linux-kernel - Re: [PATCH] x86/fpu/xstate: clear XSAVE features if DISABLED

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Message-ID: <20230531212907.GHZHe8I/DZUyzIXI2Q@fat_crate.local>
Date:   Wed, 31 May 2023 23:29:07 +0200
From:   Borislav Petkov <bp@...en8.de>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     Jon Kohler <jon@...anix.com>, Dave Hansen <dave.hansen@...el.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "x86@...nel.org" <x86@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        "Chang S. Bae" <chang.seok.bae@...el.com>,
        Kyle Huey <me@...ehuey.com>,
        "neelnatu@...gle.com" <neelnatu@...gle.com>,
        Andrew Cooper <andrew.cooper3@...rix.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        Paolo Bonzini <pbonzini@...hat.com>
Subject: Re: [PATCH] x86/fpu/xstate: clear XSAVE features if DISABLED_MASK set

On Wed, May 31, 2023 at 08:18:34PM +0000, Sean Christopherson wrote:
> Assert that the to-be-checked bit passed to cpu_feature_enabled() is a
> compile-time constant instead of applying the DISABLED_MASK_BIT_SET()
> logic if and only if the bit is a constant.  Conditioning the check on
> the bit being constant instead of requiring the bit to be constant could
> result in compiler specific kernel behavior, e.g. running on hardware that
> supports a disabled feature would return %false if the compiler resolved
> the bit to a constant, but %true if not.

Uff, more mirroring CPUID inconsistencies.

So *actually*, we should clear all those build-time disabled bits from
x86_capability so that this doesn't happen.

> All current usage of cpu_feature_enabled() specifies a hardcoded
> X86_FEATURE_* flag, so this *should* be a glorified nop.

Also, pls add here the exact example which prompted this as I'm sure we
will forget why this was done.

Thx.

-- 
Regards/Gruss,
    Boris.

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