On Sat, Dec 15, 2018 at 11:43:08AM -0800, David Miller wrote:
From: Jason Wang <jasowang@xxxxxxxxxx>Okay. Won't be the first time I'm wrong.
Date: Fri, 14 Dec 2018 12:29:54 +0800
On 2018/12/14 äå4:12, Michael S. Tsirkin wrote:Exactly, this is precisely how this kind of problem should be solved.
On Thu, Dec 13, 2018 at 06:10:19PM +0800, Jason Wang wrote:
Hi:I think the idea of speeding up userspace access is a good one.
This series tries to access virtqueue metadata through kernel virtual
address instead of copy_user() friends since they had too much
overheads like checks, spec barriers or even hardware feature
Test shows about 24% improvement on TX PPS. It should benefit other
cases as well.
However I think that moving all checks to start is way too aggressive.
So did packet and AF_XDP. Anyway, sharing address space and access
them directly is the fastest way. Performance is the major
consideration for people to choose backend. Compare to userspace
implementation, vhost does not have security advantages at any
level. If vhost is still slow, people will start to develop backends
based on e.g AF_XDP.
Michael, I strongly support the approach Jason is taking here, and I
would like to ask you to seriously reconsider your objections.
Let's say we ignore security aspects, but we need to make sure the
following all keep working (broken with this revision):
- file backed memory (I didn't see where we mark memory dirty -
if we don't we get guest memory corruption on close, if we do
then host crash as https://lwn.net/Articles/774411/ seems to apply here?)
Because vhost isn't like AF_XDP where you can just tell people "use
hugetlbfs" and "data is removed on close" - people are using it in lots
of configurations with guest memory shared between rings and unrelated
Jason, thoughts on these?