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@xxxxxxxxxx> --- 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 -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
