linux-kernel - Re: [PATCH 2/3] arm64: Consolidate hotplug notifier for instruction emulation

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Message-ID: <20150116163254.GC32155@leverpostej>
Date:	Fri, 16 Jan 2015 16:32:54 +0000
From:	Mark Rutland <mark.rutland@....com>
To:	Will Deacon <will.deacon@....com>
Cc:	Suzuki Poulose <Suzuki.Poulose@....com>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"leo.yan@...aro.org" <leo.yan@...aro.org>,
	"yexl@...vell.com" <yexl@...vell.com>,
	Catalin Marinas <Catalin.Marinas@....com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/3] arm64: Consolidate hotplug notifier for instruction
 emulation

On Fri, Jan 16, 2015 at 04:07:30PM +0000, Will Deacon wrote:
> On Thu, Jan 15, 2015 at 12:36:05PM +0000, Suzuki K. Poulose wrote:
> > From: "Suzuki K. Poulose" <suzuki.poulose@....com>
> > 
> > As of now each insn_emulation has a cpu hotplug notifier that
> > enables/disables the CPU feature bit for the functionality. This
> > patch re-arranges the code, such that there is only one notifier
> > that runs through the list of registered emulation hooks and runs
> > their corresponding set_hw_mode.
> > 
> > We do nothing when a CPU is dying as we will set the appropriate bits
> > as it comes back online based on the state of the hooks.
> > 
> > Signed-off-by: Suzuki K. Poulose <suzuki.poulose@....com>
> > Signed-off-by: Mark Rutland <mark.rutland@....com>
> > ---
> >  Documentation/arm64/legacy_instructions.txt |    4 +
> >  arch/arm64/include/asm/cpufeature.h         |    2 +
> >  arch/arm64/kernel/armv8_deprecated.c        |  113 +++++++++++++++------------
> >  3 files changed, 69 insertions(+), 50 deletions(-)
> > 
> > diff --git a/Documentation/arm64/legacy_instructions.txt b/Documentation/arm64/legacy_instructions.txt
> > index a3b3da2..0a4dc26 100644
> > --- a/Documentation/arm64/legacy_instructions.txt
> > +++ b/Documentation/arm64/legacy_instructions.txt
> > @@ -27,6 +27,10 @@ behaviours and the corresponding values of the sysctl nodes -
> >    instructions. Using hardware execution generally provides better
> >    performance, but at the loss of ability to gather runtime statistics
> >    about the use of the deprecated instructions.
> > +  Note: Emulation of a deprecated instruction depends on the availability
> > +  of the feature on all the active CPUs. In case of CPU hotplug, if a new
> > +  CPU doesn't support a feature, it could result in the abortion of the
> > +  hotplug operation.
> 
> Is this true? We should be able to *emulate* the instruction anywhere,
> it's the "hardware execution" setting that needs CPU support.

Not quite. In ARMv8 there are three possible cases for endianness
support:

(a) Mixed endian support at all levels
    ID_AA64MMFR0_EL1.BigEnd == 0b0001
    ID_AA64MMFR0_EL1.BigEnd0 == 0b0000 (RES0, invalid)

(b) Mixed endian support at EL0 only
    ID_AA64MMFR0_EL1.BigEnd == 0b0000
    ID_AA64MMFR0_EL1.BigEnd0 == 0b0001
    SCTLR_EL1.EE has a fixed value

(c) No mixed endian support at all
    ID_AA64MMFR0_EL1.BigEnd == 0b0000
    ID_AA64MMFR0_EL1.BigEnd0 == 0b0000
    SCTLR_EL1.{EE,E0E} have fixed values

We can emulate setend in cases (a) and (b), but not (c) unless we
trapped every single instruction that could access memory. I don't think
that's feasible.

Also fun in case (c) is that the kernel may not be able to run on all
CPUs (consider a BE kernel where some CPUs are fixed to LE at EL1).

I hope no-one builds a system where CPUs are mismatched w.r.t.
endianness support.

Thanks,
Mark.
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