Messages in this thread | | | From | Paolo Bonzini <> | Date | Thu, 7 Sep 2023 19:25:01 +0200 | Subject | Re: [PATCH] x86/tdx: Mark TSC reliable |
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On Fri, Aug 25, 2023 at 5:25 PM Sean Christopherson <seanjc@google.com> wrote: > > > TDX aside it might be useful to have a mechanism to select TSC over KVM > > > clock in general. > > > > Sean, Paolo, any comment on this? > > I would expect the VMM to not advertise KVM clock if the VM is going to run on > hosts with stable TSCs, i.e. the guest really shouldn't need to do anything in. > But I avoid clocks and timekeeping like the plague, so take that with a grain of > salt, e.g. maybe there's a good reason to always advertise kvmclock.
Mostly because users avoid clocks and timekeeping _even more_; advertising kvmclock is safe in general, so userspace does it in case the VM is later migrated to a machine with unstable TSC.
> For TDX and other paranoid guests, I assume the kernel command line is captured > as part of attestation. And so the existing "no-kvmclock" param should be > sufficient to ensure the guest doesn't use KVM clock over the TSC, though IIRC > TDX requires a constant, non-stop TSC, so it's likely not strictly necessary.
As Kirill said, the guest still has to protect itself, so the patch makes sense (I see a v2 has been posted in the meanwhile).
Paolo
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