Re: [PATCH RFC] uaccess: user_access_begin_after_access_ok()

Michael S. Tsirkin
2020-06-03T05:46:25Z

On Wed, Jun 03, 2020 at 01:18:54PM +0800, Jason Wang wrote:
>
> 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

It's basically vhost_translate_uaddr isn't it?

BTW now I re-read it I don't understand __vhost_get_user_slow:

static void __user *__vhost_get_user_slow(struct vhost_virtqueue *vq,
void __user *addr, unsigned int size,
int type)
{
int ret;

ret = translate_desc(vq, (u64)(uintptr_t)addr, size, vq->iotlb_iov,
ARRAY_SIZE(vq->iotlb_iov),
VHOST_ACCESS_RO);

..
}

how does this work? how can we cast a pointer to guest address without
adding any offsets?

>
> >