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

From: "Wang, Xiao W" <xiao.w.wang@intel.com>
To: "ben.dooks@codethink.co.uk" <ben.dooks@codethink.co.uk>,
	Alexandre Ghiti <alex@ghiti.fr>,
	"paul.walmsley@sifive.com" <paul.walmsley@sifive.com>,
	"palmer@dabbelt.com" <palmer@dabbelt.com>,
	"aou@eecs.berkeley.edu" <aou@eecs.berkeley.edu>
Cc: "jerry.shih@sifive.com" <jerry.shih@sifive.com>,
	"nick.knight@sifive.com" <nick.knight@sifive.com>,
	"ajones@ventanamicro.com" <ajones@ventanamicro.com>,
	"bjorn@rivosinc.com" <bjorn@rivosinc.com>,
	"andy.chiu@sifive.com" <andy.chiu@sifive.com>,
	"viro@zeniv.linux.org.uk" <viro@zeniv.linux.org.uk>,
	"cleger@rivosinc.com" <cleger@rivosinc.com>,
	"alexghiti@rivosinc.com" <alexghiti@rivosinc.com>,
	"Li, Haicheng" <haicheng.li@intel.com>,
	"akira.tsukamoto@gmail.com" <akira.tsukamoto@gmail.com>,
	"linux-riscv@lists.infradead.org"
	<linux-riscv@lists.infradead.org>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
Subject: RE: [PATCH] riscv: uaccess: Allow the last potential unrolled copy
Date: Mon, 6 May 2024 07:53:38 +0000	[thread overview]
Message-ID: <DM8PR11MB5751E4193D7B73B17DB7D7FFB81C2@DM8PR11MB5751.namprd11.prod.outlook.com> (raw)
In-Reply-To: <4a04a462-3eab-4382-83b2-ce6ed7104883@codethink.co.uk>

Hi Alex,

> -----Original Message-----
> From: ben.dooks@codethink.co.uk <ben.dooks@codethink.co.uk>
> Sent: Friday, May 3, 2024 10:30 PM
> To: Alexandre Ghiti <alex@ghiti.fr>; Wang, Xiao W <xiao.w.wang@intel.com>;
> paul.walmsley@sifive.com; palmer@dabbelt.com; aou@eecs.berkeley.edu
> Cc: jerry.shih@sifive.com; nick.knight@sifive.com;
> ajones@ventanamicro.com; bjorn@rivosinc.com; andy.chiu@sifive.com;
> viro@zeniv.linux.org.uk; cleger@rivosinc.com; alexghiti@rivosinc.com; Li,
> Haicheng <haicheng.li@intel.com>; akira.tsukamoto@gmail.com; linux-
> riscv@lists.infradead.org; linux-kernel@vger.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@intel.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@rivosinc.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@codethink.co.uk/
> 
> https://lore.kernel.org/linux-riscv/20210329095749.998940-1-
> ben.dooks@codethink.co.uk/
> --
> Ben Dooks				http://www.codethink.co.uk/
> Senior Engineer				Codethink - Providing Genius
> 
> https://www.codethink.co.uk/privacy.html

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