[PATCH v2 3/3] 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.

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>
---
 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 ++++++++++
 arch/x86/kvm/svm/svm.c  |  9 ++++++---
 5 files changed, 19 insertions(+), 5 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 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;
diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
index 38f873cb6f2c..f3813dbacb9f 100644
--- a/arch/x86/kvm/svm/svm.c
+++ b/arch/x86/kvm/svm/svm.c
@@ -5034,13 +5034,16 @@ static __init int svm_hardware_setup(void)
 	/* Setup shadow_me_value and shadow_me_mask */
 	kvm_mmu_set_me_spte_mask(sme_me_mask, sme_me_mask);
 
-	/* Note, SEV setup consumes npt_enabled. */
+	svm_adjust_mmio_mask();
+
+	/*
+	 * Note, SEV setup consumes npt_enabled and enable_mmio_caching (which
+	 * may be modified by svm_adjust_mmio_mask()).
+	 */
 	sev_hardware_setup();
 
 	svm_hv_hardware_setup();
 
-	svm_adjust_mmio_mask();
-
 	for_each_possible_cpu(cpu) {
 		r = svm_cpu_init(cpu);
 		if (r)
-- 
2.37.1.559.g78731f0fdb-goog