Re: [PATCH v1 1/2] KVM: arm64: PMU: Restore the host's PMUSERENR_EL0

Marc Zyngier
2023-03-29T07:34:30Z

On Wed, 29 Mar 2023 01:21:35 +0100,
Reiji Watanabe <reijiw@google.com> wrote:
>
> Restore the host's PMUSERENR_EL0 value instead of clearing it,
> before returning back to userspace, as the host's EL0 might have
> a direct access to PMU registers (some bits of PMUSERENR_EL0
> might not be zero).
>
> Fixes: 83a7a4d643d3 ("arm64: perf: Enable PMU counter userspace access for perf event")
> Signed-off-by: Reiji Watanabe <reijiw@google.com>
> ---
> arch/arm64/include/asm/kvm_host.h | 3 +++
> arch/arm64/kvm/hyp/include/hyp/switch.h | 3 ++-
> 2 files changed, 5 insertions(+), 1 deletion(-)
>
> diff --git a/arch/arm64/include/asm/kvm_host.h b/arch/arm64/include/asm/kvm_host.h
> index bcd774d74f34..82220ecec10e 100644
> --- a/arch/arm64/include/asm/kvm_host.h
> +++ b/arch/arm64/include/asm/kvm_host.h
> @@ -544,6 +544,9 @@ struct kvm_vcpu_arch {
>
> /* Per-vcpu CCSIDR override or NULL */
> u32 *ccsidr;
> +
> + /* the value of host's pmuserenr_el0 before guest entry */
> + u64 host_pmuserenr_el0;

I don't think we need this in each and every vcpu. Why can't this be
placed in struct kvm_host_data and accessed via the per-cpu pointer?
Maybe even use the PMUSERNR_EL0 field in the sysreg array?

There is probably a number of things that we could move there, but
let's start by not adding more unnecessary stuff to the vcpu
structure.

> };
>
> /*
> diff --git a/arch/arm64/kvm/hyp/include/hyp/switch.h b/arch/arm64/kvm/hyp/include/hyp/switch.h
> index 07d37ff88a3f..44b84fbdde0d 100644
> --- a/arch/arm64/kvm/hyp/include/hyp/switch.h
> +++ b/arch/arm64/kvm/hyp/include/hyp/switch.h
> @@ -82,6 +82,7 @@ static inline void __activate_traps_common(struct kvm_vcpu *vcpu)
> */
> if (kvm_arm_support_pmu_v3()) {
> write_sysreg(0, pmselr_el0);
> + vcpu->arch.host_pmuserenr_el0 = read_sysreg(pmuserenr_el0);
> write_sysreg(ARMV8_PMU_USERENR_MASK, pmuserenr_el0);
> }
>
> @@ -106,7 +107,7 @@ static inline void __deactivate_traps_common(struct kvm_vcpu *vcpu)
>
> write_sysreg(0, hstr_el2);
> if (kvm_arm_support_pmu_v3())
> - write_sysreg(0, pmuserenr_el0);
> + write_sysreg(vcpu->arch.host_pmuserenr_el0, pmuserenr_el0);
>
> if (cpus_have_final_cap(ARM64_SME)) {
> sysreg_clear_set_s(SYS_HFGRTR_EL2, 0,

Thanks,

M.

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