Re: [PATCH v2 2/6] KVM: arm64: unify trap setup code

From: Sebastian Ott <sebott@redhat.com>
To: Oliver Upton <oliver.upton@linux.dev>
Cc: linux-arm-kernel@lists.infradead.org, kvmarm@lists.linux.dev,
	 linux-kernel@vger.kernel.org, Marc Zyngier <maz@kernel.org>,
	 James Morse <james.morse@arm.com>,
	 Suzuki K Poulose <suzuki.poulose@arm.com>,
	 Catalin Marinas <catalin.marinas@arm.com>,
	Will Deacon <will@kernel.org>
Subject: Re: [PATCH v2 2/6] KVM: arm64: unify trap setup code
Date: Fri, 3 May 2024 17:06:54 +0200 (CEST)	[thread overview]
Message-ID: <5c17b8e9-9281-f6ef-df71-11821e19b112@redhat.com> (raw)
In-Reply-To: <ZjHmX_O7KTInLuL5@linux.dev>

On Wed, 1 May 2024, Oliver Upton wrote:
> On Fri, Apr 26, 2024 at 12:49:46PM +0200, Sebastian Ott wrote:
>> There are 2 functions to set up traps via HCR_EL2:
>
> nitpick: these functions *calculate* the trap values, but do not
> actually set them up. HCR_EL2 doesn't get written to until further down
> the line on KVM_RUN.
>
>> +	if (cpus_have_final_cap(ARM64_HAS_STAGE2_FWB)) {
>> +		vcpu->arch.hcr_el2 |= HCR_FWB;
>> +	} else {
>> +		/*
>> +		 * For non-FWB CPUs, we trap VM ops (HCR_EL2.TVM) until M+C
>> +		 * get set in SCTLR_EL1 such that we can detect when the guest
>> +		 * MMU gets turned on and do the necessary cache maintenance
>> +		 * then.
>> +		 */
>> +		vcpu->arch.hcr_el2 |= HCR_TVM;
>> +	}
>
> It seems to me like calling this once for the lifetime of a vCPU will
> break non-FWB behavior.
>
> Like the comment suggests, these traps are needed to catch the moment
> the S1 MMU is turned on and do cache maintenance to make sure D$ agrees
> with what the guest was doing before enabling the MMU.
>
> KVM_ARM_VCPU_INIT resets SCTLR_EL1, but it seems we'd miss setting
> HCR_TVM in that case.

Ugh, I didn't think about KVM_ARM_VCPU_INIT being called more than once.

But in that case don't we loose the changes done to hcr_el2 in the current
code? E.g.:

void kvm_init_sysreg(struct kvm_vcpu *vcpu)
{
...
 	if (!kvm_has_feat(kvm, ID_AA64ISAR0_EL1, TLB, OS))
 		vcpu->arch.hcr_el2 |= HCR_TTLBOS;
...
}

static inline void vcpu_reset_hcr(struct kvm_vcpu *vcpu)
{
 	vcpu->arch.hcr_el2 = HCR_GUEST_FLAGS;
...
}

Thanks,
Sebastian

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