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Date:   Fri, 6 Oct 2023 16:58:27 -0700
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Subject: Re: [PATCH v2 0/6] x86: Clean up fast syscall return validation
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From:   "H. Peter Anvin" <hpa@zytor.com>
To:     Ingo Molnar <mingo@kernel.org>, Brian Gerst <brgerst@gmail.com>
Cc:     linux-kernel@vger.kernel.org, x86@kernel.org,
        Thomas Gleixner <tglx@linutronix.de>,
        Borislav Petkov <bp@alien8.de>,
        Andy Lutomirski <luto@kernel.org>,
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        Linus Torvalds <torvalds@linux-foundation.org>,
        Uros Bizjak <ubizjak@gmail.com>,
        Denys Vlasenko <dvlasenk@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Josh Poimboeuf <jpoimboe@redhat.com>
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On 10/6/23 11:59, H. Peter Anvin wrote:
> 
> Incidentally, it is possible to save one instruction and use only *one* 
> alternative immediate:
> 
>      leaq (%rax,%rax),%rdx
>      xorq %rax,%rdx
>      shrq $(63 - LA),%rdx        # Yes, 63, not 64
>      # ZF=1 if canonical
> 
> This works because if bit [x] is set in the output, then bit [x] and 
> [x-1] in the input are different (bit [-1] considered to be zero); and 
> by definition a bit is canonical if and only if all the bits [63:LA] are 
> identical, thus bits [63:LA+1] in the output must all be zero.
> 

Yes, I'm a doofus. Bits [63:LA-1] must be identical, so 64 is correct :$)

	-hpa