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

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Date:   Tue, 28 Jun 2022 16:33:21 -0700
From:   Andy Lutomirski <luto@...nel.org>
To:     "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Peter Zijlstra <peterz@...radead.org>
Cc:     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
Subject: Re: [PATCHv3 4/8] x86/mm: Handle LAM on context switch

On 6/10/22 07:35, Kirill A. Shutemov wrote:
> Linear Address Masking mode for userspace pointers encoded in CR3 bits.
> The mode is selected per-thread. Add new thread features indicate that the
> thread has Linear Address Masking enabled.
> 
> switch_mm_irqs_off() now respects these flags and constructs CR3
> accordingly.
> 
> The active LAM mode gets recorded in the tlb_state.
> 
> Signed-off-by: Kirill A. Shutemov <kirill.shutemov@...ux.intel.com>
> ---
>   arch/x86/include/asm/mmu.h         |  1 +
>   arch/x86/include/asm/mmu_context.h | 24 ++++++++++++
>   arch/x86/include/asm/tlbflush.h    |  3 ++
>   arch/x86/mm/tlb.c                  | 62 ++++++++++++++++++++++--------
>   4 files changed, 75 insertions(+), 15 deletions(-)
> 
> diff --git a/arch/x86/include/asm/mmu.h b/arch/x86/include/asm/mmu.h
> index 5d7494631ea9..d150e92163b6 100644
> --- a/arch/x86/include/asm/mmu.h
> +++ b/arch/x86/include/asm/mmu.h
> @@ -40,6 +40,7 @@ typedef struct {
>   
>   #ifdef CONFIG_X86_64
>   	unsigned short flags;
> +	u64 lam_cr3_mask;
>   #endif
>   
>   	struct mutex lock;
> diff --git a/arch/x86/include/asm/mmu_context.h b/arch/x86/include/asm/mmu_context.h
> index b8d40ddeab00..e6eac047c728 100644
> --- a/arch/x86/include/asm/mmu_context.h
> +++ b/arch/x86/include/asm/mmu_context.h
> @@ -91,6 +91,29 @@ static inline void switch_ldt(struct mm_struct *prev, struct mm_struct *next)
>   }
>   #endif
>   
> +#ifdef CONFIG_X86_64
> +static inline u64 mm_cr3_lam_mask(struct mm_struct *mm)
> +{
> +	return mm->context.lam_cr3_mask;
> +}
> +
> +static inline void dup_lam(struct mm_struct *oldmm, struct mm_struct *mm)
> +{
> +	mm->context.lam_cr3_mask = oldmm->context.lam_cr3_mask;
> +}
> +
> +#else
> +
> +static inline u64 mm_cr3_lam_mask(struct mm_struct *mm)
> +{
> +	return 0;
> +}
> +
> +static inline void dup_lam(struct mm_struct *oldmm, struct mm_struct *mm)
> +{
> +}
> +#endif

Do we really need the ifdeffery here?  I see no real harm in having the 
field exist on 32-bit -- we don't care much about performance for 32-bit 
kernels.

> -	if (real_prev == next) {
> +	if (real_prev == next && prev_lam == new_lam) {
>   		VM_WARN_ON(this_cpu_read(cpu_tlbstate.ctxs[prev_asid].ctx_id) !=
>   			   next->context.ctx_id);

This looks wrong to me.  If we change threads within the same mm but lam 
changes (which is certainly possible by a race if nothing else) then 
this will go down the "we really are changing mms" path, not the "we're 
not changing but we might need to flush something" path.