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

Dave Martin
2018-07-25T11:50:51Z

On Thu, Jul 19, 2018 at 03:13:38PM +0200, Andrew Jones wrote:
> 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)
>
> guest_has_sve is only true when vcpu_arch->flags & KVM_ARM64_GUEST_HAS_SVE
> is true, which can only be true when system_supports_sve() is true. So
> I don't think we need system_supports_sve() here. guest_has_sve should be
> enough.
>
> > + 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)
>
> here too

As elsewhere, the system_supports_sve() check uses a static key and
should be very cheap (or free in a CONFIG_ARM64_SVE=n kernel).

The aim here is to reduce wasted effort on non-SVE systems.

Cheers
---Dave