From: LIU Zhiwei <zhiwei_liu@linux.alibaba.com>
To: Richard Henderson <richard.henderson@linaro.org>, fei2.wu@intel.com
Cc: Palmer Dabbelt <palmer@dabbelt.com>,
Alistair Francis <alistair.francis@wdc.com>,
Bin Meng <bin.meng@windriver.com>,
Weiwei Li <liweiwei@iscas.ac.cn>,
Daniel Henrique Barboza <dbarboza@ventanamicro.com>,
"open list:RISC-V TCG CPUs" <qemu-riscv@nongnu.org>,
"open list:All patches CC here" <qemu-devel@nongnu.org>
Subject: Re: [PATCH] target/riscv: reduce overhead of MSTATUS_SUM change
Date: Wed, 22 Mar 2023 09:58:08 +0800 [thread overview]
Message-ID: <209d36a3-c136-5288-a842-3c0b5f1b5d0a@linux.alibaba.com> (raw)
In-Reply-To: <609e4659-532f-0fe2-447a-f7deaa0824e5@linaro.org>
On 2023/3/22 0:10, Richard Henderson wrote:
> 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.
Hi Fei,
Let's follow Richard's advice.
Zhiwei
>
>
> r~
next prev parent reply other threads:[~2023-03-22 1:59 UTC|newest]
Thread overview: 20+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-03-21 6:37 [PATCH] target/riscv: reduce overhead of MSTATUS_SUM change fei2.wu
2023-03-21 8:28 ` liweiwei
2023-03-21 8:40 ` Wu, Fei
2023-03-21 8:50 ` liweiwei
2023-03-21 9:14 ` Wu, Fei
2023-03-21 9:47 ` liweiwei
2023-03-21 12:00 ` Wu, Fei
2023-03-21 12:46 ` liweiwei
2023-03-21 12:58 ` liweiwei
2023-03-21 13:22 ` Wu, Fei
2023-03-21 13:27 ` liweiwei
2023-03-21 16:10 ` Richard Henderson
2023-03-22 1:58 ` LIU Zhiwei [this message]
2023-03-22 2:47 ` Wu, Fei
2023-03-22 3:16 ` LIU Zhiwei
2023-03-22 3:31 ` Richard Henderson
2023-03-22 3:36 ` Wu, Fei
2023-03-22 6:40 ` Wu, Fei
2023-03-22 6:50 ` LIU Zhiwei
2023-03-22 7:04 ` Wu, Fei
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