
> queue as well as re-load the %cs, since the protected mode is just about
> to begin. But no reason to flushing for PG, since linux maps the
> addresses *identically*.
> 
> If no any reason, please remove the after paging flushing queue code,
> two near jump.

See IA32 vol 3  7.4 and 18.27.3

Anyway this code is known to work on dozen of intel/non intel processor,
how can you know if changing this code will not break an obscure clone ?

regards,
Phil

Re: [PATCH] Re: BOOT_CS

[Rene Herman]

Jamie Lokier wrote:

> Your patch uses two instructions to flush the queue (push+ret) instead
> of one (jmp or ljmp).  Is that documented as reliable?

No. The current arch manuals seem to say nothing definite, but Intel's 
386 manual (ie, the cpu, not the arch) from 1986 explicitly says "after 
setting PG, a jump must follow immediately".

> I can easily imagine an implementation which decodes one instruction
> after a mode change predictably, but not two.
> 
> I doubt that it makes a difference - we're setting PG, not changing 
> the instruction format - but I'd like us to be sure it cannot fail on
>  things like 386s and 486s, and similar non-Intel chips.

I believe you should either keep it as is, do the long jump (although I 
don't believe that to be necessary: setup.s already long jumped to us) 
or only delete the indirect jump (we've just been longjumped to, and 
identity-mapped, so everything would seem to be normalised already).

In any case, please jump directly after setting PG.

Rene.

Re: Does Flushing the Queue after PG REALLY a Necessity?

[Coywolf Qi Hunt]

Philippe Elie wrote:

> On Mon, 23 Feb 2004 at 18:27 +0000, Coywolf Qi Hunt wrote:
> 
> 
>>H. Peter Anvin wrote:
>>
>>
>>>Anyone happen to know of any legitimate reason not to reload %cs in
>>>head.S?  I think the following would be a lot cleaner, as well as a
>>>lot safer (the jump and indirect branch aren't guaranteed to have the
>>>proper effects, although technically neither should be required due to
>>>the %cr0 write):
> 
> 
> jump is sufficent when setting PG and required with cpu where cr0 write
> does not serialize.

The problem is there's two jumps in the kernel. Intel's manual only asks 
for "Execute a near JMP instruction".

> 
> 
>>Anyone happen to know of any legitimate reason to flush the prefetch
>>queue after enabling paging?
>>
>>I've read the intel manual volume 3 thoroughly. It only says that after
>>entering protected mode, flushing is required, but never says
>>specifically about whether to do flushing after enabling paging.
>>
>>Furthermore the intel example code enables protected mode and paging at
>>the same time. So does FreeBSD. There's really no more references to check.
>>
>> From the cpu's internal view, flushing for PE is to flush the prefetch
>>queue as well as re-load the %cs, since the protected mode is just about
>>to begin. But no reason to flushing for PG, since linux maps the
>>addresses *identically*.
>>
>>If no any reason, please remove the after paging flushing queue code,
>>two near jump.
> 
> 
> See IA32 vol 3  7.4 and 18.27.3
> 
> Anyway this code is known to work on dozen of intel/non intel processor,
> how can you know if changing this code will not break an obscure clone ?

Right, I also think removing the flush code is risky. Thanks very much, 
chapter 18 is what i was looking for. I recalled in an old intel 
booklet, named like something 386 system guide, says JMP after PG as 
well as PE. But I didn't have that book at hand and didn't find any e-doc.

However, in 18.27.3, "The sequence bounded by the MOV and JMP 
instructions should be identity" implies no JMP is also viable 
practically. But we needn't to be that pedantic.

If no any reason for the two jumps, the code should be fixed to remains 
only ONE near jump.


	Coywolf


-- 
Coywolf Qi Hunt
Admin of http://GreatCN.org and http://LoveCN.org

Re: Does Flushing the Queue after PG REALLY a Necessity?

[H. Peter Anvin]

Coywolf Qi Hunt wrote:
 >
> The problem is there's two jumps in the kernel. Intel's manual only asks 
> for "Execute a near JMP instruction".
> 

A far JMP is definitely sufficient, and serves to normalize EIP as well.

I have uploaded a patch which also preinitializes the GDT, which may 
make the VISWS code a bit less of a special case.

ftp://ftp.kernel.org/pub/linux/kernel/people/hpa/earlymem-4.diff

> If no any reason for the two jumps, the code should be fixed to remains 
> only ONE near jump.

Why are you so obsessed with minimality?  The performance of this code 
matters not one bit.

	-hpa

[PATCH] Remove the extra jmp

[Coywolf Qi Hunt]

[-- Attachment #1: Type: text/plain, Size: 881 bytes --]

Jamie Lokier wrote:
> 
> Your patch uses two instructions to flush the queue (push+ret) instead
> of one (jmp or ljmp).  Is that documented as reliable?  I can easily
> imagine an implementation which decodes one instruction after a mode
> change predictably, but not two.
> 
> I doubt that it makes a difference - we're setting PG, not changing
> the instruction format - but I'd like us to be sure it cannot fail on
> things like 386s and 486s, and similar non-Intel chips.

push+ret is encouraged/borrowed/stolen from FreeBSD ;) it should be 
reliable. And also, old linux uses ret. Since old linux runs on 386, it 
is quite reliable. If you still doubt, we can push before PG.



Hello Anvin,

Please either take the push+ret patch or take the one near jmp patch 
enclosed in this email. thanks

	Coywolf


-- 
Coywolf Qi Hunt
Admin of http://GreatCN.org and http://LoveCN.org

[-- Attachment #2: patch-cy0402240-2.6.3 --]
[-- Type: text/plain, Size: 339 bytes --]

--- head.S.orig	2004-02-18 11:57:16.000000000 +0800
+++ head.S	2004-02-24 11:08:34.000000000 +0800
@@ -117,9 +117,6 @@
 	movl %eax,%cr0		/* ..and set paging (PG) bit */
 	jmp 1f			/* flush the prefetch-queue */
 1:
-	movl $1f,%eax
-	jmp *%eax		/* make sure eip is relocated */
-1:
 	/* Set up the stack pointer */
 	lss stack_start,%esp
 

Re: [PATCH] Remove the extra jmp

[Brian Gerst]

Coywolf Qi Hunt wrote:
> Jamie Lokier wrote:
> 
>>
>> Your patch uses two instructions to flush the queue (push+ret) instead
>> of one (jmp or ljmp).  Is that documented as reliable?  I can easily
>> imagine an implementation which decodes one instruction after a mode
>> change predictably, but not two.
>>
>> I doubt that it makes a difference - we're setting PG, not changing
>> the instruction format - but I'd like us to be sure it cannot fail on
>> things like 386s and 486s, and similar non-Intel chips.
> 
> 
> push+ret is encouraged/borrowed/stolen from FreeBSD ;) it should be 
> reliable. And also, old linux uses ret. Since old linux runs on 386, it 
> is quite reliable. If you still doubt, we can push before PG.
> 
> 
> 
> Hello Anvin,
> 
> Please either take the push+ret patch or take the one near jmp patch 
> enclosed in this email. thanks
> 
>     Coywolf
> 
> 
> 
> ------------------------------------------------------------------------
> 
> --- head.S.orig	2004-02-18 11:57:16.000000000 +0800
> +++ head.S	2004-02-24 11:08:34.000000000 +0800
> @@ -117,9 +117,6 @@
>  	movl %eax,%cr0		/* ..and set paging (PG) bit */
>  	jmp 1f			/* flush the prefetch-queue */
>  1:
> -	movl $1f,%eax
> -	jmp *%eax		/* make sure eip is relocated */
> -1:
>  	/* Set up the stack pointer */
>  	lss stack_start,%esp
>  

This won't work, because the indirect jump is what adds PAGE_OFFSET to 
%eip (remember, call/jump use relative addressing).  Either keep just 
the indirect jump, or use "jmp __PAGE_OFFSET+1f".

--
				Brian Gerst

Re: Does Flushing the Queue after PG REALLY a Necessity?

[Randy.Dunlap]

On Tue, 24 Feb 2004 10:36:07 +0800 Coywolf Qi Hunt <coywolf@greatcn.org> wrote:

| Philippe Elie wrote:
| 
| > On Mon, 23 Feb 2004 at 18:27 +0000, Coywolf Qi Hunt wrote:
| > 
| > 
| >>H. Peter Anvin wrote:
| >>
| >>
| >>>Anyone happen to know of any legitimate reason not to reload %cs in
| >>>head.S?  I think the following would be a lot cleaner, as well as a
| >>>lot safer (the jump and indirect branch aren't guaranteed to have the
| >>>proper effects, although technically neither should be required due to
| >>>the %cr0 write):
| > 
| > 
| > jump is sufficent when setting PG and required with cpu where cr0 write
| > does not serialize.
| 
| The problem is there's two jumps in the kernel. Intel's manual only asks 
| for "Execute a near JMP instruction".
| 
| > 
| > 
| >>Anyone happen to know of any legitimate reason to flush the prefetch
| >>queue after enabling paging?
| >>
| >>I've read the intel manual volume 3 thoroughly. It only says that after
| >>entering protected mode, flushing is required, but never says
| >>specifically about whether to do flushing after enabling paging.
| >>
| >>Furthermore the intel example code enables protected mode and paging at
| >>the same time. So does FreeBSD. There's really no more references to check.
| >>
| >> From the cpu's internal view, flushing for PE is to flush the prefetch
| >>queue as well as re-load the %cs, since the protected mode is just about
| >>to begin. But no reason to flushing for PG, since linux maps the
| >>addresses *identically*.
| >>
| >>If no any reason, please remove the after paging flushing queue code,
| >>two near jump.
| > 
| > 
| > See IA32 vol 3  7.4 and 18.27.3
| > 
| > Anyway this code is known to work on dozen of intel/non intel processor,
| > how can you know if changing this code will not break an obscure clone ?
| 
| Right, I also think removing the flush code is risky. Thanks very much, 
| chapter 18 is what i was looking for. I recalled in an old intel 
| booklet, named like something 386 system guide, says JMP after PG as 
| well as PE. But I didn't have that book at hand and didn't find any e-doc.

I guess that's the 80386 System Software Writer's Guide.
Ch. 6: Initialization.
Yes, it does JMP after setting PE and after enabling PG.
Any JMP.

| However, in 18.27.3, "The sequence bounded by the MOV and JMP 
| instructions should be identity" implies no JMP is also viable 
| practically. But we needn't to be that pedantic.
| 
| If no any reason for the two jumps, the code should be fixed to remains 
| only ONE near jump.


--
~Randy

Re: Does Flushing the Queue after PG REALLY a Necessity?

[Coywolf Qi Hunt]

Randy.Dunlap wrote:

> On Tue, 24 Feb 2004 10:36:07 +0800 Coywolf Qi Hunt wrote:
> 
> | Right, I also think removing the flush code is risky. Thanks very much, 
> | chapter 18 is what i was looking for. I recalled in an old intel 
> | booklet, named like something 386 system guide, says JMP after PG as 
> | well as PE. But I didn't have that book at hand and didn't find any e-doc.
> 
> I guess that's the 80386 System Software Writer's Guide.
> Ch. 6: Initialization.
> Yes, it does JMP after setting PE and after enabling PG.
> Any JMP.

Yes, it's that booklet, very thin.

> 
> | However, in 18.27.3, "The sequence bounded by the MOV and JMP 
> | instructions should be identity" implies no JMP is also viable 
> | practically. But we needn't to be that pedantic.
> | 
> | If no any reason for the two jumps, the code should be fixed to remains 
> | only ONE near jump.


Btw, could you please do not show others email address when you reply? 
Change your mail client's configuration. I don't like my this email 
address be grabbed by spammers. thanks


	Coywolf



-- 
Coywolf Qi Hunt
Admin of http://GreatCN.org and http://LoveCN.org

Re: [PATCH] Remove the extra jmp

[Coywolf Qi Hunt]

Brian Gerst wrote:

> Coywolf Qi Hunt wrote:
> 
>> Jamie Lokier wrote:
>>
>>>
>>> Your patch uses two instructions to flush the queue (push+ret) instead
>>> of one (jmp or ljmp).  Is that documented as reliable?  I can easily
>>> imagine an implementation which decodes one instruction after a mode
>>> change predictably, but not two.
>>>
>>> I doubt that it makes a difference - we're setting PG, not changing
>>> the instruction format - but I'd like us to be sure it cannot fail on
>>> things like 386s and 486s, and similar non-Intel chips.
>>
>>
>>
>> push+ret is encouraged/borrowed/stolen from FreeBSD ;) it should be 
>> reliable. And also, old linux uses ret. Since old linux runs on 386, 
>> it is quite reliable. If you still doubt, we can push before PG.
>>
>>
>>
>> Hello Anvin,
>>
>> Please either take the push+ret patch or take the one near jmp patch 
>> enclosed in this email. thanks
>>
>>     Coywolf
>>
>>
>>
>> ------------------------------------------------------------------------
>>
>> --- head.S.orig    2004-02-18 11:57:16.000000000 +0800
>> +++ head.S    2004-02-24 11:08:34.000000000 +0800
>> @@ -117,9 +117,6 @@
>>      movl %eax,%cr0        /* ..and set paging (PG) bit */
>>      jmp 1f            /* flush the prefetch-queue */
>>  1:
>> -    movl $1f,%eax
>> -    jmp *%eax        /* make sure eip is relocated */
>> -1:
>>      /* Set up the stack pointer */
>>      lss stack_start,%esp
>>  
> 
> 
> This won't work, because the indirect jump is what adds PAGE_OFFSET to 
> %eip (remember, call/jump use relative addressing).  Either keep just 
> the indirect jump, or use "jmp __PAGE_OFFSET+1f".
> 

Any jump works. But I think you did explain very well the reason that 
the author carelessly or over carefully left the two jumps there.

	Coywolf


-- 
Coywolf Qi Hunt
Admin of http://GreatCN.org and http://LoveCN.org

Re: Does Flushing the Queue after PG REALLY a Necessity?

[Herbert Poetzl]

On Tue, Feb 24, 2004 at 05:17:18PM +0800, Coywolf Qi Hunt wrote:
> Btw, could you please do not show others email address when you reply? 
> Change your mail client's configuration. I don't like my this email 
> address be grabbed by spammers. thanks

hmm, ever heard of the evil From: header?

best,
Herbert

> 	Coywolf
> 
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> Coywolf Qi Hunt
> Admin of http://GreatCN.org and http://LoveCN.org
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Re: Does Flushing the Queue after PG REALLY a Necessity?

[Coywolf Qi Hunt]

Herbert Poetzl wrote:

> On Tue, Feb 24, 2004 at 05:17:18PM +0800, Coywolf Qi Hunt wrote:
> 
>>Btw, could you please do not show others email address when you reply? 
>>Change your mail client's configuration. I don't like my this email 
>>address be grabbed by spammers. thanks
> 
> 
> hmm, ever heard of the evil From: header?
> 

The mail address got in the body, and all the lkml archives show it in 
html pages. Then more spammers ...


	Coywolf


-- 
Coywolf Qi Hunt
Admin of http://GreatCN.org and http://LoveCN.org