[PATCHv7 5/8] kvm: eoi msi documentation

Michael S. Tsirkin
2012-06-14T13:53:07Z

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>
---
Documentation/virtual/kvm/msr.txt | 32 ++++++++++++++++++++++++++++++++
1 file changed, 32 insertions(+)

diff --git a/Documentation/virtual/kvm/msr.txt b/Documentation/virtual/kvm/msr.txt
index 96b41bd..6ae5a85 100644
--- a/Documentation/virtual/kvm/msr.txt
+++ b/Documentation/virtual/kvm/msr.txt
@@ -223,3 +223,35 @@ 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. When enabled, bits 63-1 hold 2-byte aligned physical address
+ of a 2 byte memory area which must be in guest RAM and must be zeroed.
+
+ The first, least significant bit of 2 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.
+
--
MST