On 3/22/2023 9:58 AM, LIU Zhiwei wrote: > > On 2023/3/22 0:10, Richard Henderson wrote: >> On 3/20/23 23:37, fei2...@intel.com wrote: >>> From: Fei Wu <fei2...@intel.com> >>> >>> Kernel needs to access user mode memory e.g. during syscalls, the window >>> is usually opened up for a very limited time through MSTATUS.SUM, the >>> overhead is too much if tlb_flush() gets called for every SUM change. >>> This patch saves addresses accessed when SUM=1, and flushs only these >>> pages when SUM changes to 0. If the buffer is not large enough to save >>> all the pages during SUM=1, it will fall back to tlb_flush when >>> necessary. >>> >>> The buffer size is set to 4 since in this MSTATUS.SUM open-up window, >>> most of the time kernel accesses 1 or 2 pages, it's very rare to see >>> more than 4 pages accessed. >>> >>> It's not necessary to save/restore these new added status, as >>> tlb_flush() is always called after restore. >>> >>> Result of 'pipe 10' from unixbench boosts from 223656 to 1327407. Many >>> other syscalls benefit a lot from this one too. >> >> This is not the correct approach. >> >> You should be making use of different softmmu indexes, similar to how >> ARM uses a separate index for PAN (privileged access never) mode. If >> I read the manual properly, PAN == !SUM. >> >> When you do this, you need no additional flushing. > > Hi Fei, > > Let's follow Richard's advice. >Yes, I'm thinking about how to do it, and thank Richard for the advice.
My question is: * If we ensure this separate index (S+SUM) has no overlapping tlb entries with S-mode (ignore M-mode so far), during SUM=1, we have to look into both (S+SUM) and S index for kernel address translation, that should be not desired. * If all the tlb operations are against (S+SUM) during SUM=1, then (S+SUM) could contain some duplicated tlb entries of kernel address in S index, the duplication means extra tlb lookup and fill. Also if we want to flush tlb entry of specific addr0, we have to flush both index. I will take a look at how arm handles this. Thanks, Fei. > Zhiwei > >> >> >> r~