Re: [PATCH v4 08/15] x86/sev: Provide SVSM discovery support

[Borislav Petkov]

On Wed, Apr 24, 2024 at 10:58:04AM -0500, Tom Lendacky wrote:
> The SVSM specification documents an alternative method of discovery for
> the SVSM using a reserved CPUID bit and a reserved MSR.

Yes, and all your code places where you do

	if (vmpl)

to check whether the guest is running over a SVSM should do the CPUID
check. We should not be hardcoding the VMPL level to mean a SVSM is
present.

> 
> For the CPUID support, the SNP CPUID table is updated to set bit 28 of

s/is updated/update the.../

> the EAX register of the 0x8000001f leaf when an SVSM is present. This bit
> has been reserved for use in this capacity.
> 
> For the MSR support, a new reserved MSR 0xc001f000 has been defined. A #VC
> should be generated when accessing this MSR. The #VC handler is expected
> to ignore writes to this MSR and return the physical calling area address
> (CAA) on reads of this MSR.
> 
> Signed-off-by: Tom Lendacky <thomas.lendacky@xxxxxxx>
> ---
>  arch/x86/include/asm/cpufeatures.h |  1 +
>  arch/x86/include/asm/msr-index.h   |  2 ++
>  arch/x86/kernel/sev-shared.c       | 11 +++++++++++
>  arch/x86/kernel/sev.c              | 17 +++++++++++++++++
>  4 files changed, 31 insertions(+)
> 
> diff --git a/arch/x86/include/asm/cpufeatures.h b/arch/x86/include/asm/cpufeatures.h
> index 3c7434329661..a17a81b3189b 100644
> --- a/arch/x86/include/asm/cpufeatures.h
> +++ b/arch/x86/include/asm/cpufeatures.h
> @@ -446,6 +446,7 @@
>  #define X86_FEATURE_V_TSC_AUX		(19*32+ 9) /* "" Virtual TSC_AUX */
>  #define X86_FEATURE_SME_COHERENT	(19*32+10) /* "" AMD hardware-enforced cache coherency */
>  #define X86_FEATURE_DEBUG_SWAP		(19*32+14) /* AMD SEV-ES full debug state swap support */
> +#define X86_FEATURE_SVSM_PRESENT	(19*32+28) /* "" SNP SVSM is present */

X86_FEATURE_SVSM is better right?

And then we might even want to show it in /proc/cpuinfo here to really
say that we're running over a SVSM as that might be useful info. Think
alternate injection support for one.

>  /* AMD-defined Extended Feature 2 EAX, CPUID level 0x80000021 (EAX), word 20 */
>  #define X86_FEATURE_NO_NESTED_DATA_BP	(20*32+ 0) /* "" No Nested Data Breakpoints */
> diff --git a/arch/x86/include/asm/msr-index.h b/arch/x86/include/asm/msr-index.h
> index e022e6eb766c..45ffa27569f4 100644
> --- a/arch/x86/include/asm/msr-index.h
> +++ b/arch/x86/include/asm/msr-index.h
> @@ -660,6 +660,8 @@
>  #define MSR_AMD64_RMP_BASE		0xc0010132
>  #define MSR_AMD64_RMP_END		0xc0010133
>  
> +#define MSR_SVSM_CAA			0xc001f000
> +
>  /* AMD Collaborative Processor Performance Control MSRs */
>  #define MSR_AMD_CPPC_CAP1		0xc00102b0
>  #define MSR_AMD_CPPC_ENABLE		0xc00102b1
> diff --git a/arch/x86/kernel/sev-shared.c b/arch/x86/kernel/sev-shared.c
> index b415b10a0823..50db783f151e 100644
> --- a/arch/x86/kernel/sev-shared.c
> +++ b/arch/x86/kernel/sev-shared.c
> @@ -1561,6 +1561,8 @@ static enum es_result vc_check_opcode_bytes(struct es_em_ctxt *ctxt,
>  static void __head setup_svsm_ca(const struct cc_blob_sev_info *cc_info)
>  {
>  	struct snp_secrets_page *secrets_page;
> +	struct snp_cpuid_table *cpuid_table;
> +	unsigned int i;
>  	u64 caa;
>  
>  	BUILD_BUG_ON(sizeof(*secrets_page) != PAGE_SIZE);
> @@ -1607,4 +1609,13 @@ static void __head setup_svsm_ca(const struct cc_blob_sev_info *cc_info)
>  	 */
>  	boot_svsm_caa = (struct svsm_ca *)caa;
>  	boot_svsm_caa_pa = caa;
> +
> +	/* Advertise the SVSM presence via CPUID. */
> +	cpuid_table = (struct snp_cpuid_table *)snp_cpuid_get_table();
> +	for (i = 0; i < cpuid_table->count; i++) {
> +		struct snp_cpuid_fn *fn = &cpuid_table->fn[i];
> +
> +		if (fn->eax_in == 0x8000001f)
> +			fn->eax |= BIT(28);
> +	}
>  }
> diff --git a/arch/x86/kernel/sev.c b/arch/x86/kernel/sev.c
> index 3f4342b31736..69a756781d90 100644
> --- a/arch/x86/kernel/sev.c
> +++ b/arch/x86/kernel/sev.c
> @@ -1326,12 +1326,29 @@ int __init sev_es_efi_map_ghcbs(pgd_t *pgd)
>  	return 0;
>  }
>  
> +static enum es_result vc_handle_svsm_caa_msr(struct es_em_ctxt *ctxt)

No need for that helper. And you can reuse the exit_info_1 assignment.
Diff ontop:

diff --git a/arch/x86/kernel/sev.c b/arch/x86/kernel/sev.c
index 40eb547d0d6c..7a248d66227e 100644
--- a/arch/x86/kernel/sev.c
+++ b/arch/x86/kernel/sev.c
@@ -1326,31 +1326,25 @@ int __init sev_es_efi_map_ghcbs(pgd_t *pgd)
 	return 0;
 }
 
-static enum es_result vc_handle_svsm_caa_msr(struct es_em_ctxt *ctxt)
-{
-	struct pt_regs *regs = ctxt->regs;
-
-	/* Writes to the SVSM CAA msr are ignored */
-	if (ctxt->insn.opcode.bytes[1] == 0x30)
-		return ES_OK;
-
-	regs->ax = lower_32_bits(this_cpu_read(svsm_caa_pa));
-	regs->dx = upper_32_bits(this_cpu_read(svsm_caa_pa));
-
-	return ES_OK;
-}
-
 static enum es_result vc_handle_msr(struct ghcb *ghcb, struct es_em_ctxt *ctxt)
 {
 	struct pt_regs *regs = ctxt->regs;
 	enum es_result ret;
 	u64 exit_info_1;
 
-	if (regs->cx == MSR_SVSM_CAA)
-		return vc_handle_svsm_caa_msr(ctxt);
-
 	/* Is it a WRMSR? */
-	exit_info_1 = (ctxt->insn.opcode.bytes[1] == 0x30) ? 1 : 0;
+	exit_info_1 = !!(ctxt->insn.opcode.bytes[1] == 0x30);
+
+	if (regs->cx == MSR_SVSM_CAA) {
+		/* Writes to the SVSM CAA msr are ignored */
+		if (exit_info_1)
+			return ES_OK;
+
+		regs->ax = lower_32_bits(this_cpu_read(svsm_caa_pa));
+		regs->dx = upper_32_bits(this_cpu_read(svsm_caa_pa));
+
+		return ES_OK;
+	}
 
 	ghcb_set_rcx(ghcb, regs->cx);
 	if (exit_info_1) {

-- 
Regards/Gruss,
    Boris.

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