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

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:

quoted

On 2019/3/8 下午10:12, Christoph Hellwig wrote:

quoted

On Wed, Mar 06, 2019 at 02:18:07AM -0500, Jason Wang wrote:

quoted

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()"