linux-kernel - Re: [PATCH 06/16] KVM: arm64: Force a full unmap on vpcu reinit

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Date:   Tue, 27 Jul 2021 19:11:33 +0100
From:   Will Deacon <will@...nel.org>
To:     Marc Zyngier <maz@...nel.org>
Cc:     linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.cs.columbia.edu,
        kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        qperret@...gle.com, dbrazdil@...gle.com,
        Srivatsa Vaddagiri <vatsa@...eaurora.org>,
        Shanker R Donthineni <sdonthineni@...dia.com>,
        James Morse <james.morse@....com>,
        Suzuki K Poulose <suzuki.poulose@....com>,
        Alexandru Elisei <alexandru.elisei@....com>,
        kernel-team@...roid.com
Subject: Re: [PATCH 06/16] KVM: arm64: Force a full unmap on vpcu reinit

On Thu, Jul 15, 2021 at 05:31:49PM +0100, Marc Zyngier wrote:
> As we now keep information in the S2PT, we must be careful not
> to keep it across a VM reboot, which could otherwise lead to
> interesting problems.
> 
> Make sure that the S2 is completely discarded on reset of
> a vcpu, and remove the flag that enforces the MMIO check.
> 
> Signed-off-by: Marc Zyngier <maz@...nel.org>
> ---
>  arch/arm64/kvm/arm.c | 8 +++++++-
>  1 file changed, 7 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/arm64/kvm/arm.c b/arch/arm64/kvm/arm.c
> index 97ab1512c44f..b0d2225190d2 100644
> --- a/arch/arm64/kvm/arm.c
> +++ b/arch/arm64/kvm/arm.c
> @@ -1096,12 +1096,18 @@ static int kvm_arch_vcpu_ioctl_vcpu_init(struct kvm_vcpu *vcpu,
>  	 * ensuring that the data side is always coherent. We still
>  	 * need to invalidate the I-cache though, as FWB does *not*
>  	 * imply CTR_EL0.DIC.
> +	 *
> +	 * If the MMIO guard was enabled, we pay the price of a full
> +	 * unmap to get back to a sane state (and clear the flag).
>  	 */
>  	if (vcpu->arch.has_run_once) {
> -		if (!cpus_have_final_cap(ARM64_HAS_STAGE2_FWB))
> +		if (!cpus_have_final_cap(ARM64_HAS_STAGE2_FWB) ||
> +		    test_bit(KVM_ARCH_FLAG_MMIO_GUARD, &vcpu->kvm->arch.flags))
>  			stage2_unmap_vm(vcpu->kvm);
>  		else
>  			icache_inval_all_pou();
> +
> +		clear_bit(KVM_ARCH_FLAG_MMIO_GUARD, &vcpu->kvm->arch.flags);

What prevents this racing with another vCPU trying to set the bit?

Will