linux-kernel - [RFT PATCH v3 03/21] x86/sev: Use MSR protocol only for early SVSM PVALIDATE call

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Message-ID: <20250512190834.332684-26-ardb+git@google.com>
Date: Mon, 12 May 2025 21:08:38 +0200
From: Ard Biesheuvel <ardb+git@...gle.com>
To: linux-kernel@...r.kernel.org
Cc: linux-efi@...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>, 
	Tom Lendacky <thomas.lendacky@....com>
Subject: [RFT PATCH v3 03/21] x86/sev: Use MSR protocol only for early SVSM
 PVALIDATE call

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

The early page state change API performs an SVSM call to PVALIDATE each
page when running under a SVSM, and this involves either a GHCB page
based call or a call based on the MSR protocol.

The GHCB page based variant involves VA to PA translation of the GHCB
address, and this is best avoided in the startup code, where virtual
addresses are ambiguous (1:1 or kernel virtual).

As this is the last remaining occurrence of svsm_perform_call_protocol()
in the startup code, switch to the MSR protocol exclusively in this
particular case, so that the GHCB based plumbing can be moved out of the
startup code entirely in a subsequent patch.

Signed-off-by: Ard Biesheuvel <ardb@...nel.org>
---
 arch/x86/boot/compressed/sev.c     | 20 --------------------
 arch/x86/boot/startup/sev-shared.c |  4 +++-
 2 files changed, 3 insertions(+), 21 deletions(-)

diff --git a/arch/x86/boot/compressed/sev.c b/arch/x86/boot/compressed/sev.c
index 612b443296d3..bc4ec45d9935 100644
--- a/arch/x86/boot/compressed/sev.c
+++ b/arch/x86/boot/compressed/sev.c
@@ -50,31 +50,11 @@ u64 svsm_get_caa_pa(void)
 	return boot_svsm_caa_pa;
 }
 
-int svsm_perform_call_protocol(struct svsm_call *call);
-
 u8 snp_vmpl;
 
 /* Include code for early handlers */
 #include "../../boot/startup/sev-shared.c"
 
-int svsm_perform_call_protocol(struct svsm_call *call)
-{
-	struct ghcb *ghcb;
-	int ret;
-
-	if (boot_ghcb)
-		ghcb = boot_ghcb;
-	else
-		ghcb = NULL;
-
-	do {
-		ret = ghcb ? svsm_perform_ghcb_protocol(ghcb, call)
-			   : svsm_perform_msr_protocol(call);
-	} while (ret == -EAGAIN);
-
-	return ret;
-}
-
 static bool sev_snp_enabled(void)
 {
 	return sev_status & MSR_AMD64_SEV_SNP_ENABLED;
diff --git a/arch/x86/boot/startup/sev-shared.c b/arch/x86/boot/startup/sev-shared.c
index 408e064a80d9..297d2abe8e3d 100644
--- a/arch/x86/boot/startup/sev-shared.c
+++ b/arch/x86/boot/startup/sev-shared.c
@@ -755,7 +755,9 @@ static void __head svsm_pval_4k_page(unsigned long paddr, bool validate)
 	call.rax = SVSM_CORE_CALL(SVSM_CORE_PVALIDATE);
 	call.rcx = pc_pa;
 
-	ret = svsm_perform_call_protocol(&call);
+	do {
+		ret = svsm_perform_msr_protocol(&call);
+	} while (ret == -EAGAIN);
 	if (ret)
 		sev_es_terminate(SEV_TERM_SET_LINUX, GHCB_TERM_PVALIDATE);
 
-- 
2.49.0.1045.g170613ef41-goog