linux-kernel - Re: [PATCHv3 4/8] x86/mm: Handle LAM on context switch

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Date:   Thu, 16 Jun 2022 11:08:07 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
Cc:     Dave Hansen <dave.hansen@...ux.intel.com>,
        Andy Lutomirski <luto@...nel.org>, x86@...nel.org,
        Kostya Serebryany <kcc@...gle.com>,
        Andrey Ryabinin <ryabinin.a.a@...il.com>,
        Andrey Konovalov <andreyknvl@...il.com>,
        Alexander Potapenko <glider@...gle.com>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        "H . J . Lu" <hjl.tools@...il.com>,
        Andi Kleen <ak@...ux.intel.com>,
        Rick Edgecombe <rick.p.edgecombe@...el.com>,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org, namit@...are.com
Subject: Re: [PATCHv3 4/8] x86/mm: Handle LAM on context switch

On Fri, Jun 10, 2022 at 05:35:23PM +0300, Kirill A. Shutemov wrote:

> diff --git a/arch/x86/include/asm/tlbflush.h b/arch/x86/include/asm/tlbflush.h
> index 4af5579c7ef7..5b93dad93ff4 100644
> --- a/arch/x86/include/asm/tlbflush.h
> +++ b/arch/x86/include/asm/tlbflush.h
> @@ -86,6 +86,9 @@ struct tlb_state {
>  		unsigned long		last_user_mm_spec;
>  	};
>  
> +#ifdef CONFIG_X86_64
> +	u64 lam_cr3_mask;
> +#endif
>  	u16 loaded_mm_asid;
>  	u16 next_asid;
>  

Urgh.. so there's a comment there that states:

/*
 * 6 because 6 should be plenty and struct tlb_state will fit in two cache
 * lines.
 */
#define TLB_NR_DYN_ASIDS        6

And then look at tlb_state:

struct tlb_state {
	struct mm_struct *         loaded_mm;            /*     0     8 */
	union {
		struct mm_struct * last_user_mm;         /*     8     8 */
		long unsigned int  last_user_mm_spec;    /*     8     8 */
	};                                               /*     8     8 */
	u16                        loaded_mm_asid;       /*    16     2 */
	u16                        next_asid;            /*    18     2 */
	bool                       invalidate_other;     /*    20     1 */

	/* XXX 1 byte hole, try to pack */

	short unsigned int         user_pcid_flush_mask; /*    22     2 */
	long unsigned int          cr4;                  /*    24     8 */
	struct tlb_context         ctxs[6];              /*    32    96 */

	/* size: 128, cachelines: 2, members: 8 */
	/* sum members: 127, holes: 1, sum holes: 1 */
};

If you add that u64 as you do, you'll wreck all that.

Either use that one spare byte, or find room elsewhere I suppose.