Re: [PATCH net-next 0/3] vhost: accelerate metadata access through vmap()

From: Jason Wang
Date: Mon Dec 24 2018 - 03:32:54 EST

On 2018/12/14 äå8:33, Michael S. Tsirkin wrote:
On Fri, Dec 14, 2018 at 11:42:18AM +0800, Jason Wang wrote:
On 2018/12/13 äå11:27, Michael S. Tsirkin wrote:
On Thu, Dec 13, 2018 at 06:10:19PM +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
Userspace accesses through remapping tricks and next time there's a need
for a new barrier we are left to figure it out by ourselves.

I don't get here, do you mean spec barriers?
I mean the next barrier people decide to put into userspace
memory accesses.

It's completely unnecessary for
vhost which is kernel thread.
It's defence in depth. Take a look at the commit that added them.
And yes quite possibly in most cases we actually have a spec
barrier in the validation phase. If we do let's use the
unsafe variants so they can be found.

unsafe variants can only work if you can batch userspace access. This is not necessarily the case for light load.

And even if you're right, vhost is not the
only place, there's lots of vmap() based accessing in kernel.
For sure. But if one can get by without get user pages, one
really should. Witness recently uncovered mess with file
backed storage.

We only pin metadata pages, I don't believe they will be used by any DMA.

Think in
another direction, this means we won't suffer form unnecessary barriers for
kthread like vhost in the future, we will manually pick the one we really
I personally think we should err on the side of caution not on the side of

So what you suggest may lead unnecessary performance regression (10%-20%) which is part of the goal of this series. We should audit and only use the one we really need instead of depending on copy_user() friends().

If we do it our own, it could be slow for for security fix but it's no less safe than before with performance kept.

(but it should have little possibility).
History seems to teach otherwise.

What case did you mean here?

Please notice we only access metdata through remapping not the data itself.
This idea has been used for high speed userspace backend for years, e.g
packet socket or recent AF_XDP.
I think their justification for the higher risk is that they are mostly
designed for priveledged userspace.

I think it's the same with TUN/TAP, privileged process can pass them to unprivileged ones.

The only difference is the page was remap to
from kernel to userspace.
At least that avoids the g.u.p mess.

I'm still not very clear at the point. We only pin 2 or 4 pages, they're several other cases that will pin much more.

I don't
like the idea I have to say. As a first step, why don't we switch to
unsafe_put_user/unsafe_get_user etc?

Several reasons:

- They only have x86 variant, it won't have any difference for the rest of
Is there an issue on other architectures? If yes they can be extended

Consider the unexpected amount of work and in the best case it can give the same performance to vmap(). I'm not sure it's worth.

- unsafe_put_user/unsafe_get_user is not sufficient for accessing structures
(e.g accessing descriptor) or arrays (batching).
So you want unsafe_copy_xxx_user? I can do this. Hang on will post.

- Unless we can batch at least the accessing of two places in three of
avail, used and descriptor in one run. There will be no difference. E.g we
can batch updating used ring, but it won't make any difference in this case.

So let's batch them all?

Batching might not help for the case of light load. And we need to measure the gain/cost of batching itself.

That would be more of an apples to apples comparison, would it not?

Apples to apples comparison only help if we are the No.1. But the fact is we
are not. If we want to compete with e.g dpdk or AF_XDP, vmap() is the
fastest method AFAIK.

We need to speed up the packet access itself too though.
You can't vmap all of guest memory.

This series only pin and vmap very few pages (metadata).


Test shows about 24% improvement on TX PPS. It should benefit other
cases as well.

Please review

Jason Wang (3):
vhost: generalize adding used elem
vhost: fine grain userspace memory accessors
vhost: access vq metadata through kernel virtual address

drivers/vhost/vhost.c | 281 ++++++++++++++++++++++++++++++++++++++----
drivers/vhost/vhost.h | 11 ++
2 files changed, 266 insertions(+), 26 deletions(-)