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? > I recall we had a convert access_ok() discussion with this result here: > > https://lkml.org/lkml/2018/1/17/929 Sorry let me try to clarify. IIUC speculating access_ok once is harmless. As Linus said the problem is with "_subsequent_ accesses that can then be used to perturb the cache". Thus: 1. if (!access_ok) 2. return 3. get_user 4. if (!access_ok) 5. return 6. get_user Your proposal that Linus nacked was to effectively add a barrier after lines 2 and 5 (also using the array_index_nospec trick for speed), right? Unfortunately that needs a big API change. I am asking about adding barrier_nospec within access_ok. Thus effectively before lines 1 and 4. access_ok will be slower but after all the point of access_ok is to then access the same memory multiple times. So we should be making __get_user faster and access_ok slower ... > ...but it sounds like you are proposing a smaller scope fixup for the > vhost use case? Something like barrier_nospec() in the success path > for all vhost access_ok() checks and then a get_user() variant that > disables the barrier. Maybe we'll have to. Except I hope vhost won't end up being the only user otherwise it will be hard to maintain. -- MST
