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.
Hi Paolo, Do you mean there need to add some check before setting IA32_RTIT_CTL MSR in KVM because some bits of this MSR is depend on the result of CPUID[14]. Any attempts to change these reserved bit should cause a #GP.
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.
I think kvm_arch_get_supported_cpuid() function can get the result of CPUID[14] from KVM. Is this the whitelist what you mentioned? Thanks, Luwei Kang
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