Re: [PATCH 26/26] KVM: PPC: Add Documentation about PV interface

[Alexander Graf <agraf@suse.de>]

Am 27.06.2010 um 10:14 schrieb Avi Kivity <avi@redhat.com>:

> On 06/26/2010 02:25 AM, Alexander Graf wrote:
>> We just introduced a new PV interface that screams for  
>> documentation. So here
>> it is - a shiny new and awesome text file describing the internal  
>> works of
>> the PPC KVM paravirtual interface.
>>
>
> Good, that lets people who have no idea what they're talking about  
> participate in the review.

Heh, I knew you'd like this :).

>
>> +
>> +PPC hypercalls
>> +==============
>> +
>> +The only viable ways to reliably get from guest context to host  
>> context are:
>> +
>> +    1) Call an invalid instruction
>> +    2) Call the "sc" instruction with a parameter to "sc"
>> +    3) Call the "sc" instruction with parameters in GPRs
>> +
>> +Method 1 is always a bad idea. Invalid instructions can be  
>> replaced later on
>> +by valid instructions, rendering the interface broken.
>> +
>> +Method 2 also has downfalls. If the parameter to "sc" is != 0 the  
>> spec is
>> +rather unclear if the sc is targeted directly for the hypervisor  
>> or the
>> +supervisor. It would also require that we read the syscall issuing  
>> instruction
>> +every time a syscall is issued, slowing down guest syscalls.
>> +
>> +Method 3 is what KVM uses. We pass magic constants  
>> (KVM_SC_MAGIC_R3 and
>> +KVM_SC_MAGIC_R4) in r3 and r4 respectively. If a syscall  
>> instruction with these
>> +magic values arrives from the guest's kernel mode, we take the  
>> syscall as a
>> +hypercall.
>>
>
> Is there any chance a normal syscall will have those values in r3  
> and r4?

r3 is the syscall number. So as long as the guest doesn't reuse that  
value, we're safe. Since in general syscall numbers are not randomly  
scattered throughout the number range, we should be ok here.

>
> If so, maybe it's better to use pc as they key for hypercalls.  Let  
> the guest designate one instruction address as the hypercall call  
> point; kvm can easily check it and reflect it back to the guest if  
> it doesn't match.
>

You mean the guest would tell the hv where the hypercall lies? That  
would require a hypercall, no? Defining it statically is tricky. I  
want to PV'nize osx using a kernel module later, so I don't have  
control over the physical layout.

> Is it valid and useful to issue sc from privileged mode anyway,  
> except for calling the hypervisor?

Same as a syscall on x86 really. The kernel can and does issue  
syscalls within itself.

>
>> +
>> +The parameters are as follows:
>> +
>> +    r3        KVM_SC_MAGIC_R3
>> +    r4        KVM_SC_MAGIC_R4
>> +    r5        Hypercall number
>> +    r6        First parameter
>> +    r7        Second parameter
>> +    r8        Third parameter
>> +    r9        Fourth parameter
>> +
>> +Hypercall definitions are shared in generic code, so the same  
>> hypercall numbers
>> +apply for x86 and powerpc alike.
>>
>
> Addresses passed in hypercall paramters are guest physical addresses.
>
> Do you have >32 bit physical addresses on 32-bit guests?  if so,  
> you'll need to pass physical addresses in two registers.

I think theoretically it's possible. Will we ever support it?  
Doubtful. Do we need to pass hogh memory addresses to the hv? Even  
more doubtful.

If we hit such a case, I'd just disable the hypercall for 32 bit. Or  
define param1 and param2 to contain the address if the guest is in 32- 
bit mode. No need to always make all params 64 bit imho.

>
>> +
>> +The magic page
>> +==============
>> +
>> +To enable communication between the hypervisor and guest there is  
>> a new shared
>> +page that contains parts of supervisor visible register state. The  
>> guest can
>> +map this shared page using the KVM hypercall  
>> KVM_HC_PPC_MAP_MAGIC_PAGE.
>> +
>> +With this hypercall issued the guest always gets the magic page  
>> mapped at the
>> +desired location in effective and physical address space. For now,  
>> we always
>> +map the page to -4096. This way we can access it using absolute  
>> load and store
>> +functions. The following instruction reads the first field of the  
>> magic page:
>> +
>> +    ld    rX, -4096(0)
>>
>
> Is the address guest controlled or host controlled?

Guest controlled. It's passed in to the map_magic_page hypercall.

>
>> +
>> +The interface is designed to be extensible should there be need  
>> later to add
>> +additional registers to the magic page. If you add fields to the  
>> magic page,
>> +also define a new hypercall feature to indicate that the host can  
>> give you more
>> +registers. Only if the host supports the additional features, make  
>> use of them.
>> +
>> +The magic page has the following layout as described in
>> +arch/powerpc/include/asm/kvm_para.h:
>> +
>> +struct kvm_vcpu_arch_shared {
>> +    __u64 scratch1;
>> +    __u64 scratch2;
>> +    __u64 scratch3;
>> +    __u64 critical;        /* Guest may not get interrupts if ==  
>> r1 */
>>
>
> Elaborate?

I think I have a description in the respective patch. Probably a good  
idea to add it to the documentation.

>
>> +    __u64 sprg0;
>> +    __u64 sprg1;
>> +    __u64 sprg2;
>> +    __u64 sprg3;
>> +    __u64 srr0;
>> +    __u64 srr1;
>> +    __u64 dar;
>> +    __u64 msr;
>> +    __u32 dsisr;
>> +    __u32 int_pending;    /* Tells the guest if we have an  
>> interrupt */
>> +};
>> +
>> +Additions to the page must only occur at the end. Struct fields  
>> are always 32
>> +bit aligned.
>> +
>> +Patched instructions
>> +====================
>> +
>> +The "ld" and "std" instructions are transormed to "lwz" and "stw"  
>> instructions
>> +respectively on 32 bit systems with an added offset of 4 to  
>> accomodate for big
>> +endianness.
>>
>
> Who does the patching? guest or host?

All patching is done by the guest. Probably worth mentioning, yeah.

>
>> +
>> +From            To
>> +====            ==
>> +
>> +mfmsr    rX        ld    rX, magic_page->msr
>> +mfsprg    rX, 0        ld    rX, magic_page->sprg0
>> +mfsprg    rX, 1        ld    rX, magic_page->sprg1
>> +mfsprg    rX, 2        ld    rX, magic_page->sprg2
>> +mfsprg    rX, 3        ld    rX, magic_page->sprg3
>> +mfsrr0    rX        ld    rX, magic_page->srr0
>> +mfsrr1    rX        ld    rX, magic_page->srr1
>> +mfdar    rX        ld    rX, magic_page->dar
>> +mfdsisr    rX        ld    rX, magic_page->dsisr
>> +
>> +mtmsr    rX        std    rX, magic_page->msr
>> +mtsprg    0, rX        std    rX, magic_page->sprg0
>> +mtsprg    1, rX        std    rX, magic_page->sprg1
>> +mtsprg    2, rX        std    rX, magic_page->sprg2
>> +mtsprg    3, rX        std    rX, magic_page->sprg3
>> +mtsrr0    rX        std    rX, magic_page->srr0
>> +mtsrr1    rX        std    rX, magic_page->srr1
>> +mtdar    rX        std    rX, magic_page->dar
>> +mtdsisr    rX        std    rX, magic_page->dsisr
>> +
>> +tlbsync            nop
>> +
>> +mtmsrd    rX, 0        b    <special mtmsr section>
>> +mtmsr            b    <special mtmsr section>
>> +
>> +mtmsrd    rX, 1        b    <special mtmsrd section>
>> +
>> +[BookE only]
>> +wrteei    [0|1]        b    <special wrteei section>
>>
>
> Probably the guest, as only it can arrange for special * sections.   
> Good.
>
>> +
>> +Some instructions require more logic to determine what's going on  
>> than a load
>> +or store instruction can deliver. To enable patching of those, we  
>> keep some
>> +RAM around where we can live translate instructions to. What  
>> happens is the
>> +following:
>> +
>> +    1) copy emulation code to memory
>> +    2) patch that code to fit the emulated instruction
>> +    3) patch that code to return to the original pc + 4
>> +    4) patch the original instruction to branch to the new code
>> +
>> +That way we can inject an arbitrary amount of code as replacement  
>> for a single
>> +instruction. This allows us to check for pending interrupts when  
>> setting EE=1
>> +for example.
>> +
>>
>
> Or not.
>
> What about transitions from paravirt to non-paravirt?  For example,  
> a system reset.

That ... eh ... good question. It would leave the map pending, but  
everything still continues working.

I don't really know in kvm when a reset occured. So we have to make  
qemu set the map to 0 on reset. Let's add then when we add migration  
support and actually expose all those missing states to userspace.  
Currently we only expose half the necessary state for migration  
anyway :).


Alex