Re: [RFC PATCH 16/16] KVM: arm64/sve: Report and enable SVE API extensions for userspace

Dave Martin
2018-07-26T13:23:13Z

On Thu, Jul 19, 2018 at 05:24:20PM +0200, Andrew Jones wrote:
> On Thu, Jun 21, 2018 at 03:57:40PM +0100, Dave Martin wrote:
> > This patch reports the availability of KVM SVE support to userspace
> > via a new vcpu feature flag KVM_ARM_VCPU_SVE. This flag is
> > reported via the KVM_ARM_PREFERRED_TARGET ioctl.
> >
> > Userspace can enable the feature by setting the flag for
> > KVM_ARM_VCPU_INIT. Without this flag set, SVE-related ioctls and
> > register access extensions are hidden, and SVE remains disabled
> > unconditionally for the guest. This ensures that non-SVE-aware KVM
> > userspace does not receive a vcpu that it does not understand how
> > to snapshot or restore correctly.
> >
> > Storage is allocated for the SVE register state at vcpu init time,
> > sufficient for the maximum vector length to be exposed to the vcpu.
> > No attempt is made to allocate the storage lazily for now. Also,
> > no attempt is made to resize the storage dynamically, since the
> > effective vector length of the vcpu can change at each EL0/EL1
> > transition. The storage is freed at the vcpu uninit hook.
> >
> > No particular attempt is made to prevent userspace from creating a
> > mix of vcpus some of which have SVE enabled and some of which have
> > it disabled. This may or may not be useful, but it reflects the
> > underlying architectural behaviour.
> >
> > Signed-off-by: Dave Martin <Dave.Martin@arm.com>
> > ---
> > arch/arm64/include/asm/kvm_host.h | 6 +++---
> > arch/arm64/include/uapi/asm/kvm.h | 1 +
> > arch/arm64/kvm/guest.c | 19 +++++++++++++------
> > arch/arm64/kvm/reset.c | 14 ++++++++++++++
> > 4 files changed, 31 insertions(+), 9 deletions(-)
> >
> > diff --git a/arch/arm64/include/asm/kvm_host.h b/arch/arm64/include/asm/kvm_host.h
> > index d2084ae..d956cf2 100644
> > --- a/arch/arm64/include/asm/kvm_host.h
> > +++ b/arch/arm64/include/asm/kvm_host.h
> > @@ -44,7 +44,7 @@
> >
> > #define KVM_MAX_VCPUS VGIC_V3_MAX_CPUS
> >
> > -#define KVM_VCPU_MAX_FEATURES 4
> > +#define KVM_VCPU_MAX_FEATURES 5
> >
> > #define KVM_REQ_SLEEP \
> > KVM_ARCH_REQ_FLAGS(0, KVM_REQUEST_WAIT | KVM_REQUEST_NO_WAKEUP)
> > @@ -439,8 +439,8 @@ static inline void kvm_arch_sync_events(struct kvm *kvm) {}
> > static inline void kvm_arch_sched_in(struct kvm_vcpu *vcpu, int cpu) {}
> > static inline void kvm_arch_vcpu_block_finish(struct kvm_vcpu *vcpu) {}
> >
> > -static inline int kvm_arm_arch_vcpu_init(struct kvm_vcpu *vcpu) { return 0; }
> > -static inline void kvm_arm_arch_vcpu_uninit(struct kvm_vcpu *vcpu) {}
> > +int kvm_arm_arch_vcpu_init(struct kvm_vcpu *vcpu);
> > +void kvm_arm_arch_vcpu_uninit(struct kvm_vcpu *vcpu);
> >
> > void kvm_arm_init_debug(void);
> > void kvm_arm_setup_debug(struct kvm_vcpu *vcpu);
> > diff --git a/arch/arm64/include/uapi/asm/kvm.h b/arch/arm64/include/uapi/asm/kvm.h
> > index f54a9b0..6acf276 100644
> > --- a/arch/arm64/include/uapi/asm/kvm.h
> > +++ b/arch/arm64/include/uapi/asm/kvm.h
> > @@ -101,6 +101,7 @@ struct kvm_regs {
> > #define KVM_ARM_VCPU_EL1_32BIT 1 /* CPU running a 32bit VM */
> > #define KVM_ARM_VCPU_PSCI_0_2 2 /* CPU uses PSCI v0.2 */
> > #define KVM_ARM_VCPU_PMU_V3 3 /* Support guest PMUv3 */
> > +#define KVM_ARM_VCPU_SVE 4 /* Allow SVE for guest */
> >
> > struct kvm_vcpu_init {
> > __u32 target;
> > diff --git a/arch/arm64/kvm/guest.c b/arch/arm64/kvm/guest.c
> > index 5152362..fb7f6aa 100644
> > --- a/arch/arm64/kvm/guest.c
> > +++ b/arch/arm64/kvm/guest.c
> > @@ -58,6 +58,16 @@ int kvm_arch_vcpu_setup(struct kvm_vcpu *vcpu)
> > return 0;
> > }
> >
> > +int kvm_arm_arch_vcpu_init(struct kvm_vcpu *vcpu)
> > +{
> > + return 0;
> > +}
> > +
> > +void kvm_arm_arch_vcpu_uninit(struct kvm_vcpu *vcpu)
> > +{
> > + kfree(vcpu->arch.sve_state);
> > +}
> > +
> > static u64 core_reg_offset_from_id(u64 id)
> > {
> > return id & ~(KVM_REG_ARCH_MASK | KVM_REG_SIZE_MASK | KVM_REG_ARM_CORE);
> > @@ -600,12 +610,9 @@ int kvm_vcpu_preferred_target(struct kvm_vcpu_init *init)
> >
> > memset(init, 0, sizeof(*init));
> >
> > - /*
> > - * For now, we don't return any features.
> > - * In future, we might use features to return target
> > - * specific features available for the preferred
> > - * target type.
> > - */
> > + /* KVM_ARM_VCPU_SVE understood by KVM_VCPU_INIT */
> > + init->features[0] = 1 << KVM_ARM_VCPU_SVE;
> > +
> > init->target = (__u32)target;
> >
> > return 0;
> > diff --git a/arch/arm64/kvm/reset.c b/arch/arm64/kvm/reset.c
> > index a74311b..f63a791 100644
> > --- a/arch/arm64/kvm/reset.c
> > +++ b/arch/arm64/kvm/reset.c
> > @@ -110,6 +110,20 @@ int kvm_reset_vcpu(struct kvm_vcpu *vcpu)
> > cpu_reset = &default_regs_reset;
> > }
> >
> > + if (system_supports_sve() &&
> > + test_bit(KVM_ARM_VCPU_SVE, vcpu->arch.features)) {
> > + vcpu->arch.flags |= KVM_ARM64_GUEST_HAS_SVE;
> > +
> > + vcpu->arch.sve_max_vl = sve_max_virtualisable_vl;
> > +
>
> The allocation below needs to be guarded by an if (!vcpu->arch.sve_state),
> otherwise every time the guest does a PSCI-off/PSCI-on cycle of the vcpu
> we'll have a memory leak. Or, we need to move this allocation into the new
> kvm_arm_arch_vcpu_init() function. Why did you opt for kvm_reset_vcpu()?

I think I failed to find another suitable init function that gets called
at the right time. I may have got confused though.

Good spot on the PSCI interaction. In light of that, I agree: the SVE
buffer allocation should not be done here.

I'll have a think about how to refactor this in light of the discussion.

Cheers
---Dave