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| author | Michael S. Tsirkin <mst@redhat.com> | 2012-06-24 18:24:49 +0200 |
|---|---|---|
| committer | Avi Kivity <avi@redhat.com> | 2012-06-25 11:40:34 +0200 |
| commit | c1af87dc96cd0f8f17694d0cd9be01b80b2c7a6a (patch) | |
| tree | df243a9a8ac7c5d4e825a37f8afb1bdc9eb09924 /Documentation/virtual | |
| parent | x86, bitops: note on __test_and_clear_bit atomicity (diff) | |
| download | linux-c1af87dc96cd0f8f17694d0cd9be01b80b2c7a6a.tar.xz linux-c1af87dc96cd0f8f17694d0cd9be01b80b2c7a6a.zip | |
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>
Diffstat (limited to 'Documentation/virtual')
| -rw-r--r-- | Documentation/virtual/kvm/msr.txt | 33 |
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. |