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

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Date:   Fri, 23 Sep 2022 08:44:25 -0700
From:   Ashok Raj <ashok.raj@...el.com>
To:     Dave Hansen <dave.hansen@...el.com>
CC:     Jason Gunthorpe <jgg@...dia.com>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Jacob Pan <jacob.jun.pan@...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>, Ashok Raj <ashok.raj@...el.com>
Subject: Re: [PATCHv8 00/11] Linear Address Masking enabling

On Fri, Sep 23, 2022 at 08:31:13AM -0700, Dave Hansen wrote:
> On 9/23/22 08:28, Ashok Raj wrote:
> >>
> >> I thought devices also cached address translation responses from the
> >> IOMMU and stashed them in their own device-local TLB.  If the device is
> >> unaware of the tags, then how does device TLB invalidation work?  Would
> > This is coming a full circle now :-)
> > 
> > Since the device doesn't understand tagging, SVM and tagging aren't
> > compatible. If you need SVM, you can only send sanitized pointers to the
> > device period. In fact our page-request even looks for canonical checks
> > before doing the page-faulting.
> > 
> >> all device TLB flushes be full flushes of the devices TLB?  If something
> >> tried to use single-address invalidation, it would need to invalidate
> >> every possible tag alias because the device wouldn't know that the tags
> >> *are* tags instead of actual virtual addresses.
> > Once tagging is extended into the PCI SIG, and devices know to work with
> > them, so will the IOMMU, then they can all play in the same field. Until
> > then they are isolated, or let SVM only work with untagged VA's.
> 
> But, the point that Kirill and I were getting at is still that devices
> *have* a role to play here.  The idea that this can be hidden at the
> IOMMU layer is pure fantasy.  Right?

If you *can't* send tagged memory to the device, what is the
role the device need to play? 

For now you can only send proper VA's that are canonical.