linux-kernel - RE: [PATCH] riscv: uaccess: Allow the last potential unrolled copy

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Date: Mon, 6 May 2024 07:53:38 +0000
From: "Wang, Xiao W" <xiao.w.wang@...el.com>
To: "ben.dooks@...ethink.co.uk" <ben.dooks@...ethink.co.uk>, Alexandre Ghiti
	<alex@...ti.fr>, "paul.walmsley@...ive.com" <paul.walmsley@...ive.com>,
	"palmer@...belt.com" <palmer@...belt.com>, "aou@...s.berkeley.edu"
	<aou@...s.berkeley.edu>
CC: "jerry.shih@...ive.com" <jerry.shih@...ive.com>, "nick.knight@...ive.com"
	<nick.knight@...ive.com>, "ajones@...tanamicro.com"
	<ajones@...tanamicro.com>, "bjorn@...osinc.com" <bjorn@...osinc.com>,
	"andy.chiu@...ive.com" <andy.chiu@...ive.com>, "viro@...iv.linux.org.uk"
	<viro@...iv.linux.org.uk>, "cleger@...osinc.com" <cleger@...osinc.com>,
	"alexghiti@...osinc.com" <alexghiti@...osinc.com>, "Li, Haicheng"
	<haicheng.li@...el.com>, "akira.tsukamoto@...il.com"
	<akira.tsukamoto@...il.com>, "linux-riscv@...ts.infradead.org"
	<linux-riscv@...ts.infradead.org>, "linux-kernel@...r.kernel.org"
	<linux-kernel@...r.kernel.org>
Subject: RE: [PATCH] riscv: uaccess: Allow the last potential unrolled copy

Hi Alex,

> -----Original Message-----
> From: ben.dooks@...ethink.co.uk <ben.dooks@...ethink.co.uk>
> Sent: Friday, May 3, 2024 10:30 PM
> To: Alexandre Ghiti <alex@...ti.fr>; Wang, Xiao W <xiao.w.wang@...el.com>;
> paul.walmsley@...ive.com; palmer@...belt.com; aou@...s.berkeley.edu
> Cc: jerry.shih@...ive.com; nick.knight@...ive.com;
> ajones@...tanamicro.com; bjorn@...osinc.com; andy.chiu@...ive.com;
> viro@...iv.linux.org.uk; cleger@...osinc.com; alexghiti@...osinc.com; Li,
> Haicheng <haicheng.li@...el.com>; akira.tsukamoto@...il.com; linux-
> riscv@...ts.infradead.org; linux-kernel@...r.kernel.org
> Subject: Re: [PATCH] riscv: uaccess: Allow the last potential unrolled copy
> 
> On 03/05/2024 14:02, Alexandre Ghiti wrote:
> > Hi Ben,
> >
> > On 03/05/2024 14:19, Ben Dooks wrote:
> >> On 03/05/2024 13:16, Alexandre Ghiti wrote:
> >>> Hi Xiao,
> >>>
> >>> On 13/03/2024 11:33, Xiao Wang wrote:
> >>>> When the dst buffer pointer points to the last accessible aligned
> >>>> addr, we
> >>>> could still run another iteration of unrolled copy.
> >>>>
> >>>> Signed-off-by: Xiao Wang <xiao.w.wang@...el.com>
> >>>> ---
> >>>>   arch/riscv/lib/uaccess.S | 2 +-
> >>>>   1 file changed, 1 insertion(+), 1 deletion(-)
> >>>>
> >>>> diff --git a/arch/riscv/lib/uaccess.S b/arch/riscv/lib/uaccess.S
> >>>> index 2e665f8f8fcc..1399d797d81b 100644
> >>>> --- a/arch/riscv/lib/uaccess.S
> >>>> +++ b/arch/riscv/lib/uaccess.S
> >>>> @@ -103,7 +103,7 @@ SYM_FUNC_START(fallback_scalar_usercopy)
> >>>>       fixup REG_S   t4,  7*SZREG(a0), 10f
> >>>>       addi    a0, a0, 8*SZREG
> >>>>       addi    a1, a1, 8*SZREG
> >>>> -    bltu    a0, t0, 2b
> >>>> +    bleu    a0, t0, 2b
> >>>>       addi    t0, t0, 8*SZREG /* revert to original value */
> >>>>       j    .Lbyte_copy_tail
> >>>
> >>>
> >>> I agree it is still safe to continue for another word_copy here.
> >>>
> >>> Reviewed-by: Alexandre Ghiti <alexghiti@...osinc.com>
> >>
> >> Out of interest, has anyone checked if causing a schedule event during
> >> this code breaks like the last time we had issues with the upstream
> >> testing?
> >
> >
> > I vaguely remember something, do you have a link to that discussion by
> > chance?
> >
> >
> >>
> >> I did propose saving the state of the user-access flag in the task
> >> struct
> >
> >
> > Makes sense, I just took a quick look and SR_SUM is cleared as soon as
> > we enter handle_exception() and it does not seem to be restored. Weird
> > it works, unless I missed something!

I think it's already saved/restored via pt_regs.status, using the PT_STATUS() macro in entry.S.

BRs,
Xiao

> >
> >
> >> but we mostly solved it by making sleeping functions stay
> >> away from the address calculation. This of course may have been done
> >> already or need to be done if three's long areas where the user-access
> >> flags can be disabled (generally only a few drivers did this, so we
> >> may not have come across the problem)
> >>
> > I don't understand what you mean here, would you mind expanding a bit?
> >
> 
> I think this was all gone through in the original post where
> we initially suggested saving SR_SUM and then moved as much out
> of the critical SR_SUM area by changing how the macros worked
> 
> https://lore.kernel.org/linux-riscv/20210318151010.100966-1-
> ben.dooks@...ethink.co.uk/
> 
> https://lore.kernel.org/linux-riscv/20210329095749.998940-1-
> ben.dooks@...ethink.co.uk/
> --
> Ben Dooks				http://www.codethink.co.uk/
> Senior Engineer				Codethink - Providing Genius
> 
> https://www.codethink.co.uk/privacy.html