Re: [RFC PATCH V3 0/5] Hi:

From: Jason Wang
Date: Mon Jan 07 2019 - 01:50:32 EST

On 2019/1/7 äå12:17, Michael S. Tsirkin wrote:
On Mon, Jan 07, 2019 at 11:53:41AM +0800, Jason Wang wrote:
On 2019/1/7 äå11:28, Michael S. Tsirkin wrote:
On Mon, Jan 07, 2019 at 10:19:03AM +0800, Jason Wang wrote:
On 2019/1/3 äå4:47, Michael S. Tsirkin wrote:
On Sat, Dec 29, 2018 at 08:46:51PM +0800, Jason Wang wrote:
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
Will review, thanks!
One questions that comes to mind is whether it's all about bypassing
stac/clac. Could you please include a performance comparison with

On machine without SMAP (Sandy Bridge):

Before: 4.8Mpps

After: 5.2Mpps
OK so would you say it's really unsafe versus safe accesses?
Or would you say it's just a better written code?

It's the effect of removing speculation barrier.

You mean __uaccess_begin_nospec introduced by
commit 304ec1b050310548db33063e567123fae8fd0301


So fundamentally we do access_ok checks when supplying
the memory table to the kernel thread, and we should
do the spec barrier there.

Then we can just create and use a variant of uaccess macros that does
not include the barrier?

The unsafe ones?

Or, how about moving the barrier into access_ok?
This way repeated accesses with a single access_ok get a bit faster.
CC Dan Williams on this idea.

The problem is, e.g for vhost control path. During mem table validation, we don't even want to access them there. So the spec barrier is not needed.

On machine with SMAP (Broadwell):

Before: 5.0Mpps

After: 6.1Mpps

No smap: 7.5Mpps

no smap being before or after?

Let me clarify:

Before (SMAP on): 5.0Mpps

Before (SMAP off): 7.5Mpps

After (SMAP on): 6.1Mpps

How about after + smap off?

After (SMAP off): 8.0Mpps

And maybe we want a module option just for the vhost thread to keep smap
off generally since almost all it does is copy stuff from userspace into
kernel anyway. Because what above numbers should is that we really
really want a solution that isn't limited to just meta-data access,
and I really do not see how any such solution can not also be
used to make meta-data access fast.

As we've discussed in another thread of previous version. This requires lots of changes, the main issues is SMAP state was not saved/restored on explicit schedule(). Even if it did, since vhost will call lots of net/block codes, any kind of uaccess in those codes needs understand this special request from vhost e.g you provably need to invent a new kinds of iov iterator that does not touch SMAP at all. And I'm not sure this is the only thing we need to deal with.

So I still prefer to:

1) speedup the metadata access through vmap + MMU notifier

2) speedup the datacopy with batched copy (unsafe ones or other new interfaces)