Re: [Xen-devel] [PATCH] x86/boot: Drop all use of lmsw

[Jan Beulich <jbeulich@xxxxxxxx>]

On 19.08.2019 15:42, Andrew Cooper wrote:
> lmsw is an obsolete relic of the 286 processor - so much so that it even lacks
> intercept assistance on AMD processors.
>
> Use a plain mov to %cr0 which is easier to follow, certainly faster to
> virtualise on AMD hardware, and almost certainly a faster microcode path in
> real hardware.
>
> Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
Acked-by: Jan Beulich <jbeulich@xxxxxxxx>

with one further suggestion:

> --- a/xen/arch/x86/boot/trampoline.S
> +++ b/xen/arch/x86/boot/trampoline.S
> @@ -54,9 +54,10 @@ GLOBAL(trampoline_realmode_entry)
>          lidt    bootsym(idt_48)
>          lgdt    bootsym(gdt_48)
>          mov     $1,%bl                    # EBX != 0 indicates we are an AP
> -        xor     %ax, %ax
> -        inc     %ax
> -        lmsw    %ax                       # CR0.PE = 1 (enter protected mode)
> +
> +        mov     $X86_CR0_PE, %eax
> +        mov     %eax, %cr0
Why don't you use %ebx here, allowing the move to %bl to be
dropped at the same time?

Jan

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