Re: [PATCH v2 0/6] x86: Clean up fast syscall return validation

[H. Peter Anvin]

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