On Mon, Jan 07, 2019 at 02:44:24PM -0800, Dan Williams wrote: > On Mon, Jan 7, 2019 at 2:25 PM Michael S. Tsirkin <mst@xxxxxxxxxx> wrote: > > > > On Mon, Jan 07, 2019 at 01:39:15PM -0800, Dan Williams wrote: > > > On Mon, Jan 7, 2019 at 6:11 AM Michael S. Tsirkin <mst@xxxxxxxxxx> wrote: > > > > > > > > On Sun, Jan 06, 2019 at 11:15:20PM -0800, Dan Williams wrote: > > > > > On Sun, Jan 6, 2019 at 8:17 PM Michael S. Tsirkin <mst@xxxxxxxxxx> wrote: > > > > > > > > > > > > On Mon, Jan 07, 2019 at 11:53:41AM +0800, Jason Wang wrote: > > > > > > > > > > > > > > On 2019/1/7 上午11:28, Michael S. Tsirkin wrote: > > > > > > > > On Mon, Jan 07, 2019 at 10:19:03AM +0800, Jason Wang wrote: > > > > > > > > > On 2019/1/3 上午4:47, Michael S. Tsirkin wrote: > > > > > > > > > > On Sat, Dec 29, 2018 at 08:46:51PM +0800, 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. > > > > > > > > > > Will review, thanks! > > > > > > > > > > One questions that comes to mind is whether it's all about bypassing > > > > > > > > > > stac/clac. Could you please include a performance comparison with > > > > > > > > > > nosmap? > > > > > > > > > > > > > > > > > > > On machine without SMAP (Sandy Bridge): > > > > > > > > > > > > > > > > > > Before: 4.8Mpps > > > > > > > > > > > > > > > > > > After: 5.2Mpps > > > > > > > > OK so would you say it's really unsafe versus safe accesses? > > > > > > > > Or would you say it's just a better written code? > > > > > > > > > > > > > > > > > > > > > It's the effect of removing speculation barrier. > > > > > > > > > > > > > > > > > > You mean __uaccess_begin_nospec introduced by > > > > > > commit 304ec1b050310548db33063e567123fae8fd0301 > > > > > > ? > > > > > > > > > > > > So fundamentally we do access_ok checks when supplying > > > > > > the memory table to the kernel thread, and we should > > > > > > do the spec barrier there. > > > > > > > > > > > > Then we can just create and use a variant of uaccess macros that does > > > > > > not include the barrier? > > > > > > > > > > > > Or, how about moving the barrier into access_ok? > > > > > > This way repeated accesses with a single access_ok get a bit faster. > > > > > > CC Dan Williams on this idea. > > > > > > > > > > It would be interesting to see how expensive re-doing the address > > > > > limit check is compared to the speculation barrier. I.e. just switch > > > > > vhost_get_user() to use get_user() rather than __get_user(). That will > > > > > sanitize the pointer in the speculative path without a barrier. > > > > > > > > Hmm it's way cheaper even though IIRC it's measureable. > > > > Jason, would you like to try? > > > > Although frankly __get_user being slower than get_user feels very wrong. > > > > Not yet sure what to do exactly but would you agree? > > > > > > Agree. __get_user() being faster than get_user() defeats the whole > > > point of converting code paths to the access_ok() + __get_user() > > > pattern. > > > > Did you mean the reverse? > > Hmm, no... I'll rephrase: __get_user() should have lower overhead than > get_user(). Right ... Linus, given that you just changed all users of access_ok anyway, do you still think that the access_ok() conversion to return a speculation sanitized pointer or NULL is too big a conversion? It was previously discarded here: https://lkml.org/lkml/2018/1/17/929 but at that point we didn't have numbers and there was an understandable rush to ship something safe. At this point I think that vhost can show very measureable gains from this conversion. Thanks, -- MST
