linux-kernel - Re: [PATCHv2] arm64/cpufeature: don't use mutex in bringup path

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Date:   Thu, 11 May 2017 17:08:19 +0100
From:   Marc Zyngier <marc.zyngier@....com>
To:     Suzuki K Poulose <Suzuki.Poulose@....com>,
        Mark Rutland <mark.rutland@....com>
Cc:     linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        bigeasy@...utronix.de, catalin.marinas@....com,
        peterz@...radead.org, tglx@...utronix.de, will.deacon@....com
Subject: Re: [PATCHv2] arm64/cpufeature: don't use mutex in bringup path

On 11/05/17 16:54, Suzuki K Poulose wrote:
> On 11/05/17 16:37, Mark Rutland wrote:
>> On Thu, May 11, 2017 at 04:15:38PM +0100, Suzuki K Poulose wrote:
>>> On 11/05/17 16:01, Mark Rutland wrote:
>>>> +static inline bool cpus_have_const_cap(int num)
>>>> +{
>>>> +	if (static_branch_likely(&arm64_const_caps_ready))
>>>> +		return __cpus_have_const_cap(num);
>>>> +	else
>>>> +		return cpus_have_cap(num);
>>>
>>> We use cpus_have_const_cap() from hyp code, via has_vhe() and we could potentially
>>> try to access unmapped kernel data from hyp if we fallback to cpus_have_cap().
>>> However, it looks like we have already set arm64_const_caps_ready, so should not
>>> hit it in practise. May be we could add a stricter version of the helper ?
>>>
>>> static inline cpus_have_const_cap_strict(int num)
>>> {
>>> 	BUG_ON(!static_branch_likely(&arm64_const_caps_ready);
>>> 	return __cpus_have_const_cap(num);
>>> }
>>
>> Just to check, is that the only user of cpus_have_const_cap() at hyp?
> 
> Uh, no we have one more, via system_supports_fpsimd() in __actvate_traps.

Indeed, and I'd definitely expect to see more of that trickling in (if
only to deal with errata).

I'm OK with the BUG_ON version, TBH. It's not pretty, but it will be
perfectly visible if it fires.

Thanks,

	M.
-- 
Jazz is not dead. It just smells funny...