linux-kernel - Re: [PATCHv6 06/16] efi: Disable LASS around set_virtual_address

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Message-ID: <cwbzde3qvcvwfz2bgatxfkb2v6wbdvlvoulzvtlqe5mhxr5577@ghpqtyaupkeq>
Date: Fri, 20 Jun 2025 20:50:21 +0300
From: "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
To: Dave Hansen <dave.hansen@...el.com>
Cc: Andy Lutomirski <luto@...nel.org>, 
	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>, 
	Peter Zijlstra <peterz@...radead.org>, Ard Biesheuvel <ardb@...nel.org>, 
	"Paul E. McKenney" <paulmck@...nel.org>, Josh Poimboeuf <jpoimboe@...nel.org>, 
	Xiongwei Song <xiongwei.song@...driver.com>, Xin Li <xin3.li@...el.com>, 
	"Mike Rapoport (IBM)" <rppt@...nel.org>, Brijesh Singh <brijesh.singh@....com>, 
	Michael Roth <michael.roth@....com>, Tony Luck <tony.luck@...el.com>, 
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	linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org, linux-efi@...r.kernel.org, 
	linux-mm@...ck.org
Subject: Re: [PATCHv6 06/16] efi: Disable LASS around
 set_virtual_address_map() EFI call

On Fri, Jun 20, 2025 at 08:55:29AM -0700, Dave Hansen wrote:
> On 6/20/25 06:53, Kirill A. Shutemov wrote:
> > diff --git a/arch/x86/platform/efi/efi.c b/arch/x86/platform/efi/efi.c
> > index 463b784499a8..94c335229697 100644
> > --- a/arch/x86/platform/efi/efi.c
> > +++ b/arch/x86/platform/efi/efi.c
> > @@ -825,11 +825,24 @@ static void __init __efi_enter_virtual_mode(void)
> >  
> >  	efi_sync_low_kernel_mappings();
> >  
> > +	/*
> > +	 * set_virtual_address_map is the only service located at lower
> > +	 * addresses, so we have to temporarily disable LASS around it.
> > +	 * Note that clearing EFLAGS.AC is not enough for this, the whole
> > +	 * LASS needs to be disabled.
> > +	 */
> > +	if (cpu_feature_enabled(X86_FEATURE_LASS))
> > +		cr4_clear_bits(X86_CR4_LASS);
> 
> Could we do it like this instead?
> 
> 	unsigned long lass = cr4_read_shadow() & X86_FEATURE_LASS;
> 	...
> 	cr4_clear_bits(lass);
> 
> >  	status = efi_set_virtual_address_map(efi.memmap.desc_size * count,
> >  					     efi.memmap.desc_size,
> >  					     efi.memmap.desc_version,
> >  					     (efi_memory_desc_t *)pa,
> >  					     efi_systab_phys);
> > +
> > +	if (cpu_feature_enabled(X86_FEATURE_LASS))
> > +		cr4_set_bits(X86_CR4_LASS);
> 
> and:
> 
> 	cr4_set_bits(lass);
> 
> >  	if (status != EFI_SUCCESS) {
> >  		pr_err("Unable to switch EFI into virtual mode (status=%lx)!\n",
> >  		       status);
> 
> That way, neither the presence of X86_FEATURE_LASS nor the ordering of
> setting up X86_CR4_LASS matters.
> 
> Let's say the CPU supports X86_FEATURE_LASS and this code gets called
> before the kernel is ready for LASS. It would break as written above.

Okay, will rework this.

-- 
  Kiryl Shutsemau / Kirill A. Shutemov