Re: [PATCH] x86/sev: Improve handling of writes to intercepted GUEST_TSC_FREQ

[Sean Christopherson <seanjc@google.com>]

On Mon, Jul 14, 2025, Borislav Petkov wrote:
> On Fri, Jul 11, 2025 at 09:42:00AM +0530, Nikunj A Dadhania wrote:
> > From: Sean Christopherson <seanjc@google.com>
> > 
> > For Secure TSC enabled guests, don't panic when a guest writes to
> > intercepted GUEST_TSC_FREQ MSR. Instead, ignore writes to GUEST_TSC_FREQ,
> > similar to MSR_IA32_TSC, and log a warning instead.
> 
> Why?
> 
> Nothing should poke at the TSC MSR and those who do, deserve what they get.
> 
> > Only terminate the guest when reading from intercepted GUEST_TSC_FREQ MSR
> > with Secure TSC enabled, as this indicates an unexpected hypervisor
> > configuration.
> 
> Huh, this sounds weird.
> 
> What are we "fixing" here?

Returning ES_VMM_ERROR is misleading/wrong, and panicking doesn't match how the
kernel handles every other "bad" WRMSR.  How's this for a changelog?

  For Secure TSC enabled guests, don't panic if the kernel hits a #VC due
  to attempting to write to GUEST_TSC_FREQ, and instead WARN and drop the
  write.  The kernel should never write GUEST_TSC_FREQ as it's read-only,
  but panicking with ES_VMM_ERROR is both misleading (it's entirely
  reasonable for a VMM to intercept writes to a read-only MSR), and
  unnecessary, e.g. the kernel eats #GPs with a WARN on every other "bad"
  WRMSR.