linux-kernel - Re: [Bug report] __arch

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Message-ID: <20230626210604.GLZJn9vA3gcMAuy/eP@fat_crate.local>
Date:   Mon, 26 Jun 2023 23:06:04 +0200
From:   Borislav Petkov <bp@...en8.de>
To:     Sebastian Sumpf <Sebastian.Sumpf@...ode-labs.com>
Cc:     tglx@...utronix.de, mingo@...hat.com, dave.hansen@...ux.intel.com,
        x86@...nel.org, linux-kernel@...r.kernel.org
Subject: Re: [Bug report] __arch_hweight32/64 x86

Hello,

On Mon, Jun 26, 2023 at 10:49:44PM +0200, Sebastian Sumpf wrote:
> The 'ALTERNATIVE' macro checks for the popcnt feature. In case this fails
> the '__sw_hweight' C-function is called from inline assembly with rax and
> rdi as intput/output operands. However, the code does not contain a clobber
> list of any callee safe registers that might be touched by the
> '__sw_height64' C-function.

Which registers are those? Can you be more specific?

> Therefore, these registers will not be restored
> upon function return by the compiler. This leads to varying
> exceptions/bad behavior caused by the thus corrupted registers later
> on.

How do I reproduce what you're observing so that I can take a look?

Thx.

-- 
Regards/Gruss,
    Boris.

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