[PATCH 2/4] KVM: x86/mmu: Fully re-evaluate MMIO caching when SPTE masks change

Sean Christopherson
2022-07-28T22:18:24Z

Fully re-evaluate whether or not MMIO caching can be enabled when SPTE
masks change; simply clearing enable_mmio_caching when a configuration
isn't compatible with caching fails to handle the scenario where the
masks are updated, e.g. by VMX for EPT or by SVM to account for the C-bit
location, and toggle compatibility from false=>true.

Snapshot the original module param so that re-evaluating MMIO caching
preserves userspace's desire to allow caching. Use a snapshot approach
so that enable_mmio_caching still reflects KVM's actual behavior.

Fixes: 8b9e74bfbf8c ("KVM: x86/mmu: Use enable_mmio_caching to track if MMIO caching is enabled")
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/mmu.c | 4 ++++
arch/x86/kvm/mmu/spte.c | 19 +++++++++++++++++++
arch/x86/kvm/mmu/spte.h | 1 +
3 files changed, 24 insertions(+)

diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
index 2975fcb14c86..660f58928252 100644
--- a/arch/x86/kvm/mmu/mmu.c
+++ b/arch/x86/kvm/mmu/mmu.c
@@ -6699,11 +6699,15 @@ static int set_nx_huge_pages(const char *val, const struct kernel_param *kp)
/*
* nx_huge_pages needs to be resolved to true/false when kvm.ko is loaded, as
* its default value of -1 is technically undefined behavior for a boolean.
+ * Forward the module init call to SPTE code so that it too can handle module
+ * params that need to be resolved/snapshot.
*/
void __init kvm_mmu_x86_module_init(void)
{
if (nx_huge_pages == -1)
__set_nx_huge_pages(get_nx_auto_mode());
+
+ kvm_mmu_spte_module_init();
}

/*
diff --git a/arch/x86/kvm/mmu/spte.c b/arch/x86/kvm/mmu/spte.c
index 7314d27d57a4..66f76f5a15bd 100644
--- a/arch/x86/kvm/mmu/spte.c
+++ b/arch/x86/kvm/mmu/spte.c
@@ -20,6 +20,7 @@
#include <asm/vmx.h>

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);

u64 __read_mostly shadow_host_writable_mask;
@@ -43,6 +44,18 @@ u64 __read_mostly shadow_nonpresent_or_rsvd_lower_gfn_mask;

u8 __read_mostly shadow_phys_bits;

+void __init kvm_mmu_spte_module_init(void)
+{
+ /*
+ * Snapshot userspace's desire to allow MMIO caching. Whether or not
+ * KVM can actually enable MMIO caching depends on vendor-specific
+ * hardware capabilities and other module params that can't be resolved
+ * until the vendor module is loaded, i.e. enable_mmio_caching can and
+ * will change when the vendor module is (re)loaded.
+ */
+ allow_mmio_caching = enable_mmio_caching;
+}
+
static u64 generation_mmio_spte_mask(u64 gen)
{
u64 mask;
@@ -340,6 +353,12 @@ void kvm_mmu_set_mmio_spte_mask(u64 mmio_value, u64 mmio_mask, u64 access_mask)
BUG_ON((u64)(unsigned)access_mask != access_mask);
WARN_ON(mmio_value & shadow_nonpresent_or_rsvd_lower_gfn_mask);

+ /*
+ * Reset to the original module param value to honor userspace's desire
+ * to (dis)allow MMIO caching. Update the param itself so that
+ * userspace can see whether or not KVM is actually using MMIO caching.
+ */
+ enable_mmio_caching = allow_mmio_caching;
if (!enable_mmio_caching)
mmio_value = 0;

diff --git a/arch/x86/kvm/mmu/spte.h b/arch/x86/kvm/mmu/spte.h
index cabe3fbb4f39..26b144ffd146 100644
--- a/arch/x86/kvm/mmu/spte.h
+++ b/arch/x86/kvm/mmu/spte.h
@@ -450,6 +450,7 @@ static inline u64 restore_acc_track_spte(u64 spte)

u64 kvm_mmu_changed_pte_notifier_make_spte(u64 old_spte, kvm_pfn_t new_pfn);

+void __init kvm_mmu_spte_module_init(void);
void kvm_mmu_reset_all_pte_masks(void);

#endif
--
2.37.1.455.g008518b4e5-goog