linux-kernel - Re: [PATCH v3 4/6] KVM: arm64: PMU: Reload when user modifies registers

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Message-ID: <Z9H6CWz1Z_v7DxvI@linux.dev>
Date: Wed, 12 Mar 2025 14:18:01 -0700
From: Oliver Upton <oliver.upton@...ux.dev>
To: Akihiko Odaki <akihiko.odaki@...nix.com>
Cc: Marc Zyngier <maz@...nel.org>, Joey Gouly <joey.gouly@....com>,
	Suzuki K Poulose <suzuki.poulose@....com>,
	Zenghui Yu <yuzenghui@...wei.com>,
	Catalin Marinas <catalin.marinas@....com>,
	Will Deacon <will@...nel.org>, Andrew Jones <drjones@...hat.com>,
	Shannon Zhao <shannon.zhao@...aro.org>,
	linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.linux.dev,
	linux-kernel@...r.kernel.org, devel@...nix.com
Subject: Re: [PATCH v3 4/6] KVM: arm64: PMU: Reload when user modifies
 registers

On Wed, Mar 12, 2025 at 08:55:58PM +0900, Akihiko Odaki wrote:
> Commit d0c94c49792c ("KVM: arm64: Restore PMU configuration on first
> run") added the code to reload the PMU configuration on first run.
> 
> It is also important to keep the correct state even if system registers
> are modified after first run, specifically when debugging Windows on
> QEMU with GDB; QEMU tries to write back all visible registers when
> resuming the VM execution with GDB, corrupting the PMU state. Windows
> always uses the PMU so this can cause adverse effects on that particular
> OS.
> 
> The usual register writes are already handled independently, but
> register writes from userspace and ones for reset are not covered.

Ah -- that explains why you're moving the KVM_REQ_RELOAD_PMU from
kvm_arm_pmuv3_enable().

> @@ -4259,6 +4262,9 @@ void kvm_reset_sys_regs(struct kvm_vcpu *vcpu)
>  	}
>  
>  	set_bit(KVM_ARCH_FLAG_ID_REGS_INITIALIZED, &kvm->arch.flags);
> +
> +	if (kvm_vcpu_has_pmu(vcpu))
> +		kvm_make_request(KVM_REQ_RELOAD_PMU, vcpu);

nitpick, but maybe this can be added to kvm_pmu_vcpu_reset() instead.

Thanks,
Oliver