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

> 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's not, but it's (very) easy to fix.

> >> >>> >>>> 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 >