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    SubjectRe: [PATCH RFC dontapply] kvm_para: add mmio word store hypercall
    On 03/26/2012 12:08 PM, Michael S. Tsirkin wrote:
    >
    > > > + gpa = hc_gpa(vcpu, a1, a2);
    > > > + if (!write_mmio(vcpu, gpa, 2, &a0) && run) {
    > >
    > > What's this && run thing?
    >
    > I'm not sure - copied this from another other place in emulation:
    > arch/x86/kvm/x86.c:4953: if (!write_mmio(vcpu, gpa, 2, &a0) && run)
    >
    > I assumed there's some way to trigger emulation while VCPU does not run.
    > No?

    Not the way you initialize run above.

    >
    > >
    > > > + run->exit_reason = KVM_EXIT_MMIO;
    > > > + run->mmio.phys_addr = gpa;
    > > > + memcpy(run->mmio.data, &a0, 2);
    > > > + run->mmio.len = 2;
    > > > + run->mmio.is_write = 1;
    > > > + r = 0;
    > > > + }
    > > > + goto noret;
    > >
    > > What if the address is in RAM?
    > > Note the guest can't tell if a piece of memory is direct mapped or
    > > implemented as mmio.
    >
    > True but doing hypercalls for memory which can be
    > mapped directly is bad for performance - it's
    > the reverse of what we are trying to do here.

    It's bad, but the guest can't tell.

    Suppose someone implements virtio in hardware and we pass it through to
    a guest. It should continue working, no?

    > The intent is to use this for virtio where we can explicitly let the
    > guest know whether using a hypercall is safe.
    >
    > Acceptable? What do you suggest?

    It's iffy.

    What's the performance gain from this thing?


    --
    error compiling committee.c: too many arguments to function



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