[PATCH v7 03/22] x86/sev: Use MSR protocol only for early SVSM PVALIDATE call

Ard Biesheuvel
2025-08-28T10:22:39Z

From: Ard Biesheuvel <ardb@kernel.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@kernel.org>
Reviewed-by: Tom Lendacky <thomas.lendacky@amd.com>
---
arch/x86/boot/compressed/sev.c | 20 --------------------
arch/x86/boot/startup/sev-shared.c | 9 ++++++---
2 files changed, 6 insertions(+), 23 deletions(-)

diff --git a/arch/x86/boot/compressed/sev.c b/arch/x86/boot/compressed/sev.c
index fd1b67dfea22..b71c1ab6a282 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 975d2b02926a..7bd73462c11e 100644
--- a/arch/x86/boot/startup/sev-shared.c
+++ b/arch/x86/boot/startup/sev-shared.c
@@ -741,7 +741,6 @@ static void __head svsm_pval_4k_page(unsigned long paddr, bool validate)
struct svsm_call call = {};
unsigned long flags;
u64 pc_pa;
- int ret;

/*
* This can be called very early in the boot, use native functions in
@@ -766,8 +765,12 @@ 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);
- if (ret)
+ /*
+ * Use the MSR protocol exclusively, so that this code is usable in
+ * startup code where VA/PA translations of the GHCB page's address may
+ * be problematic.
+ */
+ if (svsm_call_msr_protocol(&call))
sev_es_terminate(SEV_TERM_SET_LINUX, GHCB_TERM_PVALIDATE);

native_local_irq_restore(flags);
--
2.51.0.268.g9569e192d0-goog