linux-kernel - Re: [Qemu-devel] [PATCH RFC] 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:	Thu, 20 May 2010 09:04:37 +0300
From:	Avi Kivity <avi@...hat.com>
To:	"Michael S. Tsirkin" <mst@...hat.com>
CC:	Rusty Russell <rusty@...tcorp.com.au>,
	linux-kernel@...r.kernel.org,
	virtualization@...ts.linux-foundation.org, kvm@...r.kernel.org,
	qemu-devel@...gnu.org
Subject: Re: [Qemu-devel] [PATCH RFC] virtio: put last seen used index into
 ring itself

On 05/20/2010 01:33 AM, Michael S. Tsirkin wrote:
>
>> Virtio is already way too bouncy due to the indirection between the
>> avail/used rings and the descriptor pool.  A device with out of order
>> completion (like virtio-blk) will quickly randomize the unused
>> descriptor indexes, so every descriptor fetch will require a bounce.
>>
>> In contrast, if the rings hold the descriptors themselves instead of
>> pointers, we bounce (sizeof(descriptor)/cache_line_size) cache lines for
>> every descriptor, amortized.
>>      
> On the other hand, consider that on fast path we are never using all
> of the ring. With a good allocator we might be able to keep
> reusing only small part of the ring, instead of wrapping around
> all of it all of the time.
>    

It's still suboptimal, we have to bounce both the avail/used rings and 
the descriptor pool, compared to just the descriptor ring with a direct 
design.  Plus we don't need a fancy allocator.

When amortizing cachelines, simple data structures win.

-- 
Do not meddle in the internals of kernels, for they are subtle and quick to panic.

--
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/