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

Alexander Graf
2010-06-28T07:49:33Z

On 28.06.2010, at 09:18, Milton Miller wrote:

> On Sun Jun 27 around 19:33:52 EST 2010 Alexander Graf wrote:
>> Am 27.06.2010 um 10:14 schrieb Avi Kivity <avi at redhat.com>:
>>> On 06/26/2010 02:25 AM, Alexander Graf wrote:
>
>>>> +
>>>> +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.
>>>> +
>
> It goes to the hypervisor, and it would require the hypervisor to
> return to the supervisor, but I believe it just returns to the user with
> permission denied.

That's what I assumed, yeah :(.

>
>>>> +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.
>>
>
> No, r0 has the system call number. Registers 3 and 4 are the first
> 2 args in c abi (or first 64 bit arg in 32 bit c abi), but the linux
> syscall abi special. (In addition, it returns success or failure in
> cr0).

Oh. Ahem :)

>
>>>
>>> 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.
>>
>>
>
> I don't believe we support the kernel actually doing a syscall to itself
> anymore, at least on powerpc. The callers call the underlying system
> call function, or kernel_thread.
>
> That said, I would suggest we allocate a syscall number for this, as it
> would document the usage. (In additon to 0..nr_syscalls - 1 we have
> 0x1ebe in use).

That's actually a pretty good idea.

>
> Also, is there any desire to nest such emulation?

Nesting should just work, right? Since we only accept hypercalls from PR=0 and guests run in PR=1, we get the sc interrupt in the l1 guest by then.

The only issue I'm aware of that completely breaks when using nested KVM on PPC is the MSR_IR != MSR_DR logic. We fetch the instruction we got an interrupt on for certain interrupts in the world switch handler by keeping MSR_IR=0, but setting MSR_DR=1. And KVM speeds up MSR_DR != MSR_IR by mapping both of them lazily in a special address space. So if you access the same page as instruction and as data, you get an invalid result.

Alex