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[02/54] KVM: x86/mmu: Treat NX as used (not reserved) for all !TDP shadow MMUs

Message ID 20210622175739.3610207-3-seanjc@google.com (mailing list archive)
State New, archived
Headers show
Series KVM: x86/mmu: Bug fixes and summer cleaning | expand

Commit Message

Sean Christopherson June 22, 2021, 5:56 p.m. UTC
Mark NX as being used for all non-nested shadow MMUs, as KVM will set the
NX bit for huge SPTEs if the iTLB mutli-hit mitigation is enabled.
Checking the mitigation itself is not sufficient as it can be toggled on
at any time and KVM doesn't reset MMU contexts when that happens.  KVM
could reset the contexts, but that would require purging all SPTEs in all
MMUs, for no real benefit.  And, KVM already forces EFER.NX=1 when TDP is
disabled (for WP=0, SMEP=1, NX=0), so technically NX is never reserved
for shadow MMUs.

Fixes: b8e8c8303ff2 ("kvm: mmu: ITLB_MULTIHIT mitigation")
Cc: stable@vger.kernel.org
Signed-off-by: Sean Christopherson <seanjc@google.com>
 arch/x86/kvm/mmu/mmu.c | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)
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diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
index 84d48a33e38b..0db12f461c9d 100644
--- a/arch/x86/kvm/mmu/mmu.c
+++ b/arch/x86/kvm/mmu/mmu.c
@@ -4221,7 +4221,15 @@  static inline u64 reserved_hpa_bits(void)
 reset_shadow_zero_bits_mask(struct kvm_vcpu *vcpu, struct kvm_mmu *context)
-	bool uses_nx = context->nx ||
+	/*
+	 * KVM uses NX when TDP is disabled to handle a variety of scenarios,
+	 * notably for huge SPTEs if iTLB multi-hit mitigation is enabled and
+	 * to generate correct permissions for CR0.WP=0/CR4.SMEP=1/EFER.NX=0.
+	 * The iTLB multi-hit workaround can be toggled at any time, so assume
+	 * NX can be used by any non-nested shadow MMU to avoid having to reset
+	 * MMU contexts.  Note, KVM forces EFER.NX=1 when TDP is disabled.
+	 */
+	bool uses_nx = context->nx || !tdp_enabled ||
 	struct rsvd_bits_validate *shadow_zero_check;
 	int i;