Re: [PATCH v2 05/15] KVM: arm64: nv: Load guest hyp's ZCR into EL1 state

Marc Zyngier
2024-06-14T11:14:34Z

On Thu, 13 Jun 2024 21:17:46 +0100,
Oliver Upton <oliver.upton@linux.dev> wrote:
>
> Load the guest hypervisor's ZCR_EL2 into the corresponding EL1 register
> when restoring SVE state, as ZCR_EL2 affects the VL in the hypervisor
> context.
>
> Signed-off-by: Oliver Upton <oliver.upton@linux.dev>
> ---
> arch/arm64/include/asm/kvm_host.h | 4 ++++
> arch/arm64/kvm/hyp/include/hyp/switch.h | 3 ++-
> 2 files changed, 6 insertions(+), 1 deletion(-)
>
> diff --git a/arch/arm64/include/asm/kvm_host.h b/arch/arm64/include/asm/kvm_host.h
> index 8170c04fde91..e01e6de414f1 100644
> --- a/arch/arm64/include/asm/kvm_host.h
> +++ b/arch/arm64/include/asm/kvm_host.h
> @@ -844,6 +844,10 @@ struct kvm_vcpu_arch {
>
> #define vcpu_sve_max_vq(vcpu) sve_vq_from_vl((vcpu)->arch.sve_max_vl)
>
> +#define vcpu_sve_zcr_el1(vcpu) \
> + (unlikely(is_hyp_ctxt(vcpu)) ? __vcpu_sys_reg(vcpu, ZCR_EL2) : \
> + __vcpu_sys_reg(vcpu, ZCR_EL1))
> +

I have the feeling this abstracts the access at the wrong level. It's
not that it gives the wrong result, but it hides the register and is
only concerned with the *value*.

In turn, it makes the helper unusable with the *write* side, as shown
in patch 7.

> #define vcpu_sve_state_size(vcpu) ({ \
> size_t __size_ret; \
> unsigned int __vcpu_vq; \
> diff --git a/arch/arm64/kvm/hyp/include/hyp/switch.h b/arch/arm64/kvm/hyp/include/hyp/switch.h
> index 5ecd2600d9df..71a93e336a0c 100644
> --- a/arch/arm64/kvm/hyp/include/hyp/switch.h
> +++ b/arch/arm64/kvm/hyp/include/hyp/switch.h
> @@ -317,7 +317,8 @@ static inline void __hyp_sve_restore_guest(struct kvm_vcpu *vcpu)
> sve_cond_update_zcr_vq(vcpu_sve_max_vq(vcpu) - 1, SYS_ZCR_EL2);
> __sve_restore_state(vcpu_sve_pffr(vcpu),
> &vcpu->arch.ctxt.fp_regs.fpsr);
> - write_sysreg_el1(__vcpu_sys_reg(vcpu, ZCR_EL1), SYS_ZCR);
> +
> + write_sysreg_el1(vcpu_sve_zcr_el1(vcpu), SYS_ZCR);
> }
>
> /*

I would instead propose the following:

diff --git a/arch/arm64/include/asm/kvm_host.h b/arch/arm64/include/asm/kvm_host.h
index aeb1c567dfad..2c3eff0031eb 100644
--- a/arch/arm64/include/asm/kvm_host.h
+++ b/arch/arm64/include/asm/kvm_host.h
@@ -845,9 +845,8 @@ struct kvm_vcpu_arch {

#define vcpu_sve_max_vq(vcpu) sve_vq_from_vl((vcpu)->arch.sve_max_vl)

-#define vcpu_sve_zcr_el1(vcpu) \
- (unlikely(is_hyp_ctxt(vcpu)) ? __vcpu_sys_reg(vcpu, ZCR_EL2) : \
- __vcpu_sys_reg(vcpu, ZCR_EL1))
+#define vcpu_sve_zcr_elx(vcpu) \
+ (unlikely(is_hyp_ctxt(vcpu)) ? ZCR_EL2 : ZCR_EL1)

#define vcpu_sve_state_size(vcpu) ({ \
size_t __size_ret; \
diff --git a/arch/arm64/kvm/fpsimd.c b/arch/arm64/kvm/fpsimd.c
index bb2ef3166c63..947486a111e1 100644
--- a/arch/arm64/kvm/fpsimd.c
+++ b/arch/arm64/kvm/fpsimd.c
@@ -179,10 +179,7 @@ void kvm_arch_vcpu_put_fp(struct kvm_vcpu *vcpu)
* If the vCPU is in the hyp context then ZCR_EL1 is
* loaded with its vEL2 counterpart.
*/
- if (is_hyp_ctxt(vcpu))
- __vcpu_sys_reg(vcpu, ZCR_EL2) = zcr;
- else
- __vcpu_sys_reg(vcpu, ZCR_EL1) = zcr;
+ __vcpu_sys_reg(vcpu, vcpu_sve_zcr_elx(vcpu)) = zcr;

/*
* Restore the VL that was saved when bound to the CPU,
diff --git a/arch/arm64/kvm/hyp/include/hyp/switch.h b/arch/arm64/kvm/hyp/include/hyp/switch.h
index 0a6935a18490..ad8dec0b450b 100644
--- a/arch/arm64/kvm/hyp/include/hyp/switch.h
+++ b/arch/arm64/kvm/hyp/include/hyp/switch.h
@@ -330,7 +330,7 @@ static inline void __hyp_sve_restore_guest(struct kvm_vcpu *vcpu)
if (vcpu_has_nv(vcpu) && !is_hyp_ctxt(vcpu))
sve_cond_update_zcr_vq(__vcpu_sys_reg(vcpu, ZCR_EL2), SYS_ZCR_EL2);

- write_sysreg_el1(vcpu_sve_zcr_el1(vcpu), SYS_ZCR);
+ write_sysreg_el1(__vcpu_sys_reg(vcpu, vcpu_sve_zcr_elx(vcpu)), SYS_ZCR);
}

/*

which makes the helper select the correct guest register for the
context, and only that. In turn, the write side is much cleaner and
symmetry is restored.

Thanks,

M.

--
Without deviation from the norm, progress is not possible.