From: Suzuki K Poulose <suzuki.poulose@arm.com>
To: Mark Brown <broonie@kernel.org>
Cc: Catalin Marinas <catalin.marinas@arm.com>,
Will Deacon <will@kernel.org>,
linux-arm-kernel@lists.infradead.org
Subject: Re: [PATCH v7 3/4] arm64: Don't use KPTI where we have E0PD
Date: Thu, 7 Nov 2019 15:03:49 +0000 [thread overview]
Message-ID: <091c5d4c-0292-0dde-043f-f5f9f0fac3af@arm.com> (raw)
In-Reply-To: <20191107143750.GG6159@sirena.co.uk>
On 07/11/2019 14:37, Mark Brown wrote:
> On Thu, Nov 07, 2019 at 12:01:10PM +0000, Suzuki K Poulose wrote:
>> On 06/11/2019 13:00, Mark Brown wrote:
>
>>> + /*
>>> + * E0PD does a similar job to KPTI so can be used instead
>>> + * where available.
>>> + */
>>> + if (IS_ENABLED(CONFIG_ARM64_E0PD)) {
>>> + ftr = read_sysreg_s(SYS_ID_AA64MMFR2_EL1);
>
>> I am trying to write down the rationale of checking this per-CPU.
>
>> Given that this gets run on all individual CPUs, via unmap_kernel_at_el0()
>> and the decision of choosing KPTI is affected by the lack of the E0PD feature
>> when it is helpful, having CPU local check is fine. Also this gives us the
>> advantage of choosing an nG mapping when the boot CPU indicates the need.
>
> Well, it's mainly the fact that this runs really early on in boot before
> the cpufeature code has fully initialized so as with the existing code
> immediately below for identifying TX1 we can't rely on the cpufeature
> code being done.
Yes, I acknowledge that. I was writing it down to clear why this was
fine and why it has its own advantage. This may not be obvious for
someone who reads it later. So having this in a comment helps to
avoid staring at it.
Suzuki
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next prev parent reply other threads:[~2019-11-07 15:03 UTC|newest]
Thread overview: 13+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-11-06 13:00 [PATCH v7 0/4] E0PD support Mark Brown
2019-11-06 13:00 ` [PATCH v7 1/4] arm64: Add initial support for E0PD Mark Brown
2019-11-07 10:12 ` Suzuki K Poulose
2019-11-07 11:55 ` Mark Brown
2019-11-06 13:00 ` [PATCH v7 2/4] arm64: Factor out checks for KASLR in KPTI code into separate function Mark Brown
2019-11-06 13:00 ` [PATCH v7 3/4] arm64: Don't use KPTI where we have E0PD Mark Brown
2019-11-07 12:01 ` Suzuki K Poulose
2019-11-07 14:37 ` Mark Brown
2019-11-07 15:03 ` Suzuki K Poulose [this message]
2019-11-08 14:10 ` Mark Brown
2019-11-07 14:48 ` Mark Brown
2019-11-06 13:00 ` [PATCH v7 4/4] arm64: Use a variable to store non-global mappings decision Mark Brown
2019-11-07 11:11 ` Suzuki K Poulose
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