[PATCH v3] KVM: SVM: Override default MMIO mask if memory encryption is enabled

[Tom Lendacky]

The KVM MMIO support uses bit 51 as the reserved bit to cause nested page
faults when a guest performs MMIO. The AMD memory encryption support uses
a CPUID function to define the encryption bit position. Given this, it is
possible that these bits can conflict.

Use svm_hardware_setup() to override the MMIO mask if memory encryption
support is enabled. Various checks are performed to ensure that the mask
is properly defined and rsvd_bits() is used to generate the new mask (as
was done prior to the change that necessitated this patch).

Fixes: 28a1f3ac1d0c ("kvm: x86: Set highest physical address bits in non-present/reserved SPTEs")
Suggested-by: Sean Christopherson <sean.j.christopherson@xxxxxxxxx>
Signed-off-by: Tom Lendacky <thomas.lendacky@xxxxxxx>

---

Changes in v3:
- Add additional checks to ensure there are no conflicts between the
  encryption bit position and physical address setting.
- Use rsvd_bits() generated mask (as was previously used) instead of
  setting a single bit.

Changes in v2:
- Use of svm_hardware_setup() to override MMIO mask rather than adding an
  override callback routine.
---
 arch/x86/kvm/svm.c | 51 ++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 51 insertions(+)

diff --git a/arch/x86/kvm/svm.c b/arch/x86/kvm/svm.c
index 122d4ce3b1ab..9d6bd3fc12c8 100644
--- a/arch/x86/kvm/svm.c
+++ b/arch/x86/kvm/svm.c
@@ -1307,6 +1307,55 @@ static void shrink_ple_window(struct kvm_vcpu *vcpu)
 	}
 }
 
+/*
+ * The default MMIO mask is a single bit (excluding the present bit),
+ * which could conflict with the memory encryption bit. Check for
+ * memory encryption support and override the default MMIO masks if
+ * it is enabled.
+ */
+static __init void svm_adjust_mmio_mask(void)
+{
+	unsigned int enc_bit, mask_bit;
+	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 */
+	rdmsrl(MSR_K8_SYSCFG, msr);
+	if (!(msr & MSR_K8_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 */
+	if (enc_bit == mask_bit)
+		mask_bit++;
+
+	if (mask_bit > 51) {
+		/*
+		 * The mask bit is above 51, so use bit 51 without the present
+		 * bit.
+		 */
+		mask = BIT_ULL(51);
+	} else {
+		/*
+		 * Some bits above the physical addressing limit will always
+		 * be reserved, so use the rsvd_bits() function to generate
+		 * the mask. This mask, along with the present bit, will be
+		 * used to generate a page fault with PFER.RSV = 1.
+		 */
+		mask = rsvd_bits(mask_bit, 51);
+		mask |= BIT_ULL(0);
+	}
+
+	kvm_mmu_set_mmio_spte_mask(mask, mask,
+				   PT_WRITABLE_MASK |
+				   PT_USER_MASK);
+}
+
 static __init int svm_hardware_setup(void)
 {
 	int cpu;
@@ -1361,6 +1410,8 @@ static __init int svm_hardware_setup(void)
 		}
 	}
 
+	svm_adjust_mmio_mask();
+
 	for_each_possible_cpu(cpu) {
 		r = svm_cpu_init(cpu);
 		if (r)
-- 
2.17.1