[RFC PATCH V2 0/5] vhost: accelerate metadata access through vmap()

Jason Wang
2019-03-12T07:17:00Z

On 2019/3/12 ??11:52, Michael S. Tsirkin wrote:
> On Tue, Mar 12, 2019 at 10:59:09AM +0800, Jason Wang wrote:
>> On 2019/3/12 ??2:14, David Miller wrote:
>>> From: "Michael S. Tsirkin" <mst at redhat.com>
>>> Date: Mon, 11 Mar 2019 09:59:28 -0400
>>>
>>>> On Mon, Mar 11, 2019 at 03:13:17PM +0800, Jason Wang wrote:
>>>>> On 2019/3/8 ??10:12, Christoph Hellwig wrote:
>>>>>> On Wed, Mar 06, 2019 at 02:18:07AM -0500, 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. This is done through setup kernel address
through vmap() and
>>>>>>> resigter MMU notifier for invalidation.
>>>>>>>
>>>>>>> Test shows about 24% improvement on TX PPS.
TCP_STREAM doesn't see
>>>>>>> obvious improvement.
>>>>>> How is this going to work for CPUs with virtually
tagged caches?
>>>>> Anything different that you worry?
>>>> If caches have virtual tags then kernel and userspace view of
memory
>>>> might not be automatically in sync if they access memory
>>>> through different virtual addresses. You need to do things like
>>>> flush_cache_page, probably multiple times.
>>> "flush_dcache_page()"
>>
>> I get this. Then I think the current set_bit_to_user() is suspicious,
we
>> probably miss a flush_dcache_page() there:
>>
>>
>> static int set_bit_to_user(int nr, void __user *addr)
>> {
>>  ??????? unsigned long log = (unsigned long)addr;
>>  ??????? struct page *page;
>>  ??????? void *base;
>>  ??????? int bit = nr + (log % PAGE_SIZE) * 8;
>>  ??????? int r;
>>
>>  ??????? r = get_user_pages_fast(log, 1, 1, &page);
>>  ??????? if (r < 0)
>>  ??????????????? return r;
>>  ??????? BUG_ON(r != 1);
>>  ??????? base = kmap_atomic(page);
>>  ??????? set_bit(bit, base);
>>  ??????? kunmap_atomic(base);
>>  ??????? set_page_dirty_lock(page);
>>  ??????? put_page(page);
>>  ??????? return 0;
>> }
>>
>> Thanks
> I think you are right. The correct fix though is to re-implement
> it using asm and handling pagefault, not gup.


I agree but it needs to introduce new helpers in asm? for all archs
which is not trivial. At least for -stable, we need the flush?


> Three atomic ops per bit is way to expensive.


Yes.

Thanks