linux-kernel - [PATCH RFC v2 01/22] KVM: arm64: Document why we trap SVE access from the host

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Message-Id: <20231222-kvm-arm64-sme-v2-1-da226cb180bb@kernel.org>
Date: Fri, 22 Dec 2023 16:21:09 +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>, 
 Paolo Bonzini <pbonzini@...hat.com>, Jonathan Corbet <corbet@....net>, 
 Shuah Khan <shuah@...nel.org>
Cc: linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.linux.dev, 
 linux-kernel@...r.kernel.org, kvm@...r.kernel.org, 
 linux-doc@...r.kernel.org, linux-kselftest@...r.kernel.org, 
 Mark Brown <broonie@...nel.org>
Subject: [PATCH RFC v2 01/22] KVM: arm64: Document why we trap SVE access
 from the host

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, 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 | 1 +
 arch/arm64/kvm/hyp/nvhe/hyp-main.c   | 1 +
 2 files changed, 2 insertions(+)

diff --git a/arch/arm64/include/asm/kvm_emulate.h b/arch/arm64/include/asm/kvm_emulate.h
index 78a550537b67..14d6ff2e2a39 100644
--- a/arch/arm64/include/asm/kvm_emulate.h
+++ b/arch/arm64/include/asm/kvm_emulate.h
@@ -598,6 +598,7 @@ 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));

-- 
2.30.2