linux-kernel - Re: [PATCH v4 12/12] x86/traps: Fix up invalid PASID

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Date:   Fri, 26 Jun 2020 20:15:50 +0200
From:   Borislav Petkov <bp@...en8.de>
To:     "Luck, Tony" <tony.luck@...el.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Fenghua Yu <fenghua.yu@...el.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Joerg Roedel <joro@...tes.org>, Ingo Molnar <mingo@...hat.com>,
        H Peter Anvin <hpa@...or.com>,
        David Woodhouse <dwmw2@...radead.org>,
        Lu Baolu <baolu.lu@...ux.intel.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Jean-Philippe Brucker <jean-philippe@...aro.org>,
        Christoph Hellwig <hch@...radeed.org>,
        Ashok Raj <ashok.raj@...el.com>,
        Jacob Jun Pan <jacob.jun.pan@...el.com>,
        Dave Jiang <dave.jiang@...el.com>,
        Sohil Mehta <sohil.mehta@...el.com>,
        Ravi V Shankar <ravi.v.shankar@...el.com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        x86 <x86@...nel.org>, iommu@...ts.linux-foundation.org
Subject: Re: [PATCH v4 12/12] x86/traps: Fix up invalid PASID

On Fri, Jun 26, 2020 at 11:10:00AM -0700, Luck, Tony wrote:
> Do we have a standard way of coding for a feature that depends on multiple
> other features?  For this case the system needs to both suport the ENQCMD
> instruction, and also have kernel code that programs the IOMMU.

Yes, you need both. Consider distros enabling everything so that they
can ship a single kernel image.

> And/or guidance on when to use each of the very somewhat simlar looking
> 	boot_cpu_has()
> 	static_cpu_has()

See the comment over _static_cpu_has() in arch/x86/include/asm/cpufeature.h

In the exception path you should use the static_ variant.

> 	IS_ENABLED()

This is testing CONFIG_ settings, i.e., build time.

> 	cpu_feature_enabled()

This too, indirectly. See

arch/x86/include/asm/disabled-features.h

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette