linux-kernel - Re: [PATCH RFC] uaccess: user_access_begin_after_access

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Date:   Wed, 3 Jun 2020 14:23:08 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>
Cc:     Al Viro <viro@...iv.linux.org.uk>, linux-kernel@...r.kernel.org,
        netdev@...r.kernel.org
Subject: Re: [PATCH RFC] uaccess: user_access_begin_after_access_ok()


On 2020/6/3 下午1:46, Michael S. Tsirkin wrote:
> 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?


Yes.


>
> 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?


I'm not sure I get you here. What kind of offset did you mean?

Thanks


>
>