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    SubjectRe: [PATCH v3 00/12] KVM: Add host swap event notifications for PV guest
    On Fri, Jan 08, 2010 at 02:18:28PM -0200, Marcelo Tosatti wrote:
    > On Tue, Jan 05, 2010 at 04:12:42PM +0200, Gleb Natapov wrote:
    > > KVM virtualizes guest memory by means of shadow pages or HW assistance
    > > like NPT/EPT. Not all memory used by a guest is mapped into the guest
    > > address space or even present in a host memory at any given time.
    > > When vcpu tries to access memory page that is not mapped into the guest
    > > address space KVM is notified about it. KVM maps the page into the guest
    > > address space and resumes vcpu execution. If the page is swapped out
    > > from host memory vcpu execution is suspended till the page is not swapped
    > > into the memory again. This is inefficient since vcpu can do other work
    > > (run other task or serve interrupts) while page gets swapped in.
    > >
    > > To overcome this inefficiency this patch series implements "asynchronous
    > > page fault" for paravirtualized KVM guests. If a page that vcpu is
    > > trying to access is swapped out KVM sends an async PF to the vcpu
    > > and continues vcpu execution. Requested page is swapped in by another
    > > thread in parallel. When vcpu gets async PF it puts faulted task to
    > > sleep until "wake up" interrupt is delivered. When the page is brought
    > > to the host memory KVM sends "wake up" interrupt and the guest's task
    > > resumes execution.
    >
    > Some high level comments:
    >
    > - cr2 used as token: better use the shared region? what if:
    >
    > async pf queued
    > guest triple faults without a vmexit
    > inject async-pf-done with token in cr2
    >
    > Also, in such scenario, can't you potentially corrupt guest memory after
    > the triple fault by writing to the previously registered shared region
    > address?
    >
    After triple faults guest will reboot and this should clear all pending
    async pf injections. I'll check that this is indeed happens.

    > - The token can overflow relatively easy. Use u64?
    It not only can it frequently does, but since there can't be 2^20
    outstanding page faults per vcpu simultaneously this doesn't cause
    any problem.

    >
    > - Does it really inject interrupts for non-pv guests while waiting
    > for swapin? Can't see that. Wish it was more geared towards fv.
    >
    No it does not yet. I only started to play with this to see how it can
    work.

    > - Please share some perf numbers.
    >
    OK.

    > - Limit the number of queued async pf's per guest ?
    >
    Make sense.

    > - Unify gfn_to_pfn / gfn_to_pfn_async code in the pf handlers (easier
    > to review).
    OK.

    --
    Gleb.


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