On Fri, Oct 11, 2013 at 09:54:12AM +1030, Rusty Russell wrote:
> "Michael S. Tsirkin" <> writes:
> > On Fri, Oct 04, 2013 at 01:06:43PM +0930, Rusty Russell wrote:
> >> "Michael S. Tsirkin" <> writes:
> >> > On Tue, Sep 24, 2013 at 08:18:20PM +0930, Rusty Russell wrote:
> >> >> I don't think qemu supports changing a disk underneath a device anyway?
> >> >
> >> > QEMU supports resizing I think.
> >> >
> >> >> And in practice, I'm not sure it would ever change fast enough to
> >> >> trigger this race...
> >> >
> >> > Well it's easy to make guest very slow :)
> >> >
> >> >> So, should we try to fix it?
> >> >> Rusty.
> >> >
> >> > Not really sure but I thought I'd put it on the table.
> >>
> >> OK, I got ambitious! How's this look?
> >>
> >> Cheers,
> >> Rusty.
> >>
> >> commit 9aebe9c69be1461dd0110dfc61459a4ac7bda6f9
> >> Author: Rusty Russell <>
> >> Date: Fri Oct 4 13:01:35 2013 +0930
> >>
> >> Configuration (read) atomicity.
> >>
> >> Aka issue VIRTIO-35.
> >>
> >> This is solved per transport:
> >> 1) PCI: use the 8 bit reserved field.
> >> Assume that if you really change that fast, you'll do it lazily on
> >> config space read.
> >
> > I guess it's possible ... but isn't it easier to just make it a 32 bit field
> > than add this lazy update logic to devices?
>
> I was trying to keep it tight.
Well we always find some use for the padding space later,
or we can move some stuff around to pack it ...
> I think the lazy update logic is
> pretty trivial:
>
> bool config_changed;
> ....
>
>
> on_config_generation_read()
> {
> if (config_changed) {
> gen++;
> config_changed = false;
> }
> return gen;
> }
And something similar on migration. I agree it's not too bad.
But we'll need to document this in the spec somehow.
> >> 2) MMIO: use offset 0x18.
> >> Qemu returns 0 when they read here anyway, so it's almost 100%
> >> backwards compatible (legacy drivers should still multi-read
> >> for any transport).
> >> 3) CCW: no transport changes.
> >> They always read/write the entire thing. This just shows that
> >> Cornelia is smarter than I am.
> >>
> >> Signed-off-by: Rusty Russell <>
> >>
> >> diff --git a/virtio-v1.0-wd01-part1-specification.txt b/virtio-v1.0-wd01-part1-specification.txt
> >> index b5a5835..2945db4 100644
> >> --- a/virtio-v1.0-wd01-part1-specification.txt
> >> +++ b/virtio-v1.0-wd01-part1-specification.txt
> >> @@ -201,11 +201,36 @@ Interface' in the section title.
> >> -------------------------
> >>
> >> Configuration space is generally used for rarely-changing or
> >> -initialization-time parameters.
> >> +initialization-time parameters. Drivers must not assume reads from
> >> +fields greater than 32 bits wide are atomic, nor or reads from
> >> +multiple fields.
> >> +
> >> +Each transport provides a generation count for the configuration
> >> +space, which must change whenever there is a possibility that two
> >
> > MUST?
> >
> >> +accesses to the configuration space can see different versions of that
> >> +space.
> >> +
> >> +Thus drivers should read configuration space fields like so:
> >
> > SHOULD?
>
> The convention is to use lower case for OASIS. It says in the template
> preamble. I guess it's bold in more sophisticated formats.
>
> >> +
> >> + u32 before, after;
> >> + do {
> >> + before = get_config_generation(device);
> >> + // read config entry/entries.
> >> + after = get_config_generation(device);
> >> + } while (after != before);
> >
> > BTW this implies reads must never have side
> > effects (PCI ISR is the only exception, isn't it?).
>
> Yes. That would be weird...
>
> Cheers,
> Rusty.
