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

["Michael S. Tsirkin" <mst@redhat.com>]

On Tue, Mar 12, 2019 at 10:59:09AM +0800, Jason Wang wrote:
> 
> On 2019/3/12 上午2:14, David Miller wrote:
> > From: "Michael S. Tsirkin" <mst@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 下午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.
Three atomic ops per bit is way to expensive.

-- 
MST