[RFC] SCTLR_EL1.TIDCP toggling for performance

[RFC] SCTLR_EL1.TIDCP toggling for performance

[Liao, Chang]

Hi, Kristina

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?

Thanks

[1] https://lore.kernel.org/linux-arm-kernel/Yrw3NWkH6D0CgRsF@sirena.org.uk/T/#m5cfdb27b48d9d7e30db73e991fc6c232ba8a7349

-- 
BR
Liao, Chang

Re: [RFC] SCTLR_EL1.TIDCP toggling for performance

[Kristina Martšenko]

Hi Chang,

On 18/07/2025 03:32, Liao, Chang wrote:
> Hi, Kristina
> 
> 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?
> 
> Thanks
> 
> [1] https://lore.kernel.org/linux-arm-kernel/Yrw3NWkH6D0CgRsF@sirena.org.uk/T/#m5cfdb27b48d9d7e30db73e991fc6c232ba8a7349

I don't have any plans to add support for toggling it. You could try sending a
patch for a Kconfig option or kernel command-line parameter. I'm not really
sure what the maintainers' view on supporting IMP-DEF features is.

Thanks,
Kristina

Re: [RFC] SCTLR_EL1.TIDCP toggling for performance

[Liao, Chang]

在 2025/7/19 1:28, Kristina Martšenko 写道:
> Hi Chang,
> 
> On 18/07/2025 03:32, Liao, Chang wrote:
>> Hi, Kristina
>>
>> 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?
>>
>> Thanks
>>
>> [1] https://lore.kernel.org/linux-arm-kernel/Yrw3NWkH6D0CgRsF@sirena.org.uk/T/#m5cfdb27b48d9d7e30db73e991fc6c232ba8a7349
> 
> I don't have any plans to add support for toggling it. You could try sending a
> patch for a Kconfig option or kernel command-line parameter. I'm not really

Kristina, I am going to send a patch for command-line paramter as Catalin suggested.

Thanks.

> sure what the maintainers' view on supporting IMP-DEF features is.
> 
> Thanks,
> Kristina
> 
> 

-- 
BR
Liao, Chang

Re: [RFC] SCTLR_EL1.TIDCP toggling for performance

[Catalin Marinas]

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.

-- 
Catalin

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