KVM: SVM: Disable SEV-ES support if MMIO caching is disable

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.

Adjust KVM's MMIO masks to account for the C-bit location prior to doing
SEV(-ES) setup, and document that dependency between adjusting the MMIO
SPTE mask and SEV(-ES) setup.

Fixes: b09763da4dd8 ("KVM: x86/mmu: Add module param to disable MMIO caching (for testing)")
Reported-by: Michael Roth <michael.roth@amd.com>
Tested-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>
Message-Id: <20220803224957.1285926-4-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

1 files changed, 10 insertions, 0 deletions

diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
index b0e793e7d85c..28064060413a 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"
@@ -2221,6 +2222,15 @@ 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;