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

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Message-ID: <E4904B95-C2EC-4FAF-9CB7-8F377CEA6972@zytor.com>
Date: Thu, 25 Jan 2024 05:07:47 -0800
From: "H. Peter Anvin" <hpa@...or.com>
To: Andi Kleen <ak@...ux.intel.com>,
        "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,
        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

On January 25, 2024 1:57:18 AM PST, Andi Kleen <ak@...ux.intel.com> wrote:
>> +	/* 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
>

Agreed.