Re: [PATCH] kvm: eoi msi documentation
From: Gleb Natapov
Date: Sun May 13 2012 - 11:56:22 EST
On Sun, May 13, 2012 at 06:13:22PM +0300, Michael S. Tsirkin wrote:
> 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.
Looks good, but everything below this is here by mistake. Are You still
going to resend host side patch to address my other comment?
> +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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