linux-kernel - Re: [PATCH 2/3] x86/mm: Fix LAM inconsistency during context switch

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Date: Tue, 12 Mar 2024 15:34:55 +0000
From: Yosry Ahmed <yosryahmed@...gle.com>
To: "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
Cc: x86@...nel.org, Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>, 
	Borislav Petkov <bp@...en8.de>, Dave Hansen <dave.hansen@...ux.intel.com>, 
	"H. Peter Anvin" <hpa@...or.com>, Andy Lutomirski <luto@...nel.org>, Peter Zijlstra <peterz@...radead.org>, 
	Rick Edgecombe <rick.p.edgecombe@...el.com>, Andrew Morton <akpm@...ux-foundation.org>, 
	linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/3] x86/mm: Fix LAM inconsistency during context switch

On Tue, Mar 12, 2024 at 02:35:46PM +0200, Kirill A. Shutemov wrote:
> On Tue, Mar 12, 2024 at 03:59:50AM +0000, Yosry Ahmed wrote:
> > diff --git a/arch/x86/kernel/process_64.c b/arch/x86/kernel/process_64.c
> > index 76e91fc68c5f3..748d2b3bdb985 100644
> > --- a/arch/x86/kernel/process_64.c
> > +++ b/arch/x86/kernel/process_64.c
> > @@ -753,10 +753,12 @@ static long prctl_map_vdso(const struct vdso_image *image, unsigned long addr)
> >  static void enable_lam_func(void *__mm)
> >  {
> >  	struct mm_struct *mm = __mm;
> > +	unsigned long lam = mm_lam_cr3_mask(mm);
> > +	u64 untag_mask = mm_untag_mask(mm);
> >  
> 
> Maybe push these mm dereferences inside the if block below?
> I am not sure if compiler can re-order operations past this_cpu_read().

I am not sure either, so I will push them inside. Better safe than
sorry. Will send a v2 shortly, thanks for the reviews!

> 
> >  	if (this_cpu_read(cpu_tlbstate.loaded_mm) == mm) {
> > -		write_cr3(__read_cr3() | mm->context.lam_cr3_mask);
> > -		set_tlbstate_lam_mode(mm);
> > +		write_cr3(__read_cr3() | lam);
> > +		cpu_tlbstate_update_lam(lam, untag_mask);
> >  	}
> >  }
> >  
> 
> -- 
>   Kiryl Shutsemau / Kirill A. Shutemov