[PATCH v14 039/113] KVM: x86/mmu: Assume guest MMIOs are shared

From: isaku . yamahata
Date: Mon May 29 2023 - 00:25:17 EST

From: Chao Gao <chao.gao@xxxxxxxxx>

Guest TD doesn't necessarily invoke MAP_GPA to convert the virtual MMIO
range to shared before accessing it. When TD tries to access the virtual
device's MMIO as shared, an EPT violation is raised first.
kvm_mem_is_private() checks whether the GFN is shared or private. If
MAP_GPA is not called for the GPA, KVM thinks the GPA is private and
refuses shared access, and doesn't set up shared EPT entry. The guest
can't make progress.

Instead of requiring the guest to invoke MAP_GPA for regions of virtual
MMIOs assume regions of virtual MMIOs are shared in KVM as well (i.e., GPAs
either have no kvm_memory_slot or are backed by host MMIOs). So that guests
can access those MMIO regions.

Signed-off-by: Chao Gao <chao.gao@xxxxxxxxx>
arch/x86/kvm/mmu/mmu.c | 7 ++++++-
1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
index 544df394b085..a7bf0cecabc2 100644
--- a/arch/x86/kvm/mmu/mmu.c
+++ b/arch/x86/kvm/mmu/mmu.c
@@ -4395,7 +4395,12 @@ static int __kvm_faultin_pfn(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault

- if (fault->is_private != kvm_mem_is_private(vcpu->kvm, fault->gfn))
+ /*
+ * !fault->slot means MMIO. Don't require explicit GPA conversion for
+ * MMIO because MMIO is assigned at the boot time.
+ */
+ if (fault->slot &&
+ fault->is_private != kvm_mem_is_private(vcpu->kvm, fault->gfn))
return kvm_do_memory_fault_exit(vcpu, fault);

if (fault->is_private)