Re: [PATCH RFC dontapply] kvm_para: add mmio word store hypercall

[Avi Kivity <avi@redhat.com>]

On 03/26/2012 01:30 PM, Michael S. Tsirkin wrote:
> > >
> > > > 
> > > > > +			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.

Ok, makes sense.

> > > 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?

If we make it device specific, I guess not.

-- 
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