[RFC PATCH v1 2/6] x86/sev: Add support for NestedVirtSnpMsr

[Jeremi Piotrowski <jpiotrowski@xxxxxxxxxxxxxxxxxxx>]

The rmpupdate and psmash instructions, which are used in AMD's SEV-SNP
to update the RMP (Reverse Map) table, can't be trapped. For nested
scenarios, AMD defined MSR versions of these instructions which can be
emulated by the top-level hypervisor. One instance where these MSRs are
used are Hyper-V VMs which expose SNP isolation features to the guest.

The MSRs are defined in "AMD64 Architecture Programmer’s Manual, Volume 2:
System Programming", section 15.36.19.

Signed-off-by: Jeremi Piotrowski <jpiotrowski@xxxxxxxxxxxxxxxxxxx>
---
 arch/x86/include/asm/cpufeatures.h |  1 +
 arch/x86/include/asm/msr-index.h   |  2 +
 arch/x86/kernel/sev.c              | 62 +++++++++++++++++++++++++-----
 3 files changed, 55 insertions(+), 10 deletions(-)

diff --git a/arch/x86/include/asm/cpufeatures.h b/arch/x86/include/asm/cpufeatures.h
index 480b4eaef310..e6e2e824f67b 100644
--- a/arch/x86/include/asm/cpufeatures.h
+++ b/arch/x86/include/asm/cpufeatures.h
@@ -423,6 +423,7 @@
 #define X86_FEATURE_SEV_SNP		(19*32+ 4) /* AMD Secure Encrypted Virtualization - Secure Nested Paging */
 #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_NESTED_VIRT_SNP_MSR	(19*32+29) /* Virtualizable RMPUPDATE and PSMASH MSR available */
 
 /*
  * BUG word(s)
diff --git a/arch/x86/include/asm/msr-index.h b/arch/x86/include/asm/msr-index.h
index 35100c630617..d6103e607896 100644
--- a/arch/x86/include/asm/msr-index.h
+++ b/arch/x86/include/asm/msr-index.h
@@ -567,6 +567,8 @@
 #define MSR_AMD64_SEV_SNP_ENABLED	BIT_ULL(MSR_AMD64_SEV_SNP_ENABLED_BIT)
 #define MSR_AMD64_RMP_BASE		0xc0010132
 #define MSR_AMD64_RMP_END		0xc0010133
+#define MSR_AMD64_VIRT_RMPUPDATE	0xc001f001
+#define MSR_AMD64_VIRT_PSMASH		0xc001f002
 
 #define MSR_AMD64_VIRT_SPEC_CTRL	0xc001011f
 
diff --git a/arch/x86/kernel/sev.c b/arch/x86/kernel/sev.c
index 7fa39dc17edd..95404c7e5150 100644
--- a/arch/x86/kernel/sev.c
+++ b/arch/x86/kernel/sev.c
@@ -2566,6 +2566,24 @@ int snp_lookup_rmpentry(u64 pfn, int *level)
 }
 EXPORT_SYMBOL_GPL(snp_lookup_rmpentry);
 
+static bool virt_snp_msr(void)
+{
+	return boot_cpu_has(X86_FEATURE_NESTED_VIRT_SNP_MSR);
+}
+
+static u64 virt_psmash(u64 paddr)
+{
+	int ret;
+
+	asm volatile(
+		"wrmsr\n\t"
+		: "=a"(ret)
+		: "a"(paddr), "c"(MSR_AMD64_VIRT_PSMASH)
+		: "memory", "cc"
+	);
+	return ret;
+}
+
 /*
  * psmash is used to smash a 2MB aligned page into 4K
  * pages while preserving the Validated bit in the RMP.
@@ -2581,11 +2599,15 @@ int psmash(u64 pfn)
 	if (!cpu_feature_enabled(X86_FEATURE_SEV_SNP))
 		return -ENXIO;
 
-	/* Binutils version 2.36 supports the PSMASH mnemonic. */
-	asm volatile(".byte 0xF3, 0x0F, 0x01, 0xFF"
-		      : "=a"(ret)
-		      : "a"(paddr)
-		      : "memory", "cc");
+	if (virt_snp_msr()) {
+		ret = virt_psmash(paddr);
+	} else {
+		/* Binutils version 2.36 supports the PSMASH mnemonic. */
+		asm volatile(".byte 0xF3, 0x0F, 0x01, 0xFF"
+			      : "=a"(ret)
+			      : "a"(paddr)
+			      : "memory", "cc");
+	}
 
 	return ret;
 }
@@ -2601,6 +2623,21 @@ static int invalidate_direct_map(unsigned long pfn, int npages)
 	return set_memory_np((unsigned long)pfn_to_kaddr(pfn), npages);
 }
 
+static u64 virt_rmpupdate(unsigned long paddr, struct rmp_state *val)
+{
+	int ret;
+	register u64 hi asm("r8") = ((u64 *)val)[1];
+	register u64 lo asm("rdx") = ((u64 *)val)[0];
+
+	asm volatile(
+		"wrmsr\n\t"
+		: "=a"(ret)
+		: "a"(paddr), "c"(MSR_AMD64_VIRT_RMPUPDATE), "r"(lo), "r"(hi)
+		: "memory", "cc"
+	);
+	return ret;
+}
+
 static int rmpupdate(u64 pfn, struct rmp_state *val)
 {
 	unsigned long paddr = pfn << PAGE_SHIFT;
@@ -2626,11 +2663,16 @@ static int rmpupdate(u64 pfn, struct rmp_state *val)
 	}
 
 retry:
-	/* Binutils version 2.36 supports the RMPUPDATE mnemonic. */
-	asm volatile(".byte 0xF2, 0x0F, 0x01, 0xFE"
-		     : "=a"(ret)
-		     : "a"(paddr), "c"((unsigned long)val)
-		     : "memory", "cc");
+
+	if (virt_snp_msr()) {
+		ret = virt_rmpupdate(paddr, val);
+	} else {
+		/* Binutils version 2.36 supports the RMPUPDATE mnemonic. */
+		asm volatile(".byte 0xF2, 0x0F, 0x01, 0xFE"
+			     : "=a"(ret)
+			     : "a"(paddr), "c"((unsigned long)val)
+			     : "memory", "cc");
+	}
 
 	if (ret) {
 		if (!retries) {
-- 
2.25.1