linux-kernel - [PATCH 5.19 0041/1157] KVM: x86/mmu: Fully re-evaluate MMIO caching when SPTE masks change

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Date:   Mon, 15 Aug 2022 19:49:57 +0200
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org, Michael Roth <michael.roth@....com>,
        Tom Lendacky <thomas.lendacky@....com>,
        Sean Christopherson <seanjc@...gle.com>,
        Kai Huang <kai.huang@...el.com>,
        Paolo Bonzini <pbonzini@...hat.com>
Subject: [PATCH 5.19 0041/1157] KVM: x86/mmu: Fully re-evaluate MMIO caching when SPTE masks change

From: Sean Christopherson <seanjc@...gle.com>

commit c3e0c8c2e8b17bae30d5978bc2decdd4098f0f99 upstream.

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@....com>
Cc: Tom Lendacky <thomas.lendacky@....com>
Cc: stable@...r.kernel.org
Tested-by: Michael Roth <michael.roth@....com>
Signed-off-by: Sean Christopherson <seanjc@...gle.com>
Reviewed-by: Kai Huang <kai.huang@...el.com>
Message-Id: <20220803224957.1285926-3-seanjc@...gle.com>
Signed-off-by: Paolo Bonzini <pbonzini@...hat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
---
 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(+)

--- a/arch/x86/kvm/mmu/mmu.c
+++ b/arch/x86/kvm/mmu/mmu.c
@@ -6274,11 +6274,15 @@ static int set_nx_huge_pages(const char
 /*
  * 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();
 }
 
 /*
--- 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);
 EXPORT_SYMBOL_GPL(enable_mmio_caching);
 
@@ -43,6 +44,18 @@ u64 __read_mostly shadow_nonpresent_or_r
 
 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;
@@ -338,6 +351,12 @@ void kvm_mmu_set_mmio_spte_mask(u64 mmio
 	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;
 
--- a/arch/x86/kvm/mmu/spte.h
+++ b/arch/x86/kvm/mmu/spte.h
@@ -444,6 +444,7 @@ static inline u64 restore_acc_track_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