[PATCH v2 4/8] KVM: arm64: PMU: Disallow userspace to set PMCR.N greater than the host value

[Marc Zyngier]

On Tue, 17 Jan 2023 01:35:38 +0000,
Reiji Watanabe <reijiw at google.com> wrote:
> 
> Currently, KVM allows userspace to set PMCR_EL0 to any values
> with KVM_SET_ONE_REG for a vCPU with PMUv3 configured.
> 
> Disallow userspace to set PMCR_EL0.N to a value that is greater
> than the host value (KVM_SET_ONE_REG will fail), as KVM doesn't
> support more event counters than the host HW implements.
> Although this is an ABI change, this change only affects
> userspace setting PMCR_EL0.N to a larger value than the host.
> As accesses to unadvertised event counters indices is CONSTRAINED
> UNPREDICTABLE behavior, and PMCR_EL0.N was reset to the host value
> on every vCPU reset before this series, I can't think of any
> use case where a user space would do that.
> 
> Also, ignore writes to read-only bits that are cleared on vCPU reset,
> and RES{0,1} bits (including writable bits that KVM doesn't support
> yet), as those bits shouldn't be modified (at least with
> the current KVM).
> 
> Signed-off-by: Reiji Watanabe <reijiw at google.com>

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

	M.

-- 
Without deviation from the norm, progress is not possible.