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From: Julien Grall <julien.grall@arm.com>
To: Catalin Marinas <catalin.marinas@arm.com>
Cc: kvmarm@lists.cs.columbia.edu, ard.biesheuvel@linaro.org,
	marc.zyngier@arm.com, will.deacon@arm.com,
	linux-kernel@vger.kernel.org, Dave.Martin@arm.com,
	linux-arm-kernel@lists.infradead.org
Subject: Re: [PATCH v5 3/3] arm64/fpsimd: Don't disable softirq when touching FPSIMD/SVE state
Date: Tue, 4 Jun 2019 11:35:11 +0100
Message-ID: <5c159919-d220-f8dc-d609-b1b1b2844d4b@arm.com> (raw)
In-Reply-To: <20190603212104.mhz7vvj7afb2p3yr@mbp>

Hi Catalin,

On 6/3/19 10:21 PM, Catalin Marinas wrote:
> On Mon, Jun 03, 2019 at 05:25:34PM +0100, Catalin Marinas wrote:
>> On Tue, May 21, 2019 at 06:21:39PM +0100, Julien Grall wrote:
>>> Since a softirq is supposed to check may_use_simd() anyway before
>>> attempting to use FPSIMD/SVE, there is limited reason to keep softirq
>>> disabled when touching the FPSIMD/SVE context. Instead, we can simply
>>> disable preemption and mark the FPSIMD/SVE context as in use by setting
>>> CPU's fpsimd_context_busy flag.
>> [...]
>>> +static void get_cpu_fpsimd_context(void)
>>> +{
>>> +	preempt_disable();
>>> +	__get_cpu_fpsimd_context();
>>> +}
>>
>> Is there anything that prevents a softirq being invoked between
>> preempt_disable() and __get_cpu_fpsimd_context()?
> 
> Actually, it shouldn't matter as the softirq finishes using the fpsimd
> before the thread is resumed.

If the softirqs is handled in a thread (i.e ksoftirqd), then 
preempt_disable() will prevent them to run.

For softirq running on return from interrupt context, they will finish 
before using fpsimd before the thread is resumed.

Softirq running after __get_cpu_fpsimd_context() is called will not be 
able to use FPSIMD (may_use_simd() returns false).

Cheers,

-- 
Julien Grall
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Thread overview: 9+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-05-21 17:21 [PATCH v5 0/3] " Julien Grall
2019-05-21 17:21 ` [PATCH v5 1/3] arm64/fpsimd: Remove the prototype for sve_flush_cpu_state() Julien Grall
2019-05-21 17:21 ` [PATCH v5 2/3] arch/arm64: fpsimd: Introduce fpsimd_save_and_flush_cpu_state() and use it Julien Grall
2019-06-04 11:12   ` Catalin Marinas
2019-06-04 11:52     ` Marc Zyngier
2019-05-21 17:21 ` [PATCH v5 3/3] arm64/fpsimd: Don't disable softirq when touching FPSIMD/SVE state Julien Grall
2019-06-03 16:25   ` Catalin Marinas
2019-06-03 21:21     ` Catalin Marinas
2019-06-04 10:35       ` Julien Grall [this message]

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