linux-kernel - Re: [PATCH -fixes v3 1/2] riscv: Fix enabling cbo.zero when running in M-mode

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Message-ID: <33f16905-baab-47e8-a92f-d2543b69d704@sifive.com>
Date: Tue, 27 Feb 2024 14:01:34 -0600
From: Samuel Holland <samuel.holland@...ive.com>
To: Alexandre Ghiti <alex@...ti.fr>
Cc: Palmer Dabbelt <palmer@...belt.com>, linux-kernel@...r.kernel.org,
 Conor Dooley <conor@...nel.org>, linux-riscv@...ts.infradead.org,
 Stefan O'Rear <sorear@...tmail.com>, stable@...r.kernel.org,
 Andrew Jones <ajones@...tanamicro.com>
Subject: Re: [PATCH -fixes v3 1/2] riscv: Fix enabling cbo.zero when running
 in M-mode

Hi Alex,

On 2024-02-27 1:48 PM, Alexandre Ghiti wrote:
> Hi Samuel,
> 
> On 14/02/2024 10:28, Andrew Jones wrote:
>> On Wed, Feb 14, 2024 at 01:01:56AM -0800, Samuel Holland wrote:
>>> When the kernel is running in M-mode, the CBZE bit must be set in the
>>> menvcfg CSR, not in senvcfg.
>>>
>>> Cc: <stable@...r.kernel.org>
>>> Fixes: 43c16d51a19b ("RISC-V: Enable cbo.zero in usermode")
>>> Reviewed-by: Andrew Jones <ajones@...tanamicro.com>
>>> Signed-off-by: Samuel Holland <samuel.holland@...ive.com>
>>> ---
>>>
>>> (no changes since v1)
>>>
>>>   arch/riscv/include/asm/csr.h   | 2 ++
>>>   arch/riscv/kernel/cpufeature.c | 2 +-
>>>   2 files changed, 3 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/arch/riscv/include/asm/csr.h b/arch/riscv/include/asm/csr.h
>>> index 510014051f5d..2468c55933cd 100644
>>> --- a/arch/riscv/include/asm/csr.h
>>> +++ b/arch/riscv/include/asm/csr.h
>>> @@ -424,6 +424,7 @@
>>>   # define CSR_STATUS    CSR_MSTATUS
>>>   # define CSR_IE        CSR_MIE
>>>   # define CSR_TVEC    CSR_MTVEC
>>> +# define CSR_ENVCFG    CSR_MENVCFG
>>>   # define CSR_SCRATCH    CSR_MSCRATCH
>>>   # define CSR_EPC    CSR_MEPC
>>>   # define CSR_CAUSE    CSR_MCAUSE
>>> @@ -448,6 +449,7 @@
>>>   # define CSR_STATUS    CSR_SSTATUS
>>>   # define CSR_IE        CSR_SIE
>>>   # define CSR_TVEC    CSR_STVEC
>>> +# define CSR_ENVCFG    CSR_SENVCFG
>>>   # define CSR_SCRATCH    CSR_SSCRATCH
>>>   # define CSR_EPC    CSR_SEPC
>>>   # define CSR_CAUSE    CSR_SCAUSE
>>> diff --git a/arch/riscv/kernel/cpufeature.c b/arch/riscv/kernel/cpufeature.c
>>> index 89920f84d0a3..c5b13f7dd482 100644
>>> --- a/arch/riscv/kernel/cpufeature.c
>>> +++ b/arch/riscv/kernel/cpufeature.c
>>> @@ -950,7 +950,7 @@ arch_initcall(check_unaligned_access_all_cpus);
>>>   void riscv_user_isa_enable(void)
>>>   {
>>>       if (riscv_cpu_has_extension_unlikely(smp_processor_id(),
>>> RISCV_ISA_EXT_ZICBOZ))
>>> -        csr_set(CSR_SENVCFG, ENVCFG_CBZE);
>>> +        csr_set(CSR_ENVCFG, ENVCFG_CBZE);
>>>   }
>>>     #ifdef CONFIG_RISCV_ALTERNATIVE
>>> -- 
>>> 2.43.0
>>>
>> After our back and forth on how we determine the existence of the *envcfg
>> CSRs, I wonder if we shouldn't put a comment above this
>> riscv_user_isa_enable() function capturing the [current] decision.
>>
>> Something like
>>
>>   /*
>>    * While the [ms]envcfg CSRs weren't defined until priv spec 1.12,
>>    * they're assumed to be present when an extension is present which
>>    * specifies [ms]envcfg bit(s). Hence, we don't do any additional
>>    * priv spec version checks or CSR probes here.
>>    */
> 
> 
> I was about to read the whole discussion in v2 to understand the v3...thank you
> Drew :) I think it really makes sense to add this comment, do you intend to do
> so Samuel?

Yes, I am about to send a v4 with the changes from the previous discussion,
including a RISCV_ISA_EXT_XLINUXENVCFG bit specifically for the presence of the
[ms]envcfg CSR and a comment like the above.

Regards,
Samuel