Re: [PATCH v2] x86/sev: Improve handling of writes to intercepted TSC MSRs

[Sean Christopherson <seanjc@google.com>]

On Wed, Jul 16, 2025, Tom Lendacky wrote:
> On 7/16/25 00:53, Nikunj A Dadhania wrote:
> > From: Sean Christopherson <seanjc@google.com>
> > 
> > Currently, when a Secure TSC enabled SNP guest attempts to write to the
> > intercepted GUEST_TSC_FREQ MSR (a read-only MSR), the guest kernel response
> > incorrectly implies a VMM configuration error, when in fact it is the usual
> > VMM configuration to intercept writes to read-only MSRs, unless explicitly
> > documented.
> > 
> > Modify the intercepted TSC MSR #VC handling:
> > * Write to GUEST_TSC_FREQ will generate a #GP instead of terminating the
> >   guest
> > * Write to MSR_IA32_TSC will generate a #GP instead of silently ignoring it
> > 
> > Add a WARN_ONCE to log the incident, as well-behaved SNP guest kernels
> > should never attempt to write to these MSRs.
> > 
> > However, continue to terminate the guest when reading from intercepted
> > GUEST_TSC_FREQ MSR with Secure TSC enabled, as intercepted reads indicate
> > an improper VMM configuration for Secure TSC enabled SNP guests.
> > 
> > Signed-off-by: Sean Christopherson <seanjc@google.com>

Feel free to drop me as author and just give me a Reported-by or Suggested-by.
At this point, I ain't doing a whole lot of anything for this patch :-)

> > +	if (WARN_ON_ONCE(write)) {
> 
> Do we want to capture individual WARNs for each MSR? I guess I'm ok with
> a single WARN for either MSR, but just asking the question.

Or don't WARN at all.  If the caller is doing a bare wrmsrq(), then the kernel
will WARN in ex_handler_msr().  If the caller is doing wrmsrq_safe(), do we care
that they're being deliberately weird?