linux-kernel - Re: [PATCH] RISC-V: Enable Zicbom in usermode

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Message-ID: <mhng-cee266c4-3485-423a-b769-aae7ae034a1a@palmer-ri-x1c9>
Date: Wed, 13 Nov 2024 07:11:51 -0800 (PST)
From: Palmer Dabbelt <palmer@...belt.com>
To: Conor Dooley <conor@...nel.org>
CC: cuiyunhui@...edance.com, punit.agrawal@...edance.com,
  Paul Walmsley <paul.walmsley@...ive.com>, aou@...s.berkeley.edu, cleger@...osinc.com,
  Charlie Jenkins <charlie@...osinc.com>, Evan Green <evan@...osinc.com>, samuel.holland@...ive.com, andybnac@...il.com,
  linux-riscv@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject:     Re: [PATCH] RISC-V: Enable Zicbom in usermode

On Fri, 25 Oct 2024 03:16:44 PDT (-0700), Conor Dooley wrote:
> On Fri, Oct 25, 2024 at 05:15:27PM +0800, Yunhui Cui wrote:
>> Like Zicboz, by enabling the corresponding bits of senvcfg,
>> the instructions cbo.clean, cbo.flush, and cbo.inval can be
>> executed normally in user mode.
>> 
>> Signed-off-by: Yunhui Cui <cuiyunhui@...edance.com>
>> ---
>>  arch/riscv/kernel/cpufeature.c | 2 +-
>>  1 file changed, 1 insertion(+), 1 deletion(-)
>> 
>> diff --git a/arch/riscv/kernel/cpufeature.c b/arch/riscv/kernel/cpufeature.c
>> index 1992ea64786e..bc850518ab41 100644
>> --- a/arch/riscv/kernel/cpufeature.c
>> +++ b/arch/riscv/kernel/cpufeature.c
>> @@ -924,7 +924,7 @@ unsigned long riscv_get_elf_hwcap(void)
>>  void __init riscv_user_isa_enable(void)
>>  {
>>  	if (riscv_has_extension_unlikely(RISCV_ISA_EXT_ZICBOZ))
>> -		current->thread.envcfg |= ENVCFG_CBZE;
>> +		current->thread.envcfg |= ENVCFG_CBIE | ENVCFG_CBCFE | ENVCFG_CBZE;
>
> I believe we previously decided that userspace should not be allowed to

Ya, we didn't want to expose this because it opens up a can of worms.  
Is there a use case for this?  It's not like this is entirely impossible 
to do, it just requires a bit of thought (and should probably be gated 
behind some per-process disabling).

> use zicbom, but that not withstanding - this is wrong. It should be
> checking for Zicbom, not Zicboz.