On August 21, 2020 2:28:32 AM PDT, Thomas Gleixner <tglx@xxxxxxxxxxxxx> wrote: >On Fri, Aug 21 2020 at 10:09, Paolo Bonzini wrote: >> On 21/08/20 09:47, Borislav Petkov wrote: >>> On Thu, Aug 20, 2020 at 07:50:50PM -0700, Sean Christopherson wrote: >>>> + * Disallow RDPID if KVM is enabled as it may consume a guest's >TSC_AUX >>>> + * if an NMI arrives in KVM's run loop. KVM loads guest's >TSC_AUX on >>>> + * VM-Enter and may not restore the host's value until the CPU >returns >>>> + * to userspace, i.e. KVM depends on the kernel not using >TSC_AUX. >>>> */ >>> And frankly, this is really unfair. The kernel should be able to use >any >>> MSR. IOW, KVM needs to be fixed here. I'm sure it context-switches >other >>> MSRs so one more MSR is not a big deal. >> >> The only MSR that KVM needs to context-switch manually are XSS and >> SPEC_CTRL. They tend to be the same on host and guest in which case >> they can be optimized away. >> >> All the other MSRs (EFER and PAT are those that come to mind) are >> handled by the microcode and thus they don't have the slowness of >> RDMSR/WRMSR >> >> One more MSR *is* a big deal: KVM's vmentry+vmexit cost is around >1000 >> cycles, adding 100 clock cycles for 2 WRMSRs is a 10% increase. > >We all know that MSRs are slow, but as a general rule I have to make it >entirely clear that the kernel has precedence over KVM. > >If the kernel wants to use an MSR for it's own purposes then KVM has to >deal with that and not the other way round. Preventing the kernel from >using a facility freely is not an option ever. > >The insanities of KVM performance optimizations have bitten us more >than >once. > >For this particular case at hand I don't care much and we should just >rip the whole RDPID thing out unconditionally. We still have zero >numbers about the performance difference vs. LSL. > >Thanks, > > tglx It is hardly going to be a performance difference for paranoid entry, which is hopefully rare enough that it falls into the noise. -- Sent from my Android device with K-9 Mail. Please excuse my brevity.
