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

[David Miller]

From: "Michael S. Tsirkin" <mst@xxxxxxxxxx>
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()"