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Re: [PATCH v3 09/21] arm64: cpufeature: Add global feature override facility In-Reply-To: References: <20210111132811.2455113-1-maz@kernel.org> <20210111132811.2455113-10-maz@kernel.org> <20210111184154.GC17941@gaia> <129db8bd3913a90c96d4cfe4f55e27a0@kernel.org> User-Agent: Roundcube Webmail/1.4.9 Message-ID: X-Sender: maz@kernel.org X-SA-Exim-Connect-IP: 51.254.78.96 X-SA-Exim-Rcpt-To: suzuki.poulose@arm.com, catalin.marinas@arm.com, linux-arm-kernel@lists.infradead.org, kvmarm@lists.cs.columbia.edu, linux-kernel@vger.kernel.org, will@kernel.org, mark.rutland@arm.com, dbrazdil@google.com, alexandru.elisei@arm.com, ardb@kernel.org, jingzhangos@google.com, pajay@qti.qualcomm.com, psodagud@codeaurora.org, sramana@codeaurora.org, james.morse@arm.com, julien.thierry.kdev@gmail.com, kernel-team@android.com X-SA-Exim-Mail-From: maz@kernel.org X-SA-Exim-Scanned: No (on disco-boy.misterjones.org); SAEximRunCond expanded to false X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) 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(Exim 4.94) (envelope-from ) id 1kzIDG-006ws6-Kb; Tue, 12 Jan 2021 11:51:50 +0000 MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit Date: Tue, 12 Jan 2021 11:51:50 +0000 From: Marc Zyngier To: Suzuki K Poulose Cc: Catalin Marinas , linux-arm-kernel@lists.infradead.org, kvmarm@lists.cs.columbia.edu, linux-kernel@vger.kernel.org, Will Deacon , Mark Rutland , David Brazdil , Alexandru Elisei , Ard Biesheuvel , Jing Zhang , Ajay Patil , Prasad Sodagudi , Srinivas Ramana , James Morse , Julien Thierry , kernel-team@android.com Subject: Re: [PATCH v3 09/21] arm64: cpufeature: Add global feature override facility In-Reply-To: References: <20210111132811.2455113-1-maz@kernel.org> <20210111132811.2455113-10-maz@kernel.org> <20210111184154.GC17941@gaia> <129db8bd3913a90c96d4cfe4f55e27a0@kernel.org> User-Agent: Roundcube Webmail/1.4.9 Message-ID: X-Sender: maz@kernel.org X-SA-Exim-Connect-IP: 51.254.78.96 X-SA-Exim-Rcpt-To: suzuki.poulose@arm.com, catalin.marinas@arm.com, linux-arm-kernel@lists.infradead.org, kvmarm@lists.cs.columbia.edu, linux-kernel@vger.kernel.org, will@kernel.org, mark.rutland@arm.com, dbrazdil@google.com, alexandru.elisei@arm.com, ardb@kernel.org, jingzhangos@google.com, pajay@qti.qualcomm.com, psodagud@codeaurora.org, sramana@codeaurora.org, james.morse@arm.com, julien.thierry.kdev@gmail.com, kernel-team@android.com X-SA-Exim-Mail-From: maz@kernel.org X-SA-Exim-Scanned: No (on disco-boy.misterjones.org); SAEximRunCond expanded to false Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 2021-01-12 11:50, Marc Zyngier wrote: > Hi Suzuki, > > On 2021-01-12 09:17, Suzuki K Poulose wrote: >> Hi Marc, >> >> On 1/11/21 7:48 PM, Marc Zyngier wrote: > > [...] > >>> diff --git a/arch/arm64/kernel/cpufeature.c >>> b/arch/arm64/kernel/cpufeature.c >>> index 894af60b9669..00d99e593b65 100644 >>> --- a/arch/arm64/kernel/cpufeature.c >>> +++ b/arch/arm64/kernel/cpufeature.c >>> @@ -774,6 +774,7 @@ static void __init init_cpu_ftr_reg(u32 sys_reg, >>> u64 new) >>>      u64 strict_mask = ~0x0ULL; >>>      u64 user_mask = 0; >>>      u64 valid_mask = 0; >>> +    u64 override_val = 0, override_mask = 0; >>> >>>      const struct arm64_ftr_bits *ftrp; >>>      struct arm64_ftr_reg *reg = get_arm64_ftr_reg(sys_reg); >>> @@ -781,9 +782,35 @@ static void __init init_cpu_ftr_reg(u32 sys_reg, >>> u64 new) >>>      if (!reg) >>>          return; >>> >>> +    if (reg->override_mask && reg->override_val) { >>> +        override_mask = *reg->override_mask; >>> +        override_val = *reg->override_val; >>> +    } >>> + >>>      for (ftrp = reg->ftr_bits; ftrp->width; ftrp++) { >>>          u64 ftr_mask = arm64_ftr_mask(ftrp); >>>          s64 ftr_new = arm64_ftr_value(ftrp, new); >>> +        s64 ftr_ovr = arm64_ftr_value(ftrp, override_val); >>> + >>> +        if ((ftr_mask & override_mask) == ftr_mask) { >>> +            if (ftr_ovr < ftr_new) { >> >> Here we assume that all the features are FTR_LOWER_SAFE. We could >> probably use arm64_ftr_safe_value(ftrp, ftr_new, ftr_ovr) here ? >> That would cover us for both HIGHER_SAFE and LOWER_SAFE features. >> However that may be restrictive for FTR_EXACT, as we the safe >> value would be set to "ftr->safe_val". I guess that may be better >> than forcing to use an unsafe value for the boot CPU, which could >> anyway conflict with the other CPUs and eventually trigger the >> ftr alue to be safe_val. > > I like the idea of using the helper, as it cleanups up the code a bit. > However, not being to set a feature to a certain value could be > restrictive, > as in general, it means that we can only disable a feature and not > adjust > its level of support. > > Take PMUVER for example: with the helper, I can't override it from v8.4 > to > v8.1. I can only go to v8.0. Actually, we can only *disable* the PMU altogether. Same question though... M. > > Is it something we care about? > > Thanks, > > M. -- Jazz is not dead. It just smells funny...