linux-kernel - Re: [PATCHv3, RESEND] x86/trampoline: Bypass compat mode in trampoline

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Date: Thu, 25 Jan 2024 01:57:18 -0800
From: Andi Kleen <ak@...ux.intel.com>
To: "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
Cc: Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>,
	Borislav Petkov <bp@...en8.de>,
	Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
	"H. Peter Anvin" <hpa@...or.com>, linux-kernel@...r.kernel.org,
	Kai Huang <kai.huang@...el.com>,
	Sean Christopherson <seanjc@...gle.com>
Subject: Re: [PATCHv3, RESEND] x86/trampoline: Bypass compat mode in
 trampoline_start64() if not needed

> +	/* Paging mode is correct proceed in 64-bit mode */
> +
> +	LOCK_AND_LOAD_REALMODE_ESP lock_rip=1
> +
> +	movw	$__KERNEL_DS, %dx
> +	movl	%edx, %ss
> +	addl	$pa_real_mode_base, %esp
> +	movl	%edx, %ds
> +	movl	%edx, %es
> +	movl	%edx, %fs
> +	movl	%edx, %gs
> +
> +	movl	$pa_trampoline_pgd, %eax
> +	movq	%rax, %cr3
> +
> +	jmpq	*tr_start(%rip)

Still think we should add a far jump here so that we run on a defined
code segment. It probably doesn't matter since there are likely no
IRETs before reloading anyways, but it seems cleaner.

-Andi