linux-kernel - Re: [PATCH v4 05/11] iommu/sva: Assign a PASID to mm on PASID allocation and free it on mm exit

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Date:   Mon, 14 Feb 2022 18:24:39 +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 v4 05/11] iommu/sva: Assign a PASID to mm on PASID
 allocation and free it on mm exit

On Mon, Feb 07 2022 at 15:02, Fenghua Yu wrote:

> PASIDs are process wide. It was attempted to use refcounted PASIDs to
> free them when the last thread drops the refcount. This turned out to
> be complex and error prone. Given the fact that the PASID space is 20
> bits, which allows up to 1M processes to have a PASID associated
> concurrently, PASID resource exhaustion is not a realistic concern.
>
> Therefore it was decided to simplify the approach and stick with lazy
> on demand PASID allocation, but drop the eager free approach and make
> a allocated PASID lifetime bound to the life time of the process.
>
> Get rid of the refcounting mechanisms and replace/rename the interfaces
> to reflect this new approach.
>
> Suggested-by: Dave Hansen <dave.hansen@...ux.intel.com>
> Signed-off-by: Fenghua Yu <fenghua.yu@...el.com>
> Reviewed-by: Tony Luck <tony.luck@...el.com>

Reviewed-by: Thomas Gleixner <tglx@...utronix.de>