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

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Date:   Thu, 11 May 2017 16:54:27 +0100
From:   Suzuki K Poulose <Suzuki.Poulose@....com>
To:     Mark Rutland <mark.rutland@....com>, marc.zyngier@....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 16:37, 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.

Suzuki

>
> If so, I can do something like the above, patching <asm/virt.h> to use
> it for has_vhe().
>
> We don't have a BUG handler at hyp, but that should trigger a hyp panic,
> which I guess is good enough.
>
> Marc, thoughts?
>
> Thanks,
> Mark.
>