KVM: eoi msi documentation

Document the new EOI MSR. Couldn't decide whether this change belongs
conceptually on guest or host side, so a separate patch.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Avi Kivity <avi@redhat.com>

1 files changed, 33 insertions, 0 deletions

diff --git a/Documentation/virtual/kvm/msr.txt b/Documentation/virtual/kvm/msr.txt
index 96b41bd97523..730471048583 100644
--- a/Documentation/virtual/kvm/msr.txt
+++ b/Documentation/virtual/kvm/msr.txt
@@ -223,3 +223,36 @@ MSR_KVM_STEAL_TIME: 0x4b564d03
                 steal: the amount of time in which this vCPU did not run, in
                 nanoseconds. Time during which the vcpu is idle, will not be
                 reported as steal time.
+
+MSR_KVM_EOI_EN: 0x4b564d04
+        data: Bit 0 is 1 when PV end of interrupt is enabled on the vcpu; 0
+        when disabled.  Bit 1 is reserved and must be zero.  When PV end of
+        interrupt is enabled (bit 0 set), bits 63-2 hold a 4-byte aligned
+        physical address of a 4 byte memory area which must be in guest RAM and
+        must be zeroed.
+
+        The first, least significant bit of 4 byte memory location will be
+        written to by the hypervisor, typically at the time of interrupt
+        injection.  Value of 1 means that guest can skip writing EOI to the apic
+        (using MSR or MMIO write); instead, it is sufficient to signal
+        EOI by clearing the bit in guest memory - this location will
+        later be polled by the hypervisor.
+        Value of 0 means that the EOI write is required.
+
+        It is always safe for the guest to ignore the optimization and perform
+        the APIC EOI write anyway.
+
+        Hypervisor is guaranteed to only modify this least
+        significant bit while in the current VCPU context, this means that
+        guest does not need to use either lock prefix or memory ordering
+        primitives to synchronise with the hypervisor.
+
+        However, hypervisor can set and clear this memory bit at any time:
+        therefore to make sure hypervisor does not interrupt the
+        guest and clear the least significant bit in the memory area
+        in the window between guest testing it to detect
+        whether it can skip EOI apic write and between guest
+        clearing it to signal EOI to the hypervisor,
+        guest must both read the least significant bit in the memory area and
+        clear it using a single CPU instruction, such as test and clear, or
+        compare and exchange.