[PATCH 1/2] Revert "KVM: x86: Truncate reported guest MAXPHYADDR to C-bit if SEV is supported"

[Sean Christopherson]

It turns out that non-SEV guest can indeed use bit 47, and the unexpected
reserved #PF observed when setting bit 47 is due to a magic 12gb
HyperTransport region being off limits, even for GPAs.  Per Tom:

  I think you may be hitting a special HT region that is at the top 12GB
  of the 48-bit memory range and is reserved, even for GPAs.  Can you
  somehow get the test to use an address below 0xfffd_0000_0000? That
  would show that bit 47 is valid for the legacy guest while staying out
  of the HT region.

And indeed, accessing 0xfffd00000000 generates a reserved #PF, while
dropping down a single page to 0xfffcfffff000 does not.

This reverts commit 3675f005c87c4026713c9f863924de511fdd36c4.

Cc: Peter Gonda <pgonda@xxxxxxxxxx>
Cc: Brijesh Singh <brijesh.singh@xxxxxxx>
Cc: Tom Lendacky <thomas.lendacky@xxxxxxx>
Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
---
 arch/x86/kvm/cpuid.c   | 11 -----------
 arch/x86/kvm/svm/svm.c | 37 ++++++++-----------------------------
 arch/x86/kvm/x86.c     |  3 ---
 arch/x86/kvm/x86.h     |  1 -
 4 files changed, 8 insertions(+), 44 deletions(-)

diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
index 0edda1fc4fe7..ca7866d63e98 100644
--- a/arch/x86/kvm/cpuid.c
+++ b/arch/x86/kvm/cpuid.c
@@ -955,17 +955,6 @@ static inline int __do_cpuid_func(struct kvm_cpuid_array *array, u32 function)
 		else if (!g_phys_as)
 			g_phys_as = phys_as;
 
-		/*
-		 * The exception to the exception is if hardware supports SEV,
-		 * in which case the C-bit is reserved for non-SEV guests and
-		 * isn't a GPA bit for SEV guests.
-		 *
-		 * Note, KVM always reports '0' for the number of reduced PA
-		 * bits (see 0x8000001F).
-		 */
-		if (tdp_enabled && sev_c_bit)
-			g_phys_as = min(g_phys_as, (unsigned int)sev_c_bit);
-
 		entry->eax = g_phys_as | (virt_as << 8);
 		entry->edx = 0;
 		cpuid_entry_override(entry, CPUID_8000_0008_EBX);
diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
index 616b9679ddcc..8834822c00cd 100644
--- a/arch/x86/kvm/svm/svm.c
+++ b/arch/x86/kvm/svm/svm.c
@@ -860,26 +860,6 @@ static void shrink_ple_window(struct kvm_vcpu *vcpu)
 	}
 }
 
-static __init u8 svm_get_c_bit(bool sev_only)
-{
-	unsigned int eax, ebx, ecx, edx;
-	u64 msr;
-
-	if (cpuid_eax(0x80000000) < 0x8000001f)
-		return 0;
-
-	if (rdmsrl_safe(MSR_AMD64_SYSCFG, &msr) ||
-	    !(msr & MSR_AMD64_SYSCFG_MEM_ENCRYPT))
-		return 0;
-
-	cpuid(0x8000001f, &eax, &ebx, &ecx, &edx);
-
-	if (sev_only && !(eax & feature_bit(SEV)))
-		return 0;
-
-	return ebx & 0x3f;
-}
-
 /*
  * The default MMIO mask is a single bit (excluding the present bit),
  * which could conflict with the memory encryption bit. Check for
@@ -889,13 +869,18 @@ static __init u8 svm_get_c_bit(bool sev_only)
 static __init void svm_adjust_mmio_mask(void)
 {
 	unsigned int enc_bit, mask_bit;
-	u64 mask;
+	u64 msr, mask;
+
+	/* If there is no memory encryption support, use existing mask */
+	if (cpuid_eax(0x80000000) < 0x8000001f)
+		return;
 
 	/* If memory encryption is not enabled, use existing mask */
-	enc_bit = svm_get_c_bit(false);
-	if (!enc_bit)
+	rdmsrl(MSR_AMD64_SYSCFG, msr);
+	if (!(msr & MSR_AMD64_SYSCFG_MEM_ENCRYPT))
 		return;
 
+	enc_bit = cpuid_ebx(0x8000001f) & 0x3f;
 	mask_bit = boot_cpu_data.x86_phys_bits;
 
 	/* Increment the mask bit if it is the same as the encryption bit */
@@ -1028,12 +1013,6 @@ static __init int svm_hardware_setup(void)
 	kvm_configure_mmu(npt_enabled, get_max_npt_level(), PG_LEVEL_1G);
 	pr_info("kvm: Nested Paging %sabled\n", npt_enabled ? "en" : "dis");
 
-	/*
-	 * The SEV C-bit location is needed to correctly enumeration guest
-	 * MAXPHYADDR even if SEV is not fully supported.
-	 */
-	sev_c_bit = svm_get_c_bit(true);
-
 	/* Note, SEV setup consumes npt_enabled. */
 	sev_hardware_setup();
 
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 4a597aafe637..13905ef5bb48 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -209,9 +209,6 @@ static struct kvm_user_return_msrs __percpu *user_return_msrs;
 				| XFEATURE_MASK_BNDCSR | XFEATURE_MASK_AVX512 \
 				| XFEATURE_MASK_PKRU)
 
-u8 __read_mostly sev_c_bit;
-EXPORT_SYMBOL_GPL(sev_c_bit);
-
 u64 __read_mostly host_efer;
 EXPORT_SYMBOL_GPL(host_efer);
 
diff --git a/arch/x86/kvm/x86.h b/arch/x86/kvm/x86.h
index bc3f5c9e3708..44ae10312740 100644
--- a/arch/x86/kvm/x86.h
+++ b/arch/x86/kvm/x86.h
@@ -328,7 +328,6 @@ extern u64 host_xcr0;
 extern u64 supported_xcr0;
 extern u64 host_xss;
 extern u64 supported_xss;
-extern u8  sev_c_bit;
 
 static inline bool kvm_mpx_supported(void)
 {
-- 
2.32.0.93.g670b81a890-goog