linux-kernel - Re: [RFC] SCTLR_EL1.TIDCP toggling for performance

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Message-ID: <d84b1955-3ee5-4c12-ba3c-1a45ac4d3700@huawei.com>
Date: Tue, 22 Jul 2025 09:47:18 +0800
From: "Liao, Chang" <liaochang1@...wei.com>
To: Catalin Marinas <catalin.marinas@....com>
CC: <kristina.martsenko@....com>, <will@...nel.org>, <mark.rutland@....com>,
	<sashal@...nel.org>, <yangjiangshui@...artners.com>, <zouyipeng@...wei.com>,
	<justin.he@....com>, <zengheng4@...wei.com>, <yangyicong@...ilicon.com>,
	ruanjinjie <ruanjinjie@...wei.com>, <inux-arm-kernel@...ts.infradead.org>,
	<linux-kernel@...r.kernel.org>
Subject: Re: [RFC] SCTLR_EL1.TIDCP toggling for performance



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