Re: [kvm-unit-tests PATCH v2 04/10] x86: Move load_gdt_tss() to desc.c

[Sean Christopherson <seanjc@google.com>]

On Tue, Apr 12, 2022, Varad Gautam wrote:
> diff --git a/lib/x86/setup.c b/lib/x86/setup.c
> index 94e9f86..7dd6677 100644
> --- a/lib/x86/setup.c
> +++ b/lib/x86/setup.c
> @@ -170,7 +170,7 @@ void setup_multiboot(struct mbi_bootinfo *bi)
>  #ifdef CONFIG_EFI
>  
>  /* From x86/efi/efistart64.S */
> -extern void load_gdt_tss(size_t tss_offset);
> +extern void setup_segments64(void);
>  
>  static efi_status_t setup_memory_allocator(efi_bootinfo_t *efi_bootinfo)
>  {
> @@ -275,6 +275,8 @@ static void setup_gdt_tss(void)
>  	/* 64-bit setup_tss does not use the stacktop argument.  */
>  	tss_offset = setup_tss(NULL);
>  	load_gdt_tss(tss_offset);
> +
> +	setup_segments64();

Rather than call back into asm, how about doing this in inline asm?  It's a bit
gross no matter what, but this has the advantage of getting to use KERNEL_CS/DS
without extra magic.

And then the future patch that manipulates MSR_GS_BASE can use the wrmsr() helper.

	asm volatile("mov %0, %%ds\n\t"
		     "mov %0, %%es\n\t"
		     "mov %0, %%fs\n\t"
		     "mov %0, %%gs\n\t"
		     "mov %0, %%ss\n\t"
		     "pushq %1\n\t"
		     "lea 1f(%%rip), %0\n\t"
		     "pushq %0\n\t"
		     "lretq\n\t"
		     "1:"
		     :: "r" ((u64)KERNEL_DS), "i" (KERNEL_CS)
	);


>  efi_status_t setup_efi(efi_bootinfo_t *efi_bootinfo)
> diff --git a/x86/efi/efistart64.S b/x86/efi/efistart64.S
> index ea3d1c0..8eadca5 100644
> --- a/x86/efi/efistart64.S
> +++ b/x86/efi/efistart64.S
> @@ -26,15 +26,8 @@ ptl4:
>  .code64
>  .text
>  
> -.globl load_gdt_tss
> -load_gdt_tss:
> -	/* Load GDT */
> -	lgdt gdt_descr(%rip)
> -
> -	/* Load TSS */
> -	mov %rdi, %rax
> -	ltr %ax
> -
> +.globl setup_segments64
> +setup_segments64:
>  	/* Update data segments */
>  	mov $0x10, %ax /* 3rd entry in gdt64: 32/64-bit data segment */
>  	mov %ax, %ds
> -- 
> 2.32.0
>