linux-kernel - Re: [PATCH v12 6/8] arm64: kvm: Expose KVM_ARM_CAP

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Message-ID: <8735uhvhqz.wl-maz@kernel.org>
Date:   Thu, 20 May 2021 11:09:40 +0100
From:   Marc Zyngier <maz@...nel.org>
To:     Steven Price <steven.price@....com>
Cc:     Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
        James Morse <james.morse@....com>,
        Julien Thierry <julien.thierry.kdev@...il.com>,
        Suzuki K Poulose <suzuki.poulose@....com>,
        kvmarm@...ts.cs.columbia.edu, linux-arm-kernel@...ts.infradead.org,
        linux-kernel@...r.kernel.org, Dave Martin <Dave.Martin@....com>,
        Mark Rutland <mark.rutland@....com>,
        Thomas Gleixner <tglx@...utronix.de>, qemu-devel@...gnu.org,
        Juan Quintela <quintela@...hat.com>,
        "Dr. David Alan Gilbert" <dgilbert@...hat.com>,
        Richard Henderson <richard.henderson@...aro.org>,
        Peter Maydell <peter.maydell@...aro.org>,
        Haibo Xu <Haibo.Xu@....com>, Andrew Jones <drjones@...hat.com>
Subject: Re: [PATCH v12 6/8] arm64: kvm: Expose KVM_ARM_CAP_MTE

On Wed, 19 May 2021 14:26:31 +0100,
Steven Price <steven.price@....com> wrote:
> 
> On 17/05/2021 18:40, Marc Zyngier wrote:
> > On Mon, 17 May 2021 13:32:37 +0100,
> > Steven Price <steven.price@....com> wrote:
> >>
> >> It's now safe for the VMM to enable MTE in a guest, so expose the
> >> capability to user space.
> >>
> >> Signed-off-by: Steven Price <steven.price@....com>
> >> ---
> >>  arch/arm64/kvm/arm.c      | 9 +++++++++
> >>  arch/arm64/kvm/sys_regs.c | 3 +++
> >>  2 files changed, 12 insertions(+)
> >>
> >> diff --git a/arch/arm64/kvm/arm.c b/arch/arm64/kvm/arm.c
> >> index 1cb39c0803a4..e89a5e275e25 100644
> >> --- a/arch/arm64/kvm/arm.c
> >> +++ b/arch/arm64/kvm/arm.c
> >> @@ -93,6 +93,12 @@ int kvm_vm_ioctl_enable_cap(struct kvm *kvm,
> >>  		r = 0;
> >>  		kvm->arch.return_nisv_io_abort_to_user = true;
> >>  		break;
> >> +	case KVM_CAP_ARM_MTE:
> >> +		if (!system_supports_mte() || kvm->created_vcpus)
> >> +			return -EINVAL;
> >> +		r = 0;
> >> +		kvm->arch.mte_enabled = true;
> > 
> > As far as I can tell from the architecture, this isn't valid for a
> > 32bit guest.
> 
> Indeed, however the MTE flag is a property of the VM not of the vCPU.
> And, unless I'm mistaken, it's technically possible to create a VM where
> some CPUs are 32 bit and some 64 bit. Not that I can see much use of a
> configuration like that.

It looks that this is indeed a bug, and I'm on my way to squash it.
Can't believe we allowed that for so long...

But the architecture clearly states:

<quote>
These features are supported in AArch64 state only.
</quote>

So I'd expect something like:

diff --git a/arch/arm64/kvm/reset.c b/arch/arm64/kvm/reset.c
index 956cdc240148..50635eacfa43 100644
--- a/arch/arm64/kvm/reset.c
+++ b/arch/arm64/kvm/reset.c
@@ -220,7 +220,8 @@ int kvm_reset_vcpu(struct kvm_vcpu *vcpu)
 	switch (vcpu->arch.target) {
 	default:
 		if (test_bit(KVM_ARM_VCPU_EL1_32BIT, vcpu->arch.features)) {
-			if (!cpus_have_const_cap(ARM64_HAS_32BIT_EL1)) {
+			if (!cpus_have_const_cap(ARM64_HAS_32BIT_EL1) ||
+			    vcpu->kvm->arch.mte_enabled) {
 				ret = -EINVAL;
 				goto out;
 			}

that makes it completely impossible to create 32bit CPUs within a
MTE-enabled guest.

Thanks,

	M.

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