KVM: MMU: Decouple mmio from shadow page tables

Currently an mmio guest pte is encoded in the shadow pagetable as a
not-present trapping pte, with the SHADOW_IO_MARK bit set.  However
nothing is ever done with this information, so maintaining it is a
useless complication.

This patch moves the check for mmio to before shadow ptes are instantiated,
so the shadow code is never invoked for ptes that reference mmio.  The code
is simpler, and with future work, can be made to handle mmio concurrently.

Signed-off-by: Avi Kivity <avi@qumranet.com>

1 files changed, 8 insertions, 9 deletions

diff --git a/arch/x86/kvm/paging_tmpl.h b/arch/x86/kvm/paging_tmpl.h
index c2fd2b96144..4b55f462e2b 100644
--- a/arch/x86/kvm/paging_tmpl.h
+++ b/arch/x86/kvm/paging_tmpl.h
@@ -399,6 +399,14 @@ static int FNAME(page_fault)(struct kvm_vcpu *vcpu, gva_t addr,
 	page = gfn_to_page(vcpu->kvm, walker.gfn);
 	up_read(&current->mm->mmap_sem);
 
+	/* mmio */
+	if (is_error_page(page)) {
+		pgprintk("gfn %x is mmio\n", walker.gfn);
+		kvm_release_page_clean(page);
+		up_read(&vcpu->kvm->slots_lock);
+		return 1;
+	}
+
 	spin_lock(&vcpu->kvm->mmu_lock);
 	kvm_mmu_free_some_pages(vcpu);
 	shadow_pte = FNAME(fetch)(vcpu, addr, &walker, user_fault, write_fault,
@@ -409,15 +417,6 @@ static int FNAME(page_fault)(struct kvm_vcpu *vcpu, gva_t addr,
 	if (!write_pt)
 		vcpu->arch.last_pt_write_count = 0; /* reset fork detector */
 
-	/*
-	 * mmio: emulate if accessible, otherwise its a guest fault.
-	 */
-	if (shadow_pte && is_io_pte(*shadow_pte)) {
-		spin_unlock(&vcpu->kvm->mmu_lock);
-		up_read(&vcpu->kvm->slots_lock);
-		return 1;
-	}
-
 	++vcpu->stat.pf_fixed;
 	kvm_mmu_audit(vcpu, "post page fault (fixed)");
 	spin_unlock(&vcpu->kvm->mmu_lock);