Re: [PATCH v2 10/11] KVM: arm64: Save/restore SVE state for nVHE

On Thu, 18 Mar 2021 14:13:58 +0000,
Will Deacon [off-list ref] wrote:

On Thu, Mar 18, 2021 at 12:25:31PM +0000, Marc Zyngier wrote:

quoted

Implement the SVE save/restore for nVHE, following a similar
logic to that of the VHE implementation:

- the SVE state is switched on trap from EL1 to EL2

- no further changes to ZCR_EL2 occur as long as the guest isn't
  preempted or exit to userspace

- ZCR_EL2 is reset to its default value on the first SVE access from
  the host EL1, and ZCR_EL1 restored to the default guest value in
  vcpu_put()

Signed-off-by: Marc Zyngier <maz@kernel.org>
---
 arch/arm64/kvm/fpsimd.c                 | 10 +++++--
 arch/arm64/kvm/hyp/include/hyp/switch.h | 37 +++++++++----------------
 arch/arm64/kvm/hyp/nvhe/switch.c        |  4 +--
 3 files changed, 23 insertions(+), 28 deletions(-)

diff --git a/arch/arm64/kvm/fpsimd.c b/arch/arm64/kvm/fpsimd.c
index 14ea05c5134a..5621020b28de 100644
--- a/arch/arm64/kvm/fpsimd.c
+++ b/arch/arm64/kvm/fpsimd.c

@@ -121,11 +121,17 @@ void kvm_arch_vcpu_put_fp(struct kvm_vcpu *vcpu)
 	local_irq_save(flags);
 
 	if (vcpu->arch.flags & KVM_ARM64_FP_ENABLED) {
-		if (guest_has_sve)
+		if (guest_has_sve) {
 			__vcpu_sys_reg(vcpu, ZCR_EL1) = read_sysreg_el1(SYS_ZCR);
 
+			/* Restore the VL that was saved when bound to the CPU */
+			if (!has_vhe())
+				sve_cond_update_zcr_vq(vcpu_sve_max_vq(vcpu) - 1,
+						       SYS_ZCR_EL1);

You end up reading ZCR_EL1 twice here, but it's probably not the end of the
world.

True, but I'd expect read accesses to ZCR_EL1 to be reasonably fast,
something we can't expect from writes.

Anyway:

Acked-by: Will Deacon <will@kernel.org>

Thanks,

	M.

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

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