On Thu, 20 May 2021 13:44:34 +0100,
Mark Rutland <mark.rutland@xxxxxxx> wrote:
>
> On Thu, May 20, 2021 at 01:22:53PM +0100, Marc Zyngier wrote:
> > It looks like we have tolerated creating mixed-width VMs since...
> > forever. However, that was never the intention, and we'd rather
> > not have to support that pointless complexity.
> >
> > Forbid such a setup by making sure all the vcpus have the same
> > register width.
> >
> > Reported-by: Steven Price <steven.price@xxxxxxx>
> > Signed-off-by: Marc Zyngier <maz@xxxxxxxxxx>
> > Cc: stable@xxxxxxxxxxxxxxx
> > ---
> > arch/arm64/kvm/reset.c | 28 ++++++++++++++++++++++++----
> > 1 file changed, 24 insertions(+), 4 deletions(-)
> >
> > diff --git a/arch/arm64/kvm/reset.c b/arch/arm64/kvm/reset.c
> > index 956cdc240148..1cf308be6ef3 100644
> > --- a/arch/arm64/kvm/reset.c
> > +++ b/arch/arm64/kvm/reset.c
> > @@ -166,6 +166,25 @@ static int kvm_vcpu_enable_ptrauth(struct kvm_vcpu *vcpu)
> > return 0;
> > }
> >
> > +static bool vcpu_allowed_register_width(struct kvm_vcpu *vcpu)
> > +{
> > + struct kvm_vcpu *tmp;
> > + int i;
> > +
> > + /* Check that the vcpus are either all 32bit or all 64bit */
> > + kvm_for_each_vcpu(i, tmp, vcpu->kvm) {
> > + bool w;
> > +
> > + w = test_bit(KVM_ARM_VCPU_EL1_32BIT, tmp->arch.features);
> > + w ^= test_bit(KVM_ARM_VCPU_EL1_32BIT, vcpu->arch.features);
> > +
> > + if (w)
> > + return false;
> > + }
>
> I think this is wrong for a single-cpu VM. In that case, the loop will
> have a single iteration, and tmp == vcpu, so w must be 0 regardless of
> the value of arch.features.
I don't immediately see what is wrong with a single-cpu VM. 'w' will
be zero indeed, and we'll return that this is allowed. After all, each
VM starts by being a single-CPU VM.
But of course...
> IIUC that doesn't prevent KVM_ARM_VCPU_EL1_32BIT being set when we don't
> have the ARM64_HAS_32BIT_EL1 cap, unless that's checked elsewhere?
... I mistakenly removed the check against ARM64_HAS_32BIT_EL1...
>
> How about something like:
>
> | static bool vcpu_allowed_register_width(struct kvm_vcpu *vcpu)
> | {
> | bool is_32bit = vcpu_features_32bit(vcpu);
> | struct kvm_vcpu *tmp;
> | int i;
> |
> | if (!cpus_have_const_cap(ARM64_HAS_32BIT_EL1) && is_32bit)
> | return false;
> |
> | kvm_for_each_vcpu(i, tmp, vcpu->kvm) {
> | if (is_32bit != vcpu_features_32bit(tmp))
> | return false;
> | }
> |
> | return true;
> | }
>
> ... with a helper in <asm/kvm_emulate.h> like:
>
> | static bool vcpu_features_32bit(struct kvm_vcpu *vcpu)
> | {
> | return test_bit(KVM_ARM_VCPU_EL1_32BIT, vcpu->arch.features);
> | }
>
> ... or
>
> | static inline bool vcpu_has_feature(struct kvm_vcpu *vcpu, int feature)
> | {
> | return test_bit(feature, vcpu->arch.features);
> | }
>
> ... so that we can avoid the line splitting required by the length of
> the test_bit() expression?
Yup, looks OK to me (with a preference for the latter).
Thanks,
M.
--
Without deviation from the norm, progress is not possible.
