linux-kernel - Re: [PATCH v3 04/11] kernel/fork: Initialize mm's PASID

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Message-ID: <87wniab4kb.ffs@tglx>
Date:   Sat, 05 Feb 2022 00:22:12 +0100
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Fenghua Yu <fenghua.yu@...el.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Andy Lutomirski <luto@...nel.org>,
        Tony Luck <tony.luck@...el.com>,
        Lu Baolu <baolu.lu@...ux.intel.com>,
        Joerg Roedel <joro@...tes.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Jacob Pan <jacob.jun.pan@...ux.intel.com>,
        Ashok Raj <ashok.raj@...el.com>,
        Ravi V Shankar <ravi.v.shankar@...el.com>
Cc:     iommu@...ts.linux-foundation.org, x86 <x86@...nel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Fenghua Yu <fenghua.yu@...el.com>
Subject: Re: [PATCH v3 04/11] kernel/fork: Initialize mm's PASID

On Fri, Jan 28 2022 at 12:28, Fenghua Yu wrote:
> A new mm doesn't have a PASID yet when it's created. Initialize
> the mm's PASID on fork() or for init_mm to INVALID_IOASID (-1).

I must be missing something here.

> diff --git a/include/linux/sched/mm.h b/include/linux/sched/mm.h
> index aa5f09ca5bcf..c74d1edbac2f 100644
> --- a/include/linux/sched/mm.h
> +++ b/include/linux/sched/mm.h
> @@ -8,6 +8,7 @@
>  #include <linux/mm_types.h>
>  #include <linux/gfp.h>
>  #include <linux/sync_core.h>
> +#include <linux/ioasid.h>
>  
>  /*
>   * Routines for handling mm_structs
> @@ -433,4 +434,13 @@ static inline void membarrier_update_current_mm(struct mm_struct *next_mm)
>  }
>  #endif
>  
> +#ifdef CONFIG_IOMMU_SVA
> +static inline void mm_pasid_init(struct mm_struct *mm)
> +{
> +	mm->pasid = INVALID_IOASID;
> +}
> +#else
> +static inline void mm_pasid_init(struct mm_struct *mm) {}
> +#endif
> +
>  #endif /* _LINUX_SCHED_MM_H */

So this adds mm_pasid_init() to linux/sched/mm.h which replaces:

> -static void mm_init_pasid(struct mm_struct *mm)
> -{
> -#ifdef CONFIG_IOMMU_SVA
> -	mm->pasid = INIT_PASID;
> -#endif
> -}
> -

I.e. already existing code which is initializing mm->pasid with
INIT_PASID (0) while the replacement initializes it with INVALID_IOASID
(-1).

The change log does not have any information about why INIT_PASID is the
wrong initialization value and why this change is not having any side
effects.

It neither mentions why having this in a global available header makes
sense when the only call site is in the C file from which the already
existing function is removed.

Thanks,

        tglx