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

Brian Gerst
2023-02-28T22:48:58Z

On Tue, Feb 28, 2023 at 5:41 PM David Woodhouse <dwmw2@infradead.org> 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...

--
Brian Gerst