linux-kernel - [PATCH] x86/boot/sev: Support memory acceptance in the EFI stub under SVSM

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Message-ID: <20250428174322.2780170-2-ardb+git@google.com>
Date: Mon, 28 Apr 2025 19:43:22 +0200
From: Ard Biesheuvel <ardb+git@...gle.com>
To: linux-efi@...r.kernel.org
Cc: linux-kernel@...r.kernel.org, x86@...nel.org, 
	Ard Biesheuvel <ardb@...nel.org>, Borislav Petkov <bp@...en8.de>, Ingo Molnar <mingo@...nel.org>, 
	Dionna Amalie Glaze <dionnaglaze@...gle.com>, Kevin Loughlin <kevinloughlin@...gle.com>, 
	stable@...r.kernel.org, Tom Lendacky <thomas.lendacky@....com>
Subject: [PATCH] x86/boot/sev: Support memory acceptance in the EFI stub under SVSM

From: Ard Biesheuvel <ardb@...nel.org>

Commit

  d54d610243a4 ("x86/boot/sev: Avoid shared GHCB page for early memory acceptance")

provided a fix for SEV-SNP memory acceptance from the EFI stub when
running at VMPL #0. However, that fix was insufficient for SVSM SEV-SNP
guests running at VMPL >0, as those rely on a SVSM calling area, which
is a shared buffer whose address is programmed into a SEV-SNP MSR, and
the SEV init code that sets up this calling area executes much later
during the boot.

Given that booting via the EFI stub at VMPL >0 implies that the firmware
has configured this calling area already, reuse it for performing memory
acceptance in the EFI stub.

Cc: Borislav Petkov <bp@...en8.de>
Cc: Ingo Molnar <mingo@...nel.org>
Cc: Dionna Amalie Glaze <dionnaglaze@...gle.com>
Cc: Kevin Loughlin <kevinloughlin@...gle.com>
Cc: <stable@...r.kernel.org>
Fixes: fcd042e86422 ("x86/sev: Perform PVALIDATE using the SVSM when not at VMPL0")
Co-developed-by: Tom Lendacky <thomas.lendacky@....com>
Signed-off-by: Tom Lendacky <thomas.lendacky@....com>
Signed-off-by: Ard Biesheuvel <ardb@...nel.org>
---
Tom,

Please confirm that this works as you intended.

Thanks,

 arch/x86/boot/compressed/mem.c |  5 +--
 arch/x86/boot/compressed/sev.c | 40 ++++++++++++++++++++
 arch/x86/boot/compressed/sev.h |  2 +
 3 files changed, 43 insertions(+), 4 deletions(-)

diff --git a/arch/x86/boot/compressed/mem.c b/arch/x86/boot/compressed/mem.c
index f676156d9f3d..0e9f84ab4bdc 100644
--- a/arch/x86/boot/compressed/mem.c
+++ b/arch/x86/boot/compressed/mem.c
@@ -34,14 +34,11 @@ static bool early_is_tdx_guest(void)
 
 void arch_accept_memory(phys_addr_t start, phys_addr_t end)
 {
-	static bool sevsnp;
-
 	/* Platform-specific memory-acceptance call goes here */
 	if (early_is_tdx_guest()) {
 		if (!tdx_accept_memory(start, end))
 			panic("TDX: Failed to accept memory\n");
-	} else if (sevsnp || (sev_get_status() & MSR_AMD64_SEV_SNP_ENABLED)) {
-		sevsnp = true;
+	} else if (early_is_sevsnp_guest()) {
 		snp_accept_memory(start, end);
 	} else {
 		error("Cannot accept memory: unknown platform\n");
diff --git a/arch/x86/boot/compressed/sev.c b/arch/x86/boot/compressed/sev.c
index 89ba168f4f0f..0003e4416efd 100644
--- a/arch/x86/boot/compressed/sev.c
+++ b/arch/x86/boot/compressed/sev.c
@@ -645,3 +645,43 @@ void sev_prep_identity_maps(unsigned long top_level_pgt)
 
 	sev_verify_cbit(top_level_pgt);
 }
+
+bool early_is_sevsnp_guest(void)
+{
+	static bool sevsnp;
+
+	if (sevsnp)
+		return true;
+
+	if (!(sev_get_status() & MSR_AMD64_SEV_SNP_ENABLED))
+		return false;
+
+	sevsnp = true;
+
+	if (!snp_vmpl) {
+		unsigned int eax, ebx, ecx, edx;
+
+		/*
+		 * CPUID Fn8000_001F_EAX[28] - SVSM support
+		 */
+		eax = 0x8000001f;
+		ecx = 0;
+		native_cpuid(&eax, &ebx, &ecx, &edx);
+		if (eax & BIT(28)) {
+			struct msr m;
+
+			/* Obtain the address of the calling area to use */
+			boot_rdmsr(MSR_SVSM_CAA, &m);
+			boot_svsm_caa = (void *)m.q;
+			boot_svsm_caa_pa = m.q;
+
+			/*
+			 * The real VMPL level cannot be discovered, but the
+			 * memory acceptance routines make no use of that so
+			 * any non-zero value suffices here.
+			 */
+			snp_vmpl = U8_MAX;
+		}
+	}
+	return true;
+}
diff --git a/arch/x86/boot/compressed/sev.h b/arch/x86/boot/compressed/sev.h
index 4e463f33186d..d3900384b8ab 100644
--- a/arch/x86/boot/compressed/sev.h
+++ b/arch/x86/boot/compressed/sev.h
@@ -13,12 +13,14 @@
 bool sev_snp_enabled(void);
 void snp_accept_memory(phys_addr_t start, phys_addr_t end);
 u64 sev_get_status(void);
+bool early_is_sevsnp_guest(void);
 
 #else
 
 static inline bool sev_snp_enabled(void) { return false; }
 static inline void snp_accept_memory(phys_addr_t start, phys_addr_t end) { }
 static inline u64 sev_get_status(void) { return 0; }
+static inline bool early_is_sevsnp_guest(void) { return false; }
 
 #endif
 

base-commit: b4432656b36e5cc1d50a1f2dc15357543add530e
-- 
2.49.0.906.g1f30a19c02-goog