linux-kernel - [PATCH kernel 3/9] KVM: SVM: Rewrite sev_es_prepare_switch_to

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Date:   Thu, 15 Jun 2023 16:37:51 +1000
From:   Alexey Kardashevskiy <aik@....com>
To:     <kvm@...r.kernel.org>
CC:     <x86@...nel.org>, <linux-kernel@...r.kernel.org>,
        Tom Lendacky <thomas.lendacky@....com>,
        Sean Christopherson <seanjc@...gle.com>,
        "Alexey Kardashevskiy" <aik@....com>
Subject: [PATCH kernel 3/9] KVM: SVM: Rewrite sev_es_prepare_switch_to_guest()'s comment about swap types

From: Sean Christopherson <seanjc@...gle.com>

Rewrite the comment(s) in sev_es_prepare_switch_to_guest() to explain the
swap types employed by the CPU for SEV-ES guests, i.e. to explain why KVM
needs to save a seemingly random subset of host state, and to provide a
decoder for the APM's Type-A/B/C terminology.

Signed-off-by: Sean Christopherson <seanjc@...gle.com>
Signed-off-by: Alexey Kardashevskiy <aik@....com>
---
Changes:
v6:
* new to the series
---
 arch/x86/kvm/svm/sev.c | 25 ++++++++++++--------
 1 file changed, 15 insertions(+), 10 deletions(-)

diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
index c03bd063aecf..36fe2fcb4698 100644
--- a/arch/x86/kvm/svm/sev.c
+++ b/arch/x86/kvm/svm/sev.c
@@ -3020,19 +3020,24 @@ void sev_es_vcpu_reset(struct vcpu_svm *svm)
 void sev_es_prepare_switch_to_guest(struct sev_es_save_area *hostsa)
 {
 	/*
-	 * As an SEV-ES guest, hardware will restore the host state on VMEXIT,
-	 * of which one step is to perform a VMLOAD.  KVM performs the
-	 * corresponding VMSAVE in svm_prepare_guest_switch for both
-	 * traditional and SEV-ES guests.
+	 * All host state for SEV-ES guests is categorized into three swap types
+	 * based on how it is handled by hardware during a world switch:
+	 *
+	 * A: VMRUN:   Host state saved in host save area
+	 *    VMEXIT:  Host state loaded from host save area
+	 *
+	 * B: VMRUN:   Host state _NOT_ saved in host save area
+	 *    VMEXIT:  Host state loaded from host save area
+	 *
+	 * C: VMRUN:   Host state _NOT_ saved in host save area
+	 *    VMEXIT:  Host state initialized to default(reset) values
+	 *
+	 * Manually save type-B state, i.e. state that is loaded by VMEXIT but
+	 * isn't saved by VMRUN, that isn't already saved by VMSAVE (performed
+	 * by common SVM code).
 	 */
-
-	/* XCR0 is restored on VMEXIT, save the current host value */
 	hostsa->xcr0 = xgetbv(XCR_XFEATURE_ENABLED_MASK);
-
-	/* PKRU is restored on VMEXIT, save the current host value */
 	hostsa->pkru = read_pkru();
-
-	/* MSR_IA32_XSS is restored on VMEXIT, save the currnet host value */
 	hostsa->xss = host_xss;
 }
 
-- 
2.40.1