Re: [Xen-devel] [PATCH v2] x86/HVM: don't give the wrong impression of WRMSR succeeding

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 23/02/18 08:36, Jan Beulich wrote:
> ... for non-existent MSRs: wrmsr_hypervisor_regs()'s comment clearly
> says that the function returns 0 for unrecognized MSRs, so
> {svm,vmx}_msr_write_intercept() should not convert this into success. We
> don't want to unconditionally fail the access though, as we can't be
> certain the list of handled MSRs is complete enough for the guest types
> we care about, so instead mirror what we do on the read paths and probe
> the MSR to decide whether to raise #GP.
>
> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

Having thought this through:

At the moment, a write to any unhandled MSR is treated as silent write
discard.  This is terrible behaviour from the guests point of view.

With this patch in place, a write to any unreadable MSR yields #GP,
which is better behaviour.

The only write-only MSRs I'm aware of are in the x2apic block, and
MSR_PRED_CMD, all of which are explicitly handled.

Therefore, Reviewed-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>, as
this is an improvement in behaviour, even if the result still isn't great.

~Andrew

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