Re: [PATCH] KVM: arm64: Correctly handle RES0 bits PMEVTYPER<n>_EL0.evtCount

[Marc Zyngier <maz@kernel.org>]

AOn Thu, 13 Jul 2023 23:16:49 +0100,
Oliver Upton <oliver.upton@linux.dev> wrote:
> 
> The PMU event ID varies from 10 to 16 bits, depending on the PMU
> version. If the PMU only supports 10 bits of event ID, bits [15:10] of
> the evtCount field behave as RES0.
> 
> While the actual PMU emulation code gets this right (i.e. RES0 bits are
> masked out when programming the perf event), the sysreg emulation writes
> an unmasked value to the in-memory cpu context. The net effect is that
> guest reads and writes of PMEVTYPER<n>_EL0 will see non-RES0 behavior in
> the reserved bits of the field.
> 
> As it so happens, kvm_pmu_set_counter_event_type() already writes a
> masked value to the in-memory context that gets overwritten by
> access_pmu_evtyper(). Fix the issue by removing the unnecessary (and
> incorrect) register write in access_pmu_evtyper().
> 
> Signed-off-by: Oliver Upton <oliver.upton@linux.dev>

Reviewed-by: Marc Zyngier <maz@kernel.org>

	M.

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