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Date:   Tue, 10 Oct 2023 15:54:56 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Borislav Petkov <bp@alien8.de>
Cc:     "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
        Thomas Gleixner <tglx@linutronix.de>,
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        Fei Yang <fei.yang@intel.com>, stable@vger.kernel.org
Subject: Re: [PATCH] x86/alternatives: Disable KASAN on text_poke_early() in
 apply_alternatives()
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On Tue, Oct 10, 2023 at 03:10:54PM +0200, Borislav Petkov wrote:
> On Tue, Oct 10, 2023 at 12:10:56PM +0200, Peter Zijlstra wrote:
> > Now, obviously you really don't want boot_cpu_has() in
> > __VIRTUAL_MASK_SHIFT, that would be really bad (Linus recently
> > complained about how horrible the code-gen is around this already, must
> > not make it far worse).
> 
> You mean a MOV (%rip) and a TEST are so horrible there because it is
> a mask?
> 
> I'd experiment with it when I get a chance...

That gets you a memory-reference and potential cachemiss what should
have been an immediate :/