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

Andrew Jones
2018-07-19T15:11:19Z

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()?

Thanks,
drew

> + vcpu->arch.sve_state = kzalloc(
> + SVE_SIG_REGS_SIZE(
> + sve_vq_from_vl(vcpu->arch.sve_max_vl)),
> + GFP_KERNEL);
> + if (!vcpu->arch.sve_state)
> + return -ENOMEM;
> + }
> +
> break;
> }
>
> --
> 2.1.4
>
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