[PATCH 4/4] KVM: SVM: Disable SEV-ES support if MMIO caching is disable

[Sean Christopherson <seanjc@google.com>]

Disable SEV-ES if MMIO caching is disabled as SEV-ES relies on MMIO SPTEs
generating #NPF(RSVD), which are reflected by the CPU into the guest as
a #VC.  With SEV-ES, the untrusted host, a.k.a. KVM, doesn't have access
to the guest instruction stream or register state and so can't directly
emulate in response to a #NPF on an emulated MMIO GPA.  Disabling MMIO
caching means guest accesses to emulated MMIO ranges cause #NPF(!PRESENT),
and those flavors of #NPF cause automatic VM-Exits, not #VC.

Fixes: b09763da4dd8 ("KVM: x86/mmu: Add module param to disable MMIO caching (for testing)")
Reported-by: Michael Roth <michael.roth@amd.com>
Cc: Tom Lendacky <thomas.lendacky@amd.com>
Cc: stable@vger.kernel.org
Signed-off-by: Sean Christopherson <seanjc@google.com>
---
 arch/x86/kvm/mmu.h      |  2 ++
 arch/x86/kvm/mmu/spte.c |  1 +
 arch/x86/kvm/mmu/spte.h |  2 --
 arch/x86/kvm/svm/sev.c  | 10 ++++++++++
 4 files changed, 13 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kvm/mmu.h b/arch/x86/kvm/mmu.h
index a99acec925eb..6bdaacb6faa0 100644
--- a/arch/x86/kvm/mmu.h
+++ b/arch/x86/kvm/mmu.h
@@ -6,6 +6,8 @@
 #include "kvm_cache_regs.h"
 #include "cpuid.h"
 
+extern bool __read_mostly enable_mmio_caching;
+
 #define PT_WRITABLE_SHIFT 1
 #define PT_USER_SHIFT 2
 
diff --git a/arch/x86/kvm/mmu/spte.c b/arch/x86/kvm/mmu/spte.c
index 66f76f5a15bd..03ca740bf721 100644
--- a/arch/x86/kvm/mmu/spte.c
+++ b/arch/x86/kvm/mmu/spte.c
@@ -22,6 +22,7 @@
 bool __read_mostly enable_mmio_caching = true;
 static bool __ro_after_init allow_mmio_caching;
 module_param_named(mmio_caching, enable_mmio_caching, bool, 0444);
+EXPORT_SYMBOL_GPL(enable_mmio_caching);
 
 u64 __read_mostly shadow_host_writable_mask;
 u64 __read_mostly shadow_mmu_writable_mask;
diff --git a/arch/x86/kvm/mmu/spte.h b/arch/x86/kvm/mmu/spte.h
index 26b144ffd146..9a9414b8d1d6 100644
--- a/arch/x86/kvm/mmu/spte.h
+++ b/arch/x86/kvm/mmu/spte.h
@@ -5,8 +5,6 @@
 
 #include "mmu_internal.h"
 
-extern bool __read_mostly enable_mmio_caching;
-
 /*
  * A MMU present SPTE is backed by actual memory and may or may not be present
  * in hardware.  E.g. MMIO SPTEs are not considered present.  Use bit 11, as it
diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
index 309bcdb2f929..05bf6301acac 100644
--- a/arch/x86/kvm/svm/sev.c
+++ b/arch/x86/kvm/svm/sev.c
@@ -22,6 +22,7 @@
 #include <asm/trapnr.h>
 #include <asm/fpu/xcr.h>
 
+#include "mmu.h"
 #include "x86.h"
 #include "svm.h"
 #include "svm_ops.h"
@@ -2205,6 +2206,15 @@ void __init sev_hardware_setup(void)
 	if (!sev_es_enabled)
 		goto out;
 
+	/*
+	 * SEV-ES requires MMIO caching as KVM doesn't have access to the guest
+	 * instruction stream, i.e. can't emulate in response to a #NPF and
+	 * instead relies on #NPF(RSVD) being reflected into the guest as #VC
+	 * (the guest can then do a #VMGEXIT to request MMIO emulation).
+	 */
+	if (!enable_mmio_caching)
+		goto out;
+
 	/* Does the CPU support SEV-ES? */
 	if (!boot_cpu_has(X86_FEATURE_SEV_ES))
 		goto out;
-- 
2.37.1.455.g008518b4e5-goog