Re: [PATCH v12 07/11] x86/smpboot: Remove early_gdt_descr on 64-bit

[David Woodhouse]

On 28 February 2023 22:48:42 GMT, Brian Gerst <brgerst@xxxxxxxxx> wrote:
>On Tue, Feb 28, 2023 at 5:41 PM David Woodhouse <dwmw2@xxxxxxxxxxxxx> wrote:
>>
>> On Tue, 2023-02-28 at 21:57 +0000, David Woodhouse wrote:
>> >
>> > ----------------
>> > IN:
>> > 0xffffffffa20000b2:  48 31 d2                 xorq     %rdx, %rdx
>> > 0xffffffffa20000b5:  48 8b 82 c0 74 d5 a3     movq     -0x5c2a8b40(%rdx), %rax
>> > 0xffffffffa20000bc:  48 8b a0 58 14 00 00     movq     0x1458(%rax), %rsp
>> > 0xffffffffa20000c3:  48 83 ec 10              subq     $0x10, %rsp
>> > 0xffffffffa20000c7:  66 c7 04 24 7f 00        movw     $0x7f, (%rsp)
>> > 0xffffffffa20000cd:  48 8d 82 00 10 81 a3     leaq     -0x5c7ef000(%rdx), %rax
>> > 0xffffffffa20000d4:  48 89 44 24 02           movq     %rax, 2(%rsp)
>> > 0xffffffffa20000d9:  0f 01 14 24              lgdtq    (%rsp)
>> > 0xffffffffa20000dd:  48 83 c4 10              addq     $0x10, %rsp
>> > 0xffffffffa20000e1:  31 c0                    xorl     %eax, %eax
>> > 0xffffffffa20000e3:  8e d8                    movl     %eax, %ds
>> >
>> > I cannot work out where the value -0x5c7ef000 comes from, but it
>> > doesn't seem to be the 0xb000 you claimed, and my brain is hurting
>> > again...
>>
>> Turning off CONFIG_RANDOMIZE_BASE (or just looking at the vmlinux
>> disassembly instead as Brian did) helps to resolve that FWIW.
>>
>> I've changed it to zero all of %rdx and pushed it back to the v12bis
>> branch.
>
>xorl %edx, %edx is preferred, as a 32-bit operation zero-extends to
>the full 64-bit register.  Using xorq to clear any of the lower 8
>registers adds an unnecessary REX prefix.  Just one of many quirks of
>the x86 instruction set...

Ewww. Couldn't the assembler choose to omit the REX prefix then? It does more tricksy things than that already.

I almost prefer having the prefix but (in the morning) if you prefer I can put it back as it was with a comment about the zero-extension.