linux-kernel - Re: [PATCH 05/16] arm64: Add flags to check the safety of a capability for late CPU

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Message-ID: <20180130145636.GE5862@e103592.cambridge.arm.com>
Date:   Tue, 30 Jan 2018 14:56:37 +0000
From:   Dave Martin <Dave.Martin@....com>
To:     Suzuki K Poulose <Suzuki.Poulose@....com>
Cc:     mark.rutland@....com, ckadabi@...eaurora.org,
        ard.biesheuvel@...aro.org, marc.zyngier@....com,
        catalin.marinas@....com, will.deacon@....com,
        linux-kernel@...r.kernel.org, jnair@...iumnetworks.com,
        linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH 05/16] arm64: Add flags to check the safety of a
 capability for late CPU

On Tue, Jan 30, 2018 at 11:17:38AM +0000, Suzuki K Poulose wrote:
> On 26/01/18 10:10, Dave Martin wrote:
> >On Tue, Jan 23, 2018 at 12:27:58PM +0000, Suzuki K Poulose wrote:
> >>Add two different flags to indicate if the conflict of a capability
> >>on a late CPU with the current system state
> >>
> >>  1) Can a CPU have a capability when the system doesn't have it ?
> >>
> >>     Most arm64 features could have this set. While erratum work arounds
> >>     cannot have this, as we may miss work arounds.
> >>
> >>  2) Can a CPU miss a capability when the system has it ?
> >>     This could be set for arm64 erratum work arounds as we don't
> >>     care if a CPU doesn't need the work around. However it should
> >>     be clear for features.
> >>
> >>These flags could be added to certain entries based on their nature.
> >>
> >>Signed-off-by: Suzuki K Poulose <suzuki.poulose@....com>
> >>---
> >>  arch/arm64/include/asm/cpufeature.h | 35 +++++++++++++++++++++++++++++++----
> >>  1 file changed, 31 insertions(+), 4 deletions(-)
> >>
> >>diff --git a/arch/arm64/include/asm/cpufeature.h b/arch/arm64/include/asm/cpufeature.h
> >>index 4fd5de8ef33e..27d037bb0451 100644
> >>--- a/arch/arm64/include/asm/cpufeature.h
> >>+++ b/arch/arm64/include/asm/cpufeature.h
> >>@@ -94,10 +94,25 @@ extern struct arm64_ftr_reg arm64_ftr_reg_ctrel0;
> >>  #define SCOPE_SYSTEM				ARM64_CPUCAP_SCOPE_SYSTEM
> >>  #define SCOPE_LOCAL_CPU				ARM64_CPUCAP_SCOPE_LOCAL_CPU
> >>-/* CPU errata detected at boot time based on feature of one or more CPUs */
> >>-#define ARM64_CPUCAP_STRICT_CPU_LOCAL_ERRATUM	(ARM64_CPUCAP_SCOPE_LOCAL_CPU)
> >>-/* CPU feature detected at boot time based on system-wide value of a feature */
> >>-#define ARM64_CPUCAP_BOOT_SYSTEM_FEATURE	(ARM64_CPUCAP_SCOPE_SYSTEM)
> >>+/* Is it safe for a late CPU to have this capability when system doesn't already have */
> >>+#define ARM64_CPUCAP_LATE_CPU_SAFE_TO_HAVE	BIT(2)
> >>+/* Is it safe for a late CPU to miss this capability when system has it */
> >>+#define ARM64_CPUCAP_LATE_CPU_SAFE_TO_MISS	BIT(3)
> >
> >Maybe _OPTIONAL and _PERMITTED would be a bit less verbose?
> >
> >Alternatively,
> >	ARM64_CPUCAP_PERMITTED_FOR_LATE_CPU
> >	ARM64_CPUCAP_OPTIONAL_FOR_LATE_CPU
> 
> Sounds better than what I have. I have picked them up.

Cool, I had resigned myself to probably not winning that one ;)

Cheers
---Dave