linux-kernel - [PATCH v2] KVM: x86: use CPUID to locate host page table reserved bits

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Message-Id: <1575474037-7903-1-git-send-email-pbonzini@redhat.com>
Date:   Wed,  4 Dec 2019 16:40:37 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Cc:     thomas.lendacky@....com, sean.j.christopherson@...el.com,
        stable@...r.kernel.org
Subject: [PATCH v2] KVM: x86: use CPUID to locate host page table reserved bits

The comment in kvm_get_shadow_phys_bits refers to MKTME, but the same is actually
true of SME and SEV.  Just use CPUID[0x8000_0008].EAX[7:0] unconditionally if
available, it is simplest and works even if memory is not encrypted.

Cc: stable@...r.kernel.org
Reported-by: Tom Lendacky <thomas.lendacky@....com>
Signed-off-by: Paolo Bonzini <pbonzini@...hat.com>
---
 arch/x86/kvm/mmu/mmu.c | 20 ++++++++++++--------
 1 file changed, 12 insertions(+), 8 deletions(-)

diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
index 6f92b40d798c..1e4ee4f8de5f 100644
--- a/arch/x86/kvm/mmu/mmu.c
+++ b/arch/x86/kvm/mmu/mmu.c
@@ -538,16 +538,20 @@ void kvm_mmu_set_mask_ptes(u64 user_mask, u64 accessed_mask,
 static u8 kvm_get_shadow_phys_bits(void)
 {
 	/*
-	 * boot_cpu_data.x86_phys_bits is reduced when MKTME is detected
-	 * in CPU detection code, but MKTME treats those reduced bits as
-	 * 'keyID' thus they are not reserved bits. Therefore for MKTME
-	 * we should still return physical address bits reported by CPUID.
+	 * boot_cpu_data.x86_phys_bits is reduced when MKTME or SME are detected
+	 * in CPU detection code, but the processor treats those reduced bits as
+	 * 'keyID' thus they are not reserved bits. Therefore KVM needs to look at
+	 * the physical address bits reported by CPUID.
 	 */
-	if (!boot_cpu_has(X86_FEATURE_TME) ||
-	    WARN_ON_ONCE(boot_cpu_data.extended_cpuid_level < 0x80000008))
-		return boot_cpu_data.x86_phys_bits;
+	if (likely(boot_cpu_data.extended_cpuid_level >= 0x80000008))
+		return cpuid_eax(0x80000008) & 0xff;
 
-	return cpuid_eax(0x80000008) & 0xff;
+	/*
+	 * Quite weird to have VMX or SVM but not MAXPHYADDR; probably a VM with
+	 * custom CPUID.  Proceed with whatever the kernel found since these features
+	 * aren't virtualizable (SME/SEV also require CPUIDs higher than 0x80000008).
+	 */
+	return boot_cpu_data.x86_phys_bits;
 }
 
 static void kvm_mmu_reset_all_pte_masks(void)
-- 
1.8.3.1