Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.38 as permitted sender) client-ip=23.128.96.38;
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>,
        Ingo Molnar <mingo@redhat.com>,
        Dave Hansen <dave.hansen@linux.intel.com>, x86@kernel.org,
        "H. Peter Anvin" <hpa@zytor.com>,
        Andrey Ryabinin <ryabinin.a.a@gmail.com>,
        Alexander Potapenko <glider@google.com>,
        Andrey Konovalov <andreyknvl@gmail.com>,
        Dmitry Vyukov <dvyukov@google.com>,
        Vincenzo Frascino <vincenzo.frascino@arm.com>,
        kasan-dev@googlegroups.com, linux-kernel@vger.kernel.org,
        Fei Yang <fei.yang@intel.com>, stable@vger.kernel.org
Subject: Re: [PATCH] x86/alternatives: Disable KASAN on text_poke_early() in
 apply_alternatives()
Message-ID: <20231010135456.GL377@noisy.programming.kicks-ass.net>
References: <20231010053716.2481-1-kirill.shutemov@linux.intel.com>
 <20231010081938.GBZSUJGlSvEkFIDnES@fat_crate.local>
 <20231010101056.GF377@noisy.programming.kicks-ass.net>
 <20231010131054.GHZSVNXhruJIx0iCzq@fat_crate.local>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20231010131054.GHZSVNXhruJIx0iCzq@fat_crate.local>
Precedence: bulk

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 :/