On Mon, May 31, 2010 at 05:16:42PM +0930, Rusty Russell wrote: > On Thu, 27 May 2010 05:20:35 am Michael S. Tsirkin wrote: > > Here's a rewrite of the original patch with a new layout. > > I haven't tested it yet so no idea how this performs, but > > I think this addresses the cache bounce issue raised by Avi. > > Posting for early flames/comments. > > Sorry, not without some evidence that it'll actually reduce cacheline > bouncing. I *think* it will, but it's not obvious: the host may keep > looking at avail_idx as we're updating last_seen. Or does qemu always > look at both together anyway? > Can someone convince me this is a win? > Rusty.
What really happens is host looks at flags and last_seen together. And flags happens to be in the same cache line with avail idx. So to get an obvious win, we should put flags and last_seen in a separate cache line from avail, which us easy - just add some padding. And I'll relax the requirement from guest to only require it to update last_seen when interrupts are enabled. This way flags and last_seen are written together and read together. Makes sense? -- MST
