Re: [PATCH RFC] uaccess: user_access_begin_after_access_ok()

Jason Wang
2020-06-03T05:19:05Z

On 2020/6/3 下午12:18, Al Viro wrote:
> On Wed, Jun 03, 2020 at 11:57:11AM +0800, Jason Wang wrote:
>
>>> How widely do you hope to stretch the user_access areas, anyway?
>>
>> To have best performance for small packets like 64B, if possible, we want to
>> disable STAC not only for the metadata access done by vhost accessors but
>> also the data access via iov iterator.
> If you want to try and convince Linus to go for that, make sure to Cc
> me on that thread. Always liked quality flame...
>
> The same goes for interval tree lookups with uaccess allowed. IOW, I _really_
> doubt that it's a good idea.

I see. We are just seeking an approach to perform better in order to
compete with userspace dpdk backends.

I tried another approach of using direct mapping + mmu notifier [1] but
the synchronization with MMU notifier is not easy to perform well.

[1] https://patchwork.kernel.org/patch/11133009/

>
>>> Incidentally, who had come up with the name __vhost_get_user?
>>> Makes for lovey WTF moment for readers - esp. in vhost_put_user()...
>>
>> I think the confusion comes since it does not accept userspace pointer (when
>> IOTLB is enabled).
>>
>> How about renaming it as vhost_read()/vhost_write() ?
> Huh?
>
> __vhost_get_user() is IOTLB remapping of userland pointer. It does not access
> userland memory. Neither for read, nor for write. It is used by vhost_get_user()
> and vhost_put_user().
>
> Why would you want to rename it into vhost_read _or_ vhost_write, and in any case,
> how do you give one function two names? IDGI...

I get you know, I thought you're concerning the names of
vhost_get_user()/vhost_put_user() but actually __vhost_get_user().

Maybe something like __vhost_fetch_uaddr() is better.

Thanks

>