Re: [RFC PATCH 12/16] KVM: arm64/sve: Context switch the SVE registers

Christoffer Dall
2018-08-06T13:19:15Z

On Thu, Jun 21, 2018 at 03:57:36PM +0100, Dave Martin wrote:
> In order to give each vcpu its own view of the SVE registers, this
> patch adds context storage via a new sve_state pointer in struct
> vcpu_arch. An additional member sve_max_vl is also added for each
> vcpu, to determine the maximum vector length visible to the guest
> and thus the value to be configured in ZCR_EL2.LEN while the is
> active. This also determines the layout and size of the storage in
> sve_state, which is read and written by the same backend functions
> that are used for context-switching the SVE state for host tasks.
>
> On SVE-enabled vcpus, SVE access traps are now handled by switching
> in the vcpu's SVE context and disabling the trap before returning
> to the guest. On other vcpus, the trap is not handled and an exit
> back to the host occurs, where the handle_sve() fallback path
> reflects an undefined instruction exception back to the guest,
> consistently with the behaviour of non-SVE-capable hardware (as was
> done unconditionally prior to this patch).
>
> No SVE handling is added on non-VHE-only paths, since VHE is an
> architectural and Kconfig prerequisite of SVE.
>
> Signed-off-by: Dave Martin <Dave.Martin@arm.com>
> ---
> arch/arm64/include/asm/kvm_host.h | 2 ++
> arch/arm64/kvm/fpsimd.c | 5 +++--
> arch/arm64/kvm/hyp/switch.c | 43 ++++++++++++++++++++++++++++++---------
> 3 files changed, 38 insertions(+), 12 deletions(-)
>
> diff --git a/arch/arm64/include/asm/kvm_host.h b/arch/arm64/include/asm/kvm_host.h
> index f331abf..d2084ae 100644
> --- a/arch/arm64/include/asm/kvm_host.h
> +++ b/arch/arm64/include/asm/kvm_host.h
> @@ -211,6 +211,8 @@ typedef struct kvm_cpu_context kvm_cpu_context_t;
>
> struct kvm_vcpu_arch {
> struct kvm_cpu_context ctxt;
> + void *sve_state;
> + unsigned int sve_max_vl;
>
> /* HYP configuration */
> u64 hcr_el2;
> diff --git a/arch/arm64/kvm/fpsimd.c b/arch/arm64/kvm/fpsimd.c
> index 872008c..44cf783 100644
> --- a/arch/arm64/kvm/fpsimd.c
> +++ b/arch/arm64/kvm/fpsimd.c
> @@ -86,10 +86,11 @@ void kvm_arch_vcpu_ctxsync_fp(struct kvm_vcpu *vcpu)
>
> if (vcpu->arch.flags & KVM_ARM64_FP_ENABLED) {
> fpsimd_bind_state_to_cpu(&vcpu->arch.ctxt.gp_regs.fp_regs,
> - NULL, sve_max_vl);
> + vcpu->arch.sve_state,
> + vcpu->arch.sve_max_vl);
>
> clear_thread_flag(TIF_FOREIGN_FPSTATE);
> - clear_thread_flag(TIF_SVE);
> + update_thread_flag(TIF_SVE, vcpu_has_sve(&vcpu->arch));
> }
> }
>
> diff --git a/arch/arm64/kvm/hyp/switch.c b/arch/arm64/kvm/hyp/switch.c
> index d496ef5..98df5c1 100644
> --- a/arch/arm64/kvm/hyp/switch.c
> +++ b/arch/arm64/kvm/hyp/switch.c
> @@ -98,8 +98,13 @@ static void activate_traps_vhe(struct kvm_vcpu *vcpu)
> val = read_sysreg(cpacr_el1);
> val |= CPACR_EL1_TTA;
> val &= ~CPACR_EL1_ZEN;
> - if (!update_fp_enabled(vcpu))
> +
> + if (update_fp_enabled(vcpu)) {
> + if (vcpu_has_sve(&vcpu->arch))
> + val |= CPACR_EL1_ZEN;
> + } else {
> val &= ~CPACR_EL1_FPEN;
> + }
>
> write_sysreg(val, cpacr_el1);
>
> @@ -114,6 +119,7 @@ static void __hyp_text __activate_traps_nvhe(struct kvm_vcpu *vcpu)
>
> val = CPTR_EL2_DEFAULT;
> val |= CPTR_EL2_TTA | CPTR_EL2_TZ;
> +
> if (!update_fp_enabled(vcpu))
> val |= CPTR_EL2_TFP;
>
> @@ -329,16 +335,22 @@ static bool __hyp_text __skip_instr(struct kvm_vcpu *vcpu)
> }
> }
>
> -static bool __hyp_text __hyp_switch_fpsimd(struct kvm_vcpu *vcpu)
> +static bool __hyp_text __hyp_switch_fpsimd(struct kvm_vcpu *vcpu,
> + bool guest_has_sve)
> {
> struct user_fpsimd_state *host_fpsimd = vcpu->arch.host_fpsimd_state;
>
> - if (has_vhe())
> - write_sysreg(read_sysreg(cpacr_el1) | CPACR_EL1_FPEN,
> - cpacr_el1);
> - else
> + if (has_vhe()) {
> + u64 reg = read_sysreg(cpacr_el1) | CPACR_EL1_FPEN;
> +
> + if (system_supports_sve() && guest_has_sve)
> + reg |= CPACR_EL1_ZEN;
> +
> + write_sysreg(reg, cpacr_el1);
> + } else {
> write_sysreg(read_sysreg(cptr_el2) & ~(u64)CPTR_EL2_TFP,
> cptr_el2);
> + }
>
> isb();
>
> @@ -361,7 +373,13 @@ static bool __hyp_text __hyp_switch_fpsimd(struct kvm_vcpu *vcpu)
> vcpu->arch.flags &= ~KVM_ARM64_FP_HOST;
> }
>
> - __fpsimd_restore_state(&vcpu->arch.ctxt.gp_regs.fp_regs);
> + if (system_supports_sve() && guest_has_sve)
> + sve_load_state((char *)vcpu->arch.sve_state +
> + sve_ffr_offset(vcpu->arch.sve_max_vl),

nit: would it make sense to have a macro 'vcpu_get_sve_state_ptr(vcpu)'
to make this first argument more pretty?

> + &vcpu->arch.ctxt.gp_regs.fp_regs.fpsr,
> + sve_vq_from_vl(vcpu->arch.sve_max_vl) - 1);
> + else
> + __fpsimd_restore_state(&vcpu->arch.ctxt.gp_regs.fp_regs);
>
> /* Skip restoring fpexc32 for AArch64 guests */
> if (!(read_sysreg(hcr_el2) & HCR_RW))
> @@ -380,6 +398,8 @@ static bool __hyp_text __hyp_switch_fpsimd(struct kvm_vcpu *vcpu)
> */
> static bool __hyp_text fixup_guest_exit(struct kvm_vcpu *vcpu, u64 *exit_code)
> {
> + bool guest_has_sve;
> +
> if (ARM_EXCEPTION_CODE(*exit_code) != ARM_EXCEPTION_IRQ)
> vcpu->arch.fault.esr_el2 = read_sysreg_el2(esr);
>
> @@ -397,10 +417,13 @@ static bool __hyp_text fixup_guest_exit(struct kvm_vcpu *vcpu, u64 *exit_code)
> * and restore the guest context lazily.
> * If FP/SIMD is not implemented, handle the trap and inject an
> * undefined instruction exception to the guest.
> + * Similarly for trapped SVE accesses.
> */
> - if (system_supports_fpsimd() &&
> - kvm_vcpu_trap_get_class(vcpu) == ESR_ELx_EC_FP_ASIMD)
> - return __hyp_switch_fpsimd(vcpu);
> + guest_has_sve = vcpu_has_sve(&vcpu->arch);
> + if ((system_supports_fpsimd() &&
> + kvm_vcpu_trap_get_class(vcpu) == ESR_ELx_EC_FP_ASIMD) ||
> + (guest_has_sve && kvm_vcpu_trap_get_class(vcpu) == ESR_ELx_EC_SVE))

nit: this may also be folded nicely into a static bool
__trap_fpsimd_sve_access() check.

> + return __hyp_switch_fpsimd(vcpu, guest_has_sve);
>
> if (!__populate_fault_info(vcpu))
> return true;
> --
> 2.1.4
>

Thanks,
-Christoffer