Re: [RFC] SCTLR_EL1.TIDCP toggling for performance

[Liao, Chang]

在 2025/7/19 3:03, Catalin Marinas 写道:
> On Fri, Jul 18, 2025 at 10:32:00AM +0800, Liao, Chang wrote:
>> I've reviewed your patch [1] for FEAT_TIDCP1 support, which by default traps EL0
>> accesses to implementation-defined system registers and instructions at EL1/EL2.
>>
>> Do you have any plans to add support for toggling the SCTLR_EL1.TIDCP1 bit? I'm
>> encountering performance degradation on CPU where certain implementation-defined
>> registers and instructions are designed for EL0 performance use. The trapping
>> overhead is substantial enough to compromise any benefits, and it's even worse
>> in virtualization. Therefore, I'm hoping there's a way to clear the SCTLR_EL1.TIDCP1
>> bit on such platforms, perhaps via a kernel config option or command-line parameter.
>> Alternatively, do you have a better solution for gracefully toggling this bit on
>> and off?
> 
> Given that we don't know what imp def functionality is available, what
> side-effects it has, we should not allow user-space to toggle such bit,
> nor allow the user access to those registers.
> 
> System-wide, passing id_aa64mmfr1.tidcp1=0 on the kernel command line
> may work.

Thanks for the suggestion. I'll prepare a patch to support id_aa64mmfr1.tidcp1=0
on the kernel command line.

> 

-- 
BR
Liao, Chang