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Re: [libvirt] [PATCH RESEND v1 1/2] i386: Add Intel Processor Trace feature support



On 18/01/2018 15:37, Eduardo Habkost wrote:
> On Thu, Jan 18, 2018 at 02:39:57PM +0100, Paolo Bonzini wrote:
>> On 18/01/2018 14:24, Eduardo Habkost wrote:
>>> However, if there's a simple way to make it possible to migrate
>>> between hosts with different CPUID[14h] data, it would be even
>>> better.  With the current KVM intel-pt implementation, what
>>> happens if the CPUID[14h] data seen by the guest doesn't match
>>> exactly the CPUID[14h] leaves from the host?
>>
>> Some bits in there can be treated as CPU features (e.g. EBX bit 0 "CR3
>> filtering support").  Probably we should handle these in KVM right now.
>> KVM needs to compute a mask of valid 1 bits for IA32_RTIT_CTL based on
>> CPUID, and apply it when the MSR is written.
> 
> Does this mean QEMU can't set CPUID values that won't match the
> host with the existing implementation, or this won't matter for
> well-behaved guests that don't try to set reserved bits on the
> MSRs?

All the features could be handled exactly like regular feature bits.  If
QEMU sets them incorrectly and "enforce" is not used, bad things happen
but it's the user's fault.

> 
>>                                               It also needs to whitelist
>> bits like we do for other feature words.  These include:
>>
>> - CPUID[EAX=14h,ECX=0].EBX
>>
>> - CPUID[EAX=14h,ECX=0].ECX except bit 31
>>
>> - CPUID[EAX=14h,ECX=1].EAX bits 16:31 (if CPUID[EAX=14h,ECX=0].EBX[3]=1)
>>
>> - CPUID[EAX=14h,ECX=1].EBX (if CPUID[EAX=14h,ECX=0].EBX[1]=1)
> 
> What do you mean by whitelist?

KVM needs to tell QEMU the bits it knows about.

>> Others, currently only CPUID[EAX=14h,ECX=0].ECX[31] must match, there is
>> no way to emulate the "wrong" value.
> 
> In this case we could make it configurable but require the host
> and guest value to always match.
> 
> This might be an obstacle to enabling intel-pt by default
> (because it could make VMs not migratable to newer hosts), but
> may allow the feature to be configured in a predictable
> way.

Yeah, but consider that virtualized PT anyway would only be enabled on
Ice Lake processors.  It's a few years away anyway!

>> Others, currently only CPUID[EAX=14h,ECX=1].EAX[2:0] are numeric values,
>> and it's possible to emulate a lower value than the one in the processor.
> 
> This could be handled by QEMU.  There's no requirement that all
> GET_SUPPORTED_CPUID values should be validated by simple bit
> masking.

Good!

Paolo


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