linux-kernel - Re: [PATCHv2] x86/trampoline: Bypass compat mode in trampoline

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Date: Mon, 8 Jan 2024 18:26:03 -0800
From: Andi Kleen <ak@...ux.intel.com>
To: "Huang, Kai" <kai.huang@...el.com>
Cc: "kirill.shutemov@...ux.intel.com" <kirill.shutemov@...ux.intel.com>,
	"tglx@...utronix.de" <tglx@...utronix.de>,
	"mingo@...hat.com" <mingo@...hat.com>,
	"bp@...en8.de" <bp@...en8.de>,
	"dave.hansen@...ux.intel.com" <dave.hansen@...ux.intel.com>,
	"hpa@...or.com" <hpa@...or.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"x86@...nel.org" <x86@...nel.org>,
	"seanjc@...gle.com" <seanjc@...gle.com>
Subject: Re: [PATCHv2] x86/trampoline: Bypass compat mode in
 trampoline_start64() if not needed

> This seems depends on the BIOS will always use 4-level paging.  Can we make such
> assumption?

Yes I believe it's fine. All BIOS on 5 level capable systems currently 
only use 4-level when passing control to someone else.

(although I cannot find the quote in the UEFI spec currently, will check
on that)

The UEFI run time environment is defined as 4-level. Changing that would
break compatibility OS supprt at least for run time services.


> 
> > +
> > +	/* 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)
> 
> IIUC you won't be using __KERNEL_CS in this case?  Not sure whether this matters
> though, because the spec says in 64-bit mode the hardware treats CS,DS,ES,SS as
> zero.

That's a good catch. Might be better to use __KERNEL_CS. Otherwise if a
IRET happens later and it tries to reload CS it might fault. Probably
doesn't happen before another reload happens anyways, but it's better
to avoid it.

-Andi