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

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Date:   Mon, 12 Sep 2022 17:23:16 -0700
From:   Ashok Raj <ashok.raj@...el.com>
To:     Dave Hansen <dave.hansen@...el.com>
CC:     Jacob Pan <jacob.jun.pan@...el.com>,
        Ashok Raj <ashok_raj@...ux.intel.com>,
        "Kirill A. Shutemov" <kirill@...temov.name>,
        "Kirill A. Shutemov" <kirill.shutemov@...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>,
        Ashok Raj <ashok.raj@...el.com>
Subject: Re: [PATCHv8 00/11] Linear Address Masking enabling

On Mon, Sep 12, 2022 at 02:41:56PM -0700, Dave Hansen wrote:
> On 9/12/22 13:39, Jacob Pan wrote:
> >>> +	if (pasid_valid(mm->pasid) && !forced) {
> > I don't think this works since we have lazy pasid free.  for example,
> > after all the devices did sva_unbind, mm->pasid  we'll remain valid until
> > mmdrop(). LAM  should be supported in this case.
> 
> Nah, it works fine.
> 
> It just means that the rules are "you can't do LAM if your process
> *EVER* got a PASID" instead of "you can't do LAM if you are actively
> using your PASID".
> 
> We knew that PASID use would be a one-way trip for a process when we
> moved to the simplified implementation.  This is just more fallout from
> that.  It's fine.

Agree.

> 
> > Perhaps, we could introduce another prctl flag for SVA, PR_GET_SVA?
> > Both iommu driver and LAM can set/query the flag. LAM applications may not
> > be the only ones want to know if share virtual addressing is  on.
> 
> I don't think it's a good idea to add yet more UABI around this issue.
> Won't the IOMMU folks eventually get their hardware in line with LAM?
> Isn't this situation temporary?

This is more than just the IOMMU change, since this involves device end,
ability to report tagging feature, communicating the width to ignore
etc. I suspect PCIe changes are required for the device end which would
be a long pole. 

I suspect this would be moderately permanent :-) memory tagging is more
of a niche use case, and hurting general IO devices has lots of design
touch points that makes it difficult to close in short order.