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

[Eduardo Habkost <ehabkost@redhat.com>]

On Thu, Jan 18, 2018 at 03:44:49PM +0100, Paolo Bonzini wrote:
> 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.

Oh, I mean setting the bit to 0 when it's 1 on the host (if it's
0 on the host, QEMU would never set it anyway).  Is it safe to do
it with the current KVM intel-pt implementation?


> 
> > 
> >>                                               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.

So KVM isn't currently doing it on GET_SUPPORTED_CPUID?  Oops.


> 
> >> 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

-- 
Eduardo