From: Sean Christopherson Date: Fri, 23 May 2025 01:17:52 +0000 (-0700) Subject: KVM: x86: Avoid calling kvm_is_mmio_pfn() when kvm_x86_ops.get_mt_mask is NULL X-Git-Url: http://git.ipfire.org/?a=commitdiff_plain;h=c126b46e6fa87eb27e08e2120a732ec988f20eb2;p=thirdparty%2Fkernel%2Flinux.git KVM: x86: Avoid calling kvm_is_mmio_pfn() when kvm_x86_ops.get_mt_mask is NULL Guard the call to kvm_x86_call(get_mt_mask) with an explicit check on kvm_x86_ops.get_mt_mask so as to avoid unnecessarily calling kvm_is_mmio_pfn(), which is moderately expensive for some backing types. E.g. lookup_memtype() conditionally takes a system-wide spinlock if KVM ends up being call pat_pfn_immune_to_uc_mtrr(), e.g. for DAX memory. While the call to kvm_x86_ops.get_mt_mask() itself is elided, the compiler still needs to compute all parameters, as it can't know at build time that the call will be squashed. <+243>: call 0xffffffff812ad880 <+248>: mov %r13,%rsi <+251>: mov %rbx,%rdi <+254>: movzbl %al,%edx <+257>: call 0xffffffff81c26af0 <__SCT__kvm_x86_get_mt_mask> Fixes: 3fee4837ef40 ("KVM: x86: remove shadow_memtype_mask") Tested-by: Pawan Gupta Link: https://lore.kernel.org/r/20250523011756.3243624-2-seanjc@google.com Signed-off-by: Sean Christopherson --- diff --git a/arch/x86/kvm/mmu/spte.c b/arch/x86/kvm/mmu/spte.c index cfce03d8f1234..f262c380f40ef 100644 --- a/arch/x86/kvm/mmu/spte.c +++ b/arch/x86/kvm/mmu/spte.c @@ -209,7 +209,9 @@ bool make_spte(struct kvm_vcpu *vcpu, struct kvm_mmu_page *sp, if (level > PG_LEVEL_4K) spte |= PT_PAGE_SIZE_MASK; - spte |= kvm_x86_call(get_mt_mask)(vcpu, gfn, kvm_is_mmio_pfn(pfn)); + if (kvm_x86_ops.get_mt_mask) + spte |= kvm_x86_call(get_mt_mask)(vcpu, gfn, kvm_is_mmio_pfn(pfn)); + if (host_writable) spte |= shadow_host_writable_mask; else