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

Jason Wang
2019-01-08T11:42:43Z

On 2019/1/7 下午10:11, Michael S. Tsirkin wrote:
> On Sun, Jan 06, 2019 at 11:15:20PM -0800, Dan Williams wrote:
>> On Sun, Jan 6, 2019 at 8:17 PM Michael S. Tsirkin <mst@redhat.com> wrote:
>>> On Mon, Jan 07, 2019 at 11:53:41AM +0800, Jason Wang wrote:
>>>> On 2019/1/7 上午11:28, Michael S. Tsirkin wrote:
>>>>> On Mon, Jan 07, 2019 at 10:19:03AM +0800, Jason Wang wrote:
>>>>>> On 2019/1/3 上午4:47, 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
>>>>>>>> toggling.
>>>>>>> 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
>>>>>>> nosmap?
>>>>>>>
>>>>>> 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?
>>>
>>> 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.
>> It would be interesting to see how expensive re-doing the address
>> limit check is compared to the speculation barrier. I.e. just switch
>> vhost_get_user() to use get_user() rather than __get_user(). That will
>> sanitize the pointer in the speculative path without a barrier.
> Hmm it's way cheaper even though IIRC it's measureable.
> Jason, would you like to try?

0.5% regression after using get_user()/put_user()/...

> Although frankly __get_user being slower than get_user feels very wrong.
> Not yet sure what to do exactly but would you agree?
>
>
>> I recall we had a convert access_ok() discussion with this result here:
>>
>> https://lkml.org/lkml/2018/1/17/929
> Sorry let me try to clarify. IIUC speculating access_ok once
> is harmless. As Linus said the problem is with "_subsequent_
> accesses that can then be used to perturb the cache".
>
> Thus:
>
> 1. if (!access_ok)
> 2. return
> 3. get_user
> 4. if (!access_ok)
> 5. return
> 6. get_user
>
> Your proposal that Linus nacked was to effectively add a barrier after
> lines 2 and 5 (also using the array_index_nospec trick for speed),
> right? Unfortunately that needs a big API change.
>
> I am asking about adding barrier_nospec within access_ok.
> Thus effectively before lines 1 and 4.
> access_ok will be slower but after all the point of access_ok is
> to then access the same memory multiple times.
> So we should be making __get_user faster and access_ok slower ...

And the barrier_nospec() was completely necessary if you want to do
write instead read.

Thanks

>
>> ...but it sounds like you are proposing a smaller scope fixup for the
>> vhost use case? Something like barrier_nospec() in the success path
>> for all vhost access_ok() checks and then a get_user() variant that
>> disables the barrier.
> Maybe we'll have to. Except I hope vhost won't end up being the
> only user otherwise it will be hard to maintain.
>
>