Re: [PATCH] target/riscv: reduce overhead of MSTATUS_SUM change

[Richard Henderson <richard.henderson@linaro.org>]

On 3/20/23 23:37, fei2.wu@intel.com wrote:
> From: Fei Wu <fei2.wu@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.


r~