2 files changed, 37 insertions, 1 deletions

diff --git a/Documentation/kvm/cpuid.txt b/Documentation/kvm/cpuid.txt
index 14a12ea92b7..882068538c9 100644
--- a/Documentation/kvm/cpuid.txt
+++ b/Documentation/kvm/cpuid.txt
@@ -36,6 +36,9 @@ KVM_FEATURE_MMU_OP                 ||     2 || deprecated.
 KVM_FEATURE_CLOCKSOURCE2           ||     3 || kvmclock available at msrs
                                    ||       || 0x4b564d00 and 0x4b564d01
 ------------------------------------------------------------------------------
+KVM_FEATURE_ASYNC_PF               ||     4 || async pf can be enabled by
+                                   ||       || writing to msr 0x4b564d02
+------------------------------------------------------------------------------
 KVM_FEATURE_CLOCKSOURCE_STABLE_BIT ||    24 || host will warn if no guest-side
                                    ||       || per-cpu warps are expected in
                                    ||       || kvmclock.
diff --git a/Documentation/kvm/msr.txt b/Documentation/kvm/msr.txt
index 8ddcfe84c09..e67b4a8783d 100644
--- a/Documentation/kvm/msr.txt
+++ b/Documentation/kvm/msr.txt
@@ -3,7 +3,6 @@ Glauber Costa <glommer@redhat.com>, Red Hat Inc, 2010
 =====================================================
 
 KVM makes use of some custom MSRs to service some requests.
-At present, this facility is only used by kvmclock.
 
 Custom MSRs have a range reserved for them, that goes from
 0x4b564d00 to 0x4b564dff. There are MSRs outside this area,
@@ -151,3 +150,37 @@ MSR_KVM_SYSTEM_TIME: 0x12
                         return PRESENT;
                 } else
                         return NON_PRESENT;
+
+MSR_KVM_ASYNC_PF_EN: 0x4b564d02
+        data: Bits 63-6 hold 64-byte aligned physical address of a
+        64 byte memory area which must be in guest RAM and must be
+        zeroed. Bits 5-1 are reserved and should be zero. Bit 0 is 1
+        when asynchronous page faults are enabled on the vcpu 0 when
+        disabled.
+
+        First 4 byte of 64 byte memory location will be written to by
+        the hypervisor at the time of asynchronous page fault (APF)
+        injection to indicate type of asynchronous page fault. Value
+        of 1 means that the page referred to by the page fault is not
+        present. Value 2 means that the page is now available. Disabling
+        interrupt inhibits APFs. Guest must not enable interrupt
+        before the reason is read, or it may be overwritten by another
+        APF. Since APF uses the same exception vector as regular page
+        fault guest must reset the reason to 0 before it does
+        something that can generate normal page fault.  If during page
+        fault APF reason is 0 it means that this is regular page
+        fault.
+
+        During delivery of type 1 APF cr2 contains a token that will
+        be used to notify a guest when missing page becomes
+        available. When page becomes available type 2 APF is sent with
+        cr2 set to the token associated with the page. There is special
+        kind of token 0xffffffff which tells vcpu that it should wake
+        up all processes waiting for APFs and no individual type 2 APFs
+        will be sent.
+
+        If APF is disabled while there are outstanding APFs, they will
+        not be delivered.
+
+        Currently type 2 APF will be always delivered on the same vcpu as
+        type 1 was, but guest should not rely on that.