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

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Date:   Thu, 11 May 2017 16:37:20 +0100
From:   Mark Rutland <mark.rutland@....com>
To:     Suzuki K Poulose <Suzuki.Poulose@....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 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?

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.