Re: [PATCH 07/16] KVM: arm64: Wire MMIO guard hypercalls

From: Will Deacon <will@kernel.org>
To: Marc Zyngier <maz@kernel.org>
Cc: linux-arm-kernel@lists.infradead.org,
	kvmarm@lists.cs.columbia.edu, kvm@vger.kernel.org,
	linux-kernel@vger.kernel.org, qperret@google.com,
	dbrazdil@google.com, Srivatsa Vaddagiri <vatsa@codeaurora.org>,
	Shanker R Donthineni <sdonthineni@nvidia.com>,
	James Morse <james.morse@arm.com>,
	Suzuki K Poulose <suzuki.poulose@arm.com>,
	Alexandru Elisei <alexandru.elisei@arm.com>,
	kernel-team@android.com
Subject: Re: [PATCH 07/16] KVM: arm64: Wire MMIO guard hypercalls
Date: Tue, 27 Jul 2021 19:11:46 +0100	[thread overview]
Message-ID: <20210727181145.GF19173@willie-the-truck> (raw)
In-Reply-To: <20210715163159.1480168-8-maz@kernel.org>

On Thu, Jul 15, 2021 at 05:31:50PM +0100, Marc Zyngier wrote:
> Plumb in the hypercall interface to allow a guest to discover,
> enroll, map and unmap MMIO regions.
> 
> Signed-off-by: Marc Zyngier <maz@kernel.org>
> ---
>  arch/arm64/kvm/hypercalls.c | 20 ++++++++++++++++++++
>  include/linux/arm-smccc.h   | 28 ++++++++++++++++++++++++++++
>  2 files changed, 48 insertions(+)
> 
> diff --git a/arch/arm64/kvm/hypercalls.c b/arch/arm64/kvm/hypercalls.c
> index 30da78f72b3b..a3deeb907fdd 100644
> --- a/arch/arm64/kvm/hypercalls.c
> +++ b/arch/arm64/kvm/hypercalls.c
> @@ -5,6 +5,7 @@
>  #include <linux/kvm_host.h>
>  
>  #include <asm/kvm_emulate.h>
> +#include <asm/kvm_mmu.h>
>  
>  #include <kvm/arm_hypercalls.h>
>  #include <kvm/arm_psci.h>
> @@ -129,10 +130,29 @@ int kvm_hvc_call_handler(struct kvm_vcpu *vcpu)
>  	case ARM_SMCCC_VENDOR_HYP_KVM_FEATURES_FUNC_ID:
>  		val[0] = BIT(ARM_SMCCC_KVM_FUNC_FEATURES);
>  		val[0] |= BIT(ARM_SMCCC_KVM_FUNC_PTP);
> +		val[0] |= BIT(ARM_SMCCC_KVM_FUNC_MMIO_GUARD_INFO);
> +		val[0] |= BIT(ARM_SMCCC_KVM_FUNC_MMIO_GUARD_ENROLL);
> +		val[0] |= BIT(ARM_SMCCC_KVM_FUNC_MMIO_GUARD_MAP);
> +		val[0] |= BIT(ARM_SMCCC_KVM_FUNC_MMIO_GUARD_UNMAP);
>  		break;
>  	case ARM_SMCCC_VENDOR_HYP_KVM_PTP_FUNC_ID:
>  		kvm_ptp_get_time(vcpu, val);
>  		break;
> +	case ARM_SMCCC_VENDOR_HYP_KVM_MMIO_GUARD_INFO_FUNC_ID:
> +		val[0] = PAGE_SIZE;
> +		break;

I get the nagging feeling that querying the stage-2 page-size outside of
MMIO guard is going to be useful once we start looking at memory sharing,
so perhaps rename this to something more generic?

> +	case ARM_SMCCC_VENDOR_HYP_KVM_MMIO_GUARD_ENROLL_FUNC_ID:
> +		set_bit(KVM_ARCH_FLAG_MMIO_GUARD, &vcpu->kvm->arch.flags);
> +		val[0] = SMCCC_RET_SUCCESS;
> +		break;
> +	case ARM_SMCCC_VENDOR_HYP_KVM_MMIO_GUARD_MAP_FUNC_ID:
> +		if (kvm_install_ioguard_page(vcpu, vcpu_get_reg(vcpu, 1)))
> +			val[0] = SMCCC_RET_SUCCESS;
> +		break;
> +	case ARM_SMCCC_VENDOR_HYP_KVM_MMIO_GUARD_UNMAP_FUNC_ID:
> +		if (kvm_remove_ioguard_page(vcpu, vcpu_get_reg(vcpu, 1)))
> +			val[0] = SMCCC_RET_SUCCESS;
> +		break;

I think there's a slight discrepancy between MAP and UNMAP here in that
calling UNMAP on something that hasn't been mapped will fail, whereas
calling MAP on something that's already been mapped will succeed. I think
that might mean you can't reason about the final state of the page if two
vCPUs race to call these functions in some cases (and both succeed).

Will