[RFT PATCH v2 12/23] x86/sev: Unify SEV-SNP hypervisor feature check

Ard Biesheuvel
2025-05-04T09:53:30Z

From: Ard Biesheuvel <ardb@kernel.org>

The decompressor and the core kernel both check the hypervisor feature
mask exposed by the hypervisor, but test it in slightly different ways.

This disparity seems unintentional, and simply a result of the fact that
the decompressor and the core kernel evolve differently over time when
it comes to setting up the SEV-SNP execution context.

So move the HV feature check into a helper function and call that
instead. For the core kernel, move the check to an earlier boot stage,
right after the point where it is established that the guest is
executing in SEV-SNP mode.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
---
arch/x86/boot/compressed/sev.c | 19 +----------
arch/x86/boot/startup/sev-shared.c | 33 +++++++++++++++-----
arch/x86/boot/startup/sme.c | 2 ++
arch/x86/coco/sev/core.c | 11 -------
arch/x86/include/asm/sev-internal.h | 2 +-
arch/x86/include/asm/sev.h | 2 ++
6 files changed, 32 insertions(+), 37 deletions(-)

diff --git a/arch/x86/boot/compressed/sev.c b/arch/x86/boot/compressed/sev.c
index 18ba81c0b0ed..79550736ad2a 100644
--- a/arch/x86/boot/compressed/sev.c
+++ b/arch/x86/boot/compressed/sev.c
@@ -414,24 +414,7 @@ void sev_enable(struct boot_params *bp)
sev_es_terminate(SEV_TERM_SET_GEN, GHCB_SEV_ES_PROT_UNSUPPORTED);
}

- /*
- * SNP is supported in v2 of the GHCB spec which mandates support for HV
- * features.
- */
- if (sev_status & MSR_AMD64_SEV_SNP_ENABLED) {
- u64 hv_features;
-
- hv_features = get_hv_features();
- if (!(hv_features & GHCB_HV_FT_SNP))
- sev_es_terminate(SEV_TERM_SET_GEN, GHCB_SNP_UNSUPPORTED);
-
- /*
- * Running at VMPL0 is required unless an SVSM is present and
- * the hypervisor supports the required SVSM GHCB events.
- */
- if (snp_vmpl > 0 && !(hv_features & GHCB_HV_FT_SNP_MULTI_VMPL))
- sev_es_terminate(SEV_TERM_SET_LINUX, GHCB_TERM_NOT_VMPL0);
- }
+ snp_check_hv_features();

if (snp && !(sev_status & MSR_AMD64_SEV_SNP_ENABLED))
error("SEV-SNP supported indicated by CC blob, but not SEV status MSR.");
diff --git a/arch/x86/boot/startup/sev-shared.c b/arch/x86/boot/startup/sev-shared.c
index b1f4b9b15045..b72c2b9c40c3 100644
--- a/arch/x86/boot/startup/sev-shared.c
+++ b/arch/x86/boot/startup/sev-shared.c
@@ -78,16 +78,10 @@ sev_es_terminate(unsigned int set, unsigned int reason)
asm volatile("hlt\n" : : : "memory");
}

-/*
- * The hypervisor features are available from GHCB version 2 onward.
- */
-u64 get_hv_features(void)
+static u64 __head get_hv_features(void)
{
u64 val;

- if (ghcb_version < 2)
- return 0;
-
sev_es_wr_ghcb_msr(GHCB_MSR_HV_FT_REQ);
VMGEXIT();

@@ -98,6 +92,31 @@ u64 get_hv_features(void)
return GHCB_MSR_HV_FT_RESP_VAL(val);
}

+u64 __head snp_check_hv_features(void)
+{
+ /*
+ * SNP is supported in v2 of the GHCB spec which mandates support for HV
+ * features.
+ */
+ if (sev_status & MSR_AMD64_SEV_SNP_ENABLED) {
+ u64 hv_features;
+
+ hv_features = get_hv_features();
+ if (!(hv_features & GHCB_HV_FT_SNP))
+ sev_es_terminate(SEV_TERM_SET_GEN, GHCB_SNP_UNSUPPORTED);
+
+ /*
+ * Running at VMPL0 is required unless an SVSM is present and
+ * the hypervisor supports the required SVSM GHCB events.
+ */
+ if (snp_vmpl > 0 && !(hv_features & GHCB_HV_FT_SNP_MULTI_VMPL))
+ sev_es_terminate(SEV_TERM_SET_LINUX, GHCB_TERM_NOT_VMPL0);
+
+ return hv_features;
+ }
+ return 0;
+}
+
bool sev_es_negotiate_protocol(void)
{
u64 val;
diff --git a/arch/x86/boot/startup/sme.c b/arch/x86/boot/startup/sme.c
index ade5ad5060e9..7fc6a689cefe 100644
--- a/arch/x86/boot/startup/sme.c
+++ b/arch/x86/boot/startup/sme.c
@@ -524,6 +524,8 @@ void __head sme_enable(struct boot_params *bp)
if (snp_en ^ !!(msr & MSR_AMD64_SEV_SNP_ENABLED))
snp_abort();

+ sev_hv_features = snp_check_hv_features();
+
/* Check if memory encryption is enabled */
if (feature_mask == AMD_SME_BIT) {
if (!(bp->hdr.xloadflags & XLF_MEM_ENCRYPTION))
diff --git a/arch/x86/coco/sev/core.c b/arch/x86/coco/sev/core.c
index 36edf670ff19..106c231d8ded 100644
--- a/arch/x86/coco/sev/core.c
+++ b/arch/x86/coco/sev/core.c
@@ -1264,17 +1264,6 @@ void __init sev_es_init_vc_handling(void)
if (!sev_es_check_cpu_features())
panic("SEV-ES CPU Features missing");

- /*
- * SNP is supported in v2 of the GHCB spec which mandates support for HV
- * features.
- */
- if (cc_platform_has(CC_ATTR_GUEST_SEV_SNP)) {
- sev_hv_features = get_hv_features();
-
- if (!(sev_hv_features & GHCB_HV_FT_SNP))
- sev_es_terminate(SEV_TERM_SET_GEN, GHCB_SNP_UNSUPPORTED);
- }
-
/* Initialize per-cpu GHCB pages */
for_each_possible_cpu(cpu) {
alloc_runtime_data(cpu);
diff --git a/arch/x86/include/asm/sev-internal.h b/arch/x86/include/asm/sev-internal.h
index 4335711274e3..3aa59ad288ce 100644
--- a/arch/x86/include/asm/sev-internal.h
+++ b/arch/x86/include/asm/sev-internal.h
@@ -84,6 +84,6 @@ static __always_inline void sev_es_wr_ghcb_msr(u64 val)

bool sev_es_negotiate_protocol(void);
bool sev_es_check_cpu_features(void);
-u64 get_hv_features(void);
+void check_hv_features(void);

const struct snp_cpuid_table *snp_cpuid_get_table(void);
diff --git a/arch/x86/include/asm/sev.h b/arch/x86/include/asm/sev.h
index ca7168cc2118..ca914f7384c6 100644
--- a/arch/x86/include/asm/sev.h
+++ b/arch/x86/include/asm/sev.h
@@ -418,6 +418,7 @@ struct svsm_call {
#ifdef CONFIG_AMD_MEM_ENCRYPT

extern u8 snp_vmpl;
+extern u64 sev_hv_features;

extern void __sev_es_ist_enter(struct pt_regs *regs);
extern void __sev_es_ist_exit(void);
@@ -495,6 +496,7 @@ void snp_set_memory_private(unsigned long vaddr, unsigned long npages);
void snp_set_wakeup_secondary_cpu(void);
bool snp_init(struct boot_params *bp);
void __noreturn snp_abort(void);
+u64 snp_check_hv_features(void);
void snp_dmi_setup(void);
int snp_issue_svsm_attest_req(u64 call_id, struct svsm_call *call, struct svsm_attest_call *input);
void snp_accept_memory(phys_addr_t start, phys_addr_t end);
--
2.49.0.906.g1f30a19c02-goog