Re: [PATCH v17 4/6] KVM: arm64: Expose KVM_ARM_CAP_MTE

[Marc Zyngier]

On Tue, 22 Jun 2021 09:07:51 +0100,
Fuad Tabba <tabba@google.com> wrote:
> 
> Hi,
> 
> On Mon, Jun 21, 2021 at 12:18 PM Steven Price <steven.price@arm.com> wrote:
> >
> > It's now safe for the VMM to enable MTE in a guest, so expose the
> > capability to user space.
> >
> > Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
> > Signed-off-by: Steven Price <steven.price@arm.com>
> > ---
> >  arch/arm64/kvm/arm.c      | 9 +++++++++
> >  arch/arm64/kvm/reset.c    | 4 ++++
> >  arch/arm64/kvm/sys_regs.c | 3 +++
> >  3 files changed, 16 insertions(+)
> >
> > diff --git a/arch/arm64/kvm/arm.c b/arch/arm64/kvm/arm.c
> > index e720148232a0..28ce26a68f09 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;
> > +               break;
> >         default:
> >                 r = -EINVAL;
> >                 break;
> > @@ -237,6 +243,9 @@ int kvm_vm_ioctl_check_extension(struct kvm *kvm, long 
> > ext)
> >                  */
> >                 r = 1;
> >                 break;
> > +       case KVM_CAP_ARM_MTE:
> > +               r = system_supports_mte();
> > +               break;
> >         case KVM_CAP_STEAL_TIME:
> >                 r = kvm_arm_pvtime_supported();
> >                 break;
> > diff --git a/arch/arm64/kvm/reset.c b/arch/arm64/kvm/reset.c
> > index d37ebee085cf..9e6922b9503a 100644
> > --- a/arch/arm64/kvm/reset.c
> > +++ b/arch/arm64/kvm/reset.c
> > @@ -244,6 +244,10 @@ 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 (vcpu->kvm->arch.mte_enabled) {
> > +                               ret = -EINVAL;
> > +                               goto out;
> > +                       }
> >                         pstate = VCPU_RESET_PSTATE_SVC;
> >                 } else {
> >                         pstate = VCPU_RESET_PSTATE_EL1;
> 
> nit: I was wondering whether this check would be better suited in
> kvm_vcpu_set_target, rather than here (kvm_reset_vcpu). kvm_reset_vcpu
> is called by kvm_vcpu_set_target, but kvm_vcpu_set_target is where
> checking for supported features happens. It might be better to group
> all such checks together. I don't think that there is any risk of this
> feature being toggled by the other call path to kvm_reset_vcpu (via
> check_vcpu_requests).

We already group the 32bit related compatibility checks in
vcpu_allowed_register_width(), and this is where I think this should
move to. I've provisionally added the change below.

        M.

diff --git a/arch/arm64/kvm/reset.c b/arch/arm64/kvm/reset.c
index 9e6922b9503a..cba7872d69a8 100644
--- a/arch/arm64/kvm/reset.c
+++ b/arch/arm64/kvm/reset.c
@@ -176,6 +176,10 @@ static bool vcpu_allowed_register_width(struct kvm_vcpu 
*vcpu)
        if (!cpus_have_const_cap(ARM64_HAS_32BIT_EL1) && is32bit)
                return false;
 
+       /* MTE is incompatible with AArch32 */
+       if (kvm_has_mte(vcpu->kvm) && is32bit)
+               return false;
+
        /* Check that the vcpus are either all 32bit or all 64bit */
        kvm_for_each_vcpu(i, tmp, vcpu->kvm) {
                if (vcpu_has_feature(tmp, KVM_ARM_VCPU_EL1_32BIT) != is32bit)
@@ -244,10 +248,6 @@ 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 (vcpu->kvm->arch.mte_enabled) {
-                               ret = -EINVAL;
-                               goto out;
-                       }
                        pstate = VCPU_RESET_PSTATE_SVC;
                } else {
                        pstate = VCPU_RESET_PSTATE_EL1;


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