On Sun, Jun 03, 2012 at 10:28:21AM +0300, Michael S. Tsirkin wrote: > 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 files changed, 32 insertions(+), 0 deletions(-) > > diff --git a/Documentation/virtual/kvm/msr.txt b/Documentation/virtual/kvm/msr.txt > index 96b41bd..f202a22 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 sgnificant bit in the memory area and typo > + clear it using a single CPU instruction, such as test and clear, or > + compare and exchange. > + > -- -- 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
