[PATCH v2 3/4] x86/sev: Unify SEV-SNP hypervisor feature check

Ard Biesheuvel
2025-04-16T16:58:07Z

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/sme.c | 2 ++
arch/x86/coco/sev/core.c | 11 -------
arch/x86/coco/sev/shared.c | 33 +++++++++++++++-----
arch/x86/include/asm/sev-internal.h | 3 +-
arch/x86/include/asm/sev.h | 2 ++
6 files changed, 32 insertions(+), 38 deletions(-)

diff --git a/arch/x86/boot/compressed/sev.c b/arch/x86/boot/compressed/sev.c
index 26ba997c45fa..aeee011a6486 100644
--- a/arch/x86/boot/compressed/sev.c
+++ b/arch/x86/boot/compressed/sev.c
@@ -611,24 +611,7 @@ 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/sme.c b/arch/x86/boot/startup/sme.c
index 5738b31c8e60..11caa343790d 100644
--- a/arch/x86/boot/startup/sme.c
+++ b/arch/x86/boot/startup/sme.c
@@ -533,6 +533,8 @@ 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 aeb7731862c0..4618ab64c767 100644
--- a/arch/x86/coco/sev/core.c
+++ b/arch/x86/coco/sev/core.c
@@ -1582,17 +1582,6 @@ 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/coco/sev/shared.c b/arch/x86/coco/sev/shared.c
index a7c94020e384..514e2d42700b 100644
--- a/arch/x86/coco/sev/shared.c
+++ b/arch/x86/coco/sev/shared.c
@@ -99,16 +99,10 @@ 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();

@@ -119,6 +113,31 @@ 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 (RIP_REL_REF(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;
+}
+
void snp_register_ghcb_early(unsigned long paddr)
{
unsigned long pfn = paddr >> PAGE_SHIFT;
diff --git a/arch/x86/include/asm/sev-internal.h b/arch/x86/include/asm/sev-internal.h
index 73cb774c3639..81abef86b1d0 100644
--- a/arch/x86/include/asm/sev-internal.h
+++ b/arch/x86/include/asm/sev-internal.h
@@ -4,7 +4,6 @@

extern struct ghcb boot_ghcb_page;
extern struct ghcb *boot_ghcb;
-extern u64 sev_hv_features;

/* #VC handler runtime per-CPU data */
struct sev_es_runtime_data {
@@ -117,6 +116,6 @@ enum es_result sev_es_ghcb_hv_call(struct ghcb *ghcb,
void snp_register_ghcb_early(unsigned long paddr);
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 a8661dfc9a9a..8637a65973ef 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);
@@ -494,6 +495,7 @@ 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);
--
2.49.0.805.g082f7c87e0-goog