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

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Date:   Mon, 12 Sep 2022 14:41:56 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     Jacob Pan <jacob.jun.pan@...el.com>,
        Ashok Raj <ashok_raj@...ux.intel.com>
Cc:     "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 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.

> 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?