linux-kernel - Re: [PATCHv2-RFC 0/2] virtio: put last seen used index into ring itself

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Date:	Mon, 31 May 2010 15:20:56 +0300
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	Rusty Russell <rusty@...tcorp.com.au>
Cc:	linux-kernel@...r.kernel.org,
	virtualization@...ts.linux-foundation.org, kvm@...r.kernel.org,
	qemu-devel@...gnu.org
Subject: Re: [PATCHv2-RFC 0/2] virtio: put last seen used index into ring
	itself

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
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/