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

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Date:   Tue, 20 Sep 2022 11:41:04 -0700
From:   Jacob Pan <jacob.jun.pan@...ux.intel.com>
To:     Jason Gunthorpe <jgg@...dia.com>
Cc:     Dave Hansen <dave.hansen@...el.com>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Jacob Pan <jacob.jun.pan@...el.com>,
        Ashok Raj <ashok.raj@...el.com>,
        "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,
        Joerg Roedel <joro@...tes.org>, jacob.jun.pan@...ux.intel.com
Subject: Re: [PATCHv8 00/11] Linear Address Masking enabling

Hi Jason,

On Tue, 20 Sep 2022 13:27:27 -0300, Jason Gunthorpe <jgg@...dia.com> wrote:

> On Tue, Sep 20, 2022 at 09:06:32AM -0700, Dave Hansen wrote:
> > On 9/20/22 06:14, Jason Gunthorpe wrote:  
> > > For this I would rather have a function that queries the format of the
> > > page table under the mm_struct and we have enum values like
> > > INTEL_NORMAL and INTEL_LAM as possible values.
> > > 
> > > The iommu driver will block incompatible page table formats, and when
> > > it starts up it should assert something that blocks changing the
> > > format.  
> > 
> > That doesn't sound too bad.  Except, please don't call it a "page table
> > format".  The format of the page tables does not change with LAM.  It's
> > entirely how the CPU interprets addresses that changes.  
> 
> Sure it does. The rules for how the page table is walked change. The
> actual bits stored in memory might not be different, but that doesn't
> mean the format didn't change. If it didn't change we wouldn't have an
> incompatibility with the IOMMU HW walker.

There are many CPU-IOMMU compatibility checks before we do for SVA,e.g. we
check paging mode in sva_bind. We are delegating these checks in
arch/platform code. So why can't we let arch code decide how to convey
mm-IOMMU SVA compatibility? let it be a flag ( as in this patch) or some
callback.

Perhaps a more descriptive name
s/arch_can_alloc_pasid(mm)/arch_can_support_sva(mm)/ is all we disagreeing
:)

Thanks,

Jacob