Re: [PATCH] x86/msr: fix handling of MSR_IA32_PERF_{STATUS/CTL}

[Jan Beulich <jbeulich@xxxxxxxx>]

On 06.10.2020 18:23, Roger Pau Monne wrote:
> Currently a PV hardware domain can also be given control over the CPU
> frequency, and such guest is allowed to write to MSR_IA32_PERF_CTL.
> However since commit 322ec7c89f6 the default behavior has been changed
> to reject accesses to not explicitly handled MSRs, preventing PV
> guests that manage CPU frequency from reading
> MSR_IA32_PERF_{STATUS/CTL}.
> 
> Additionally some HVM guests (Windows at least) will attempt to read
> MSR_IA32_PERF_CTL and will panic if given back a #GP fault:
> 
> vmx.c:3035:d8v0 RDMSR 0x00000199 unimplemented
> d8v0 VIRIDIAN CRASH: 3b c0000096 fffff806871c1651 ffffda0253683720 0
> 
> Move the handling of MSR_IA32_PERF_{STATUS/CTL} to the common MSR
> handling shared between HVM and PV guests, and add an explicit case
> for reads to MSR_IA32_PERF_{STATUS/CTL}.
> 
> Restore previous behavior and allow PV guests with the required
> permissions to read the contents of the mentioned MSRs. Non privileged
> guests will get 0 when trying to read those registers, as writes to
> MSR_IA32_PERF_CTL by such guest will already be silently dropped.
> 
> Fixes: 322ec7c89f6 ('x86/pv: disallow access to unknown MSRs')
> Fixes: 84e848fd7a1 ('x86/hvm: disallow access to unknown MSRs')
> Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>

I would have given this my R-b, but Andrew's "straight up broken"
comment needs resolving first, one way or another.

Jan