[PATCH] kvm: eoi msi documentation

From: Michael S. Tsirkin
Date: Sun May 13 2012 - 11:13:27 EST

Document the new EOI MSR.

Signed-off-by: Michael S. Tsirkin <mst@xxxxxxxxxx>

This documents my PV EOI patchset and applies on top.
Will make it part of the patchset on the next respin.

Documentation/virtual/kvm/msr.txt | 56 +++++++++++++++++++++++++++++++++++++
1 files changed, 56 insertions(+), 0 deletions(-)

diff --git a/Documentation/virtual/kvm/msr.txt b/Documentation/virtual/kvm/msr.txt
index 5031780..bdbd337 100644
--- a/Documentation/virtual/kvm/msr.txt
+++ b/Documentation/virtual/kvm/msr.txt
@@ -219,3 +219,59 @@ 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 sgnificant bit in the memory area and
+ clear it using a single CPU instruction, such as test and clear, or
+ compare and exchange.
+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.
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