On Mon, Mar 26, 2012 at 12:16:14PM +0200, Avi Kivity wrote:
> 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.
Thanks for pointing this out, I'll drop the test.
> >
> > >
> > > > + 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?
Why would we want hypercalls then?
As I see it, virtio device would have a capability
that tells the guest to use hypercalls for access.
An actual PCI device won't expose this capability,
as would a device on a host which lacks the hypercall.
> > 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.
Question is, do we want a bunch of dead code sitting there
just in case? And what are the chances it'll work correctly
when we need it to?
> What's the performance gain from this thing?
I'll test and post separately.
>
> --
> error compiling committee.c: too many arguments to function
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