linux-kernel - Re: [PATCHv8 00/11] Linear Address Masking enabling

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Date:   Wed, 14 Sep 2022 08:31:56 -0700
From:   Ashok Raj <ashok.raj@...el.com>
To:     "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
CC:     "Kirill A. Shutemov" <kirill@...temov.name>,
        Ashok Raj <ashok_raj@...ux.intel.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "Andy Lutomirski" <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.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>,
        Taras Madan <tarasmadan@...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>,
        Jacon Jun Pan <jacob.jun.pan@...el.com>,
        Jason Gunthorpe <jgg@...dia.com>,
        Joerg Roedel <joro@...tes.org>, Ashok Raj <ashok.raj@...el.com>
Subject: Re: [PATCHv8 00/11] Linear Address Masking enabling

On Wed, Sep 14, 2022 at 06:18:18PM +0300, Kirill A. Shutemov wrote:
> > > > > 
> > > > > The patch below implements something like this. It is PoC, build-tested only.
> > > > > 
> > > > > To be honest, I hate it. It is clearly a layering violation. It feels
> > > > > dirty. But I don't see any better way as we tie orthogonal features
> > > > > together.
> > > > > 
> > > > > Also I have no idea how to make forced PASID allocation if LAM enabled.
> > > > > What the API has to look like?
> > > > 
> > > > Jacob, Ashok, any comment on this part?
> > > > 
> > > > I expect in many cases LAM will be enabled very early (like before malloc
> > > > is functinal) in process start and it makes PASID allocation always fail.
> > > > 
> > > > Any way out?
> > > 
> > > We need closure on this to proceed. Any clue?
> > 
> > Failing PASID allocation seems like the right thing to do here. If the
> > application is explicitly allocating PASID's it can opt-out using the
> > similar mechanism you have for LAM enabling. So user takes
> > responsibility for sanitizing pointers. 
> > 
> > If some library is using an accelerator without application knowledge,
> > that would use the failure as a mechanism to use an alternate path if
> > one exists.
> > 
> > I don't know if both LAM and SVM need a separate forced opt-in (or i
> > don't have an opinion rather). Is this what you were asking? 
> > 
> > + Joerg, JasonG in case they have an opinion.
> 
> My point is that the patch provides a way to override LAM vs. PASID mutual
> exclusion, but only if PASID allocated first. If we enabled LAM before
> PASID is allcoated there's no way to forcefully allocate PASID, bypassing
> LAM check. I think there should be one, no?

Yes, we should have one for force enabling SVM too if the application
asks for forgiveness.