KVM: introduce readonly memslot

In current code, if we map a readonly memory space from host to guest
and the page is not currently mapped in the host, we will get a fault
pfn and async is not allowed, then the vm will crash

We introduce readonly memory region to map ROM/ROMD to the guest, read access
is happy for readonly memslot, write access on readonly memslot will cause
KVM_EXIT_MMIO exit

Signed-off-by: Xiao Guangrong <xiaoguangrong@linux.vnet.ibm.com>
Signed-off-by: Avi Kivity <avi@redhat.com>

3 files changed, 11 insertions, 0 deletions

diff --git a/arch/x86/include/asm/kvm.h b/arch/x86/include/asm/kvm.h
index 246617efd67..521bf252e34 100644
--- a/arch/x86/include/asm/kvm.h
+++ b/arch/x86/include/asm/kvm.h
@@ -25,6 +25,7 @@
 #define __KVM_HAVE_DEBUGREGS
 #define __KVM_HAVE_XSAVE
 #define __KVM_HAVE_XCRS
+#define __KVM_HAVE_READONLY_MEM
 
 /* Architectural interrupt line count. */
 #define KVM_NR_INTERRUPTS 256
diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
index 5548971ae80..8e312a2e141 100644
--- a/arch/x86/kvm/mmu.c
+++ b/arch/x86/kvm/mmu.c
@@ -2647,6 +2647,15 @@ static void kvm_send_hwpoison_signal(unsigned long address, struct task_struct *
 
 static int kvm_handle_bad_page(struct kvm_vcpu *vcpu, gfn_t gfn, pfn_t pfn)
 {
+        /*
+         * Do not cache the mmio info caused by writing the readonly gfn
+         * into the spte otherwise read access on readonly gfn also can
+         * caused mmio page fault and treat it as mmio access.
+         * Return 1 to tell kvm to emulate it.
+         */
+        if (pfn == KVM_PFN_ERR_RO_FAULT)
+                return 1;
+
         if (pfn == KVM_PFN_ERR_HWPOISON) {
                 kvm_send_hwpoison_signal(gfn_to_hva(vcpu->kvm, gfn), current);
                 return 0;
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 704680d0fa3..42bbf4187d2 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -2175,6 +2175,7 @@ int kvm_dev_ioctl_check_extension(long ext)
         case KVM_CAP_GET_TSC_KHZ:
         case KVM_CAP_PCI_2_3:
         case KVM_CAP_KVMCLOCK_CTRL:
+        case KVM_CAP_READONLY_MEM:
                 r = 1;
                 break;
         case KVM_CAP_COALESCED_MMIO: