linux-kernel - Re: [PATCH 01/13] arm/arm64: Add new is_kernel_in_hyp

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Message-ID: <20150709101244.GC20105@leverpostej>
Date:	Thu, 9 Jul 2015 11:12:44 +0100
From:	Mark Rutland <mark.rutland@....com>
To:	Marc Zyngier <marc.zyngier@....com>
Cc:	Catalin Marinas <Catalin.Marinas@....com>,
	Will Deacon <Will.Deacon@....com>,
	Christoffer Dall <christoffer.dall@...aro.org>,
	"kvmarm@...ts.cs.columbia.edu" <kvmarm@...ts.cs.columbia.edu>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"kvm@...r.kernel.org" <kvm@...r.kernel.org>
Subject: Re: [PATCH 01/13] arm/arm64: Add new is_kernel_in_hyp_mode predicate

On Thu, Jul 09, 2015 at 11:05:34AM +0100, Marc Zyngier wrote:
> On 09/07/15 10:42, Mark Rutland wrote:
> > Hi,
> > 
> >> +static inline bool is_kernel_in_hyp_mode(void)
> >> +{
> >> +	u64 el;
> >> +
> >> +	asm("mrs %0, CurrentEL" : "=r" (el));
> >> +	return el == CurrentEL_EL2;
> >> +}
> > 
> > If you can include cputype.h, I think this can be:
> > 
> > static inline bool is_kernel_in_hyp_mode(void)
> > {
> > 	return read_cpuid(CurrentEL) == CurrentEL_EL2;
> > }
> 
> This would indeed work, but CurrentEL is hardly an ID register. I feel
> slightly uncomfortable using read_cpuid (which might return a cached
> version at some point) for random system registers.
> 
> Thoughts?

I have no strong feelings either way, but I agree with the general
uneasiness w.r.t. what read_cpuid can be expected to do.

Elsewhere we just use inline asm to read non CPUID system registers, so
let's leave your patch as it was.

Thanks,
Mark.
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