linux-kernel - Re: [PATCHv14 04/17] x86/mm: Handle LAM on context switch

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Message-ID: <20230111141459.ah7ijfbm733c4g7m@box.shutemov.name>
Date:   Wed, 11 Jan 2023 17:14:59 +0300
From:   "Kirill A. Shutemov" <kirill@...temov.name>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     "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>,
        the arch/x86 maintainers <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>,
        Bharata B Rao <bharata@....com>,
        Jacob Pan <jacob.jun.pan@...ux.intel.com>,
        Ashok Raj <ashok.raj@...el.com>, Linux-MM <linux-mm@...ck.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCHv14 04/17] x86/mm: Handle LAM on context switch

On Wed, Jan 11, 2023 at 07:49:30AM -0600, Linus Torvalds wrote:
> Sorry, I'm traveling, and right now only on mobile. So I'm reading patches
> on my phone, and answering on it too, so html garbage..
> 
> On Wed, Jan 11, 2023, 07:24 Kirill A. Shutemov <
> kirill.shutemov@...ux.intel.com> wrote:
> 
> >
> > +static inline unsigned long mm_lam_cr3_mask(struct mm_struct *mm)
> > +{
> > +       return READ_ONCE(mm->context.lam_cr3_mask);
> > +}
> >
> 
> I mentioned this before - in the original version this needed (but didn't
> have, iirc) that READ_ONCE, but in the new non-thread situation I don't
> think that's true. There should be no concurrent changes that can interfere
> with the read, no?

It should be safe, yes. 

It is reachable from iommu_sva_bind_device(), but it only called with mm
== current->mm.

I will drop the READ_ONCE().

-- 
  Kiryl Shutsemau / Kirill A. Shutemov