Re: [RFC 7/11] virtio_pci: new, capability-aware driver.

On Fri, 13 Jan 2012 04:19:30 +0200, "Michael S. Tsirkin" [off-list ref] wrote:

On Thu, Jan 12, 2012 at 12:12:17PM +1030, Rusty Russell wrote:

quoted

On Thu, 12 Jan 2012 00:02:33 +0200, "Michael S. Tsirkin" [off-list ref] wrote:

quoted

Look, we have a race currently. Let us not tie a bug fix to a huge
rewrite with unclear performance benefits, please.

In theory, yes.  In practice, we bandaid it.

I think in the short term we change ->get to get the entire sequence
twice, and check it's the same.  Theoretically, still racy, but it does
cut the window.  And we haven't seen the bug yet, either.

I thought about this some more. Since we always get
an interrupt on config changes, it seems that a rather
robust method would be to just synchronize against that.
Something like the below (warning - completely untested).
Still need to think about memory barriers, overflow etc.
What do you think?

Ben's point about arbitrary delays in irq delivery still applies.  If
qemu changes config space the injects, there's a window there, too.

But in practice, like my suggested double-read, it probably slows things
down enough that it would be hard to hit.  Testing required...

Obviously, I'd like to fix this properly, but for legacy this might work
fine.

Cheers,
Rusty.