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.
Zhiwei
r~
