linux-kernel - [PATCH] KVM: arm64: Document why we trap SVE access from the host in nVHE

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Message-Id: <20231221-kvm-arm64-nvhe-sve-trap-v1-1-332ecc56f694@kernel.org>
Date: Thu, 21 Dec 2023 21:21:09 +0000
From: Mark Brown <broonie@...nel.org>
To: Marc Zyngier <maz@...nel.org>, Oliver Upton <oliver.upton@...ux.dev>, 
 James Morse <james.morse@....com>, 
 Suzuki K Poulose <suzuki.poulose@....com>, 
 Catalin Marinas <catalin.marinas@....com>, Will Deacon <will@...nel.org>
Cc: linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.linux.dev, 
 linux-kernel@...r.kernel.org, Mark Brown <broonie@...nel.org>
Subject: [PATCH] KVM: arm64: Document why we trap SVE access from the host
 in nVHE

When we exit from a SVE guest we leave the SVE configuration in EL2 as it
was for the guest, only switching back to the host configuration on next
use by the host. This is perhaps a little surprising when encountered in
the code by itself, add comments explaining what is going on both in the
trap handler and when we configure the traps.

Signed-off-by: Mark Brown <broonie@...nel.org>
---
 arch/arm64/include/asm/kvm_emulate.h | 2 ++
 arch/arm64/kvm/hyp/nvhe/hyp-main.c   | 1 +
 2 files changed, 3 insertions(+)

diff --git a/arch/arm64/include/asm/kvm_emulate.h b/arch/arm64/include/asm/kvm_emulate.h
index 78a550537b67..edd807a77e19 100644
--- a/arch/arm64/include/asm/kvm_emulate.h
+++ b/arch/arm64/include/asm/kvm_emulate.h
@@ -590,6 +590,7 @@ static __always_inline u64 kvm_get_reset_cptr_el2(struct kvm_vcpu *vcpu)
 	} else if (has_hvhe()) {
 		val = (CPACR_EL1_FPEN_EL0EN | CPACR_EL1_FPEN_EL1EN);
 
+		/* Leave traps enabled, we will restore EL2 lazily */
 		if (!vcpu_has_sve(vcpu) ||
 		    (vcpu->arch.fp_state != FP_STATE_GUEST_OWNED))
 			val |= CPACR_EL1_ZEN_EL1EN | CPACR_EL1_ZEN_EL0EN;
@@ -598,6 +599,7 @@ static __always_inline u64 kvm_get_reset_cptr_el2(struct kvm_vcpu *vcpu)
 	} else {
 		val = CPTR_NVHE_EL2_RES1;
 
+		/* Leave traps enabled, we will restore EL2 lazily */
 		if (vcpu_has_sve(vcpu) &&
 		    (vcpu->arch.fp_state == FP_STATE_GUEST_OWNED))
 			val |= CPTR_EL2_TZ;
diff --git a/arch/arm64/kvm/hyp/nvhe/hyp-main.c b/arch/arm64/kvm/hyp/nvhe/hyp-main.c
index 2385fd03ed87..84deed83e580 100644
--- a/arch/arm64/kvm/hyp/nvhe/hyp-main.c
+++ b/arch/arm64/kvm/hyp/nvhe/hyp-main.c
@@ -420,6 +420,7 @@ void handle_trap(struct kvm_cpu_context *host_ctxt)
 		handle_host_smc(host_ctxt);
 		break;
 	case ESR_ELx_EC_SVE:
+		/* Handle lazy restore of the host VL */
 		if (has_hvhe())
 			sysreg_clear_set(cpacr_el1, 0, (CPACR_EL1_ZEN_EL1EN |
 							CPACR_EL1_ZEN_EL0EN));

---
base-commit: 2cc14f52aeb78ce3f29677c2de1f06c0e91471ab
change-id: 20231218-kvm-arm64-nvhe-sve-trap-f65a3dd46a6a

Best regards,
-- 
Mark Brown <broonie@...nel.org>