Re: [Qemu-ppc] [Qemu-devel] [PATCH v3 0/4] target-ppc: Add FWNMI support in qemu for powerKVM guests

[Aravinda Prasad]

On Thursday 03 September 2015 11:52 AM, Sam Bobroff wrote:
> On Thu, Sep 03, 2015 at 03:05:21PM +1000, David Gibson wrote:
> 
> [snip]
> 
>> Hm.. so why can't the hypervisor code do the retrying?
> 
> Aravinda replied to this earlier in the thread:
> 
> "Retrying cannot be done internally in h_report_mc_err hcall: only one
> thread can succeed entering qemu upon parallel hcall and hence retrying
> inside the hcall will not allow the ibm,nmi-interlock from first CPU to
> succeed."
> 
> I assume that this means that the big QEMU lock is held while an hcall is
> processed by QEMU, but I haven't checked the code myself. Actually, even if 
> the
> lock is normally held, I don't see why these particular hcalls couldn't 
> release
> the lock. I'll look into this.

I am not sure whether we can release this lock inside an hcall. I need
to check.

> 
>>>> Also, it looks like the vector will need at least one scratch register
>>>> (for the hcall number, if nothing else).  Does PAPR specify what SPRGs
>>>> the vector can clobber?  Obviously it can't be anything the guest
>>>> kernel uses.
>>>
>>> PAPR only says SPRGs 0 to 3 are for software use, but the kernel (see
>>> arch/powerpc/include/asm/reg.h) defines SPRG2 as an exception scratch 
>>> register
>>> so it should be the right one to use here.
>>
>> Uh.. no.  If 0..3 are for software (i.e. OS) use, then this needs to
>> use a different one, since it's being used as a firmware resource
>> here.  Linux might treat SPRG2 as scratch, but another OS would be
>> within its rights to use it for something persistent.
>>
>> Although, as paulus points out, sc 1 will clobber SRR0/1 anyway, and
>> if we use a special illegal instruction, then you no longer need a
>> scratch register.
>>
>>>> Btw, does anyone know what happens with the VPA (and dispatch trace
>>>> log and so forth) on kexec() - it could be subject to the same stale
>>>> address problem, and rewriting vectors won't save us there.
>>>
>>> I asked Michael Ellerman this one and he thinks kexec probably frees and
>>> re-allocates the VPA.
>>
>> Ok.  So the question is: if an explicit deregister is good enough for
>> the VPA, is it also good enough for the FWNMI vector, in which case
>> doing it with just a qemu exit and not bouncing through the guest space
>> is back on the table.
>>
>> I guess that's still problematic because there are existing guests
>> that assume a kexec() will magically wipe the fwnmi vectors away.
> 
> Yes, but I think we could handle this separately if necessary: even if we 
> don't
> need to write anything to the vector, we could still insert a magic value and
> check for it later. If it's been clobbered by a kexec, go back to the old
> method.

"> check for it later" - But does QEMU is informed or get to know when
kexec() is issued?

Regards,
Aravinda

> 
> Sam.
> 

-- 
Regards,
Aravinda