linux-kernel - Re: [PATCH] mmu_notifiers: Notify on pte permission upgrades

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Date:   Mon, 22 May 2023 23:45:12 -0700
From:   Christoph Hellwig <hch@...radead.org>
To:     Alistair Popple <apopple@...dia.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, robin.murphy@....com,
        will@...nel.org, nicolinc@...dia.com,
        linux-arm-kernel@...ts.infradead.org, kvm@...r.kernel.org,
        kvmarm@...ts.cs.columbia.edu, jgg@...dia.com,
        John Hubbard <jhubbard@...dia.com>
Subject: Re: [PATCH] mmu_notifiers: Notify on pte permission upgrades

On Mon, May 22, 2023 at 04:37:25PM +1000, Alistair Popple wrote:
> This problem was discovered during testing of an ARM SMMU
> implementation that does not support broadcast TLB maintenance
> (BTM). In this case the SMMU driver uses notifiers to issue TLB
> invalidates. For read-only to read-write pte upgrades the SMMU
> continually returned a read-only PTE to the device, even though the
> CPU had a read-write PTE installed.
> 
> Sending a mmu notifier event to the SMMU driver fixes the problem by
> flushing secondary TLB entries. A new notifier event type is added so
> drivers may filter out these invalidations if not required. Note a
> driver should never upgrade or install a PTE in response to this mmu
> notifier event as it is not synchronised against other PTE operations.

I don't see these SMMU driver changes anywhere.  I.e. you're adding dead
code as far as I can tell.