linux-kernel - Re: [PATCH v6 26/34] iommu/amd: Allow the AMD IOMMU to work with memory encryption

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Date:   Wed, 21 Jun 2017 17:37:22 +0200
From:   Joerg Roedel <joro@...tes.org>
To:     Borislav Petkov <bp@...en8.de>
Cc:     Tom Lendacky <thomas.lendacky@....com>, linux-arch@...r.kernel.org,
        linux-efi@...r.kernel.org, kvm@...r.kernel.org,
        linux-doc@...r.kernel.org, x86@...nel.org,
        kexec@...ts.infradead.org, linux-kernel@...r.kernel.org,
        kasan-dev@...glegroups.com, linux-mm@...ck.org,
        iommu@...ts.linux-foundation.org, Rik van Riel <riel@...hat.com>,
        Radim Krčmář <rkrcmar@...hat.com>,
        Toshimitsu Kani <toshi.kani@....com>,
        Arnd Bergmann <arnd@...db.de>,
        Jonathan Corbet <corbet@....net>,
        Matt Fleming <matt@...eblueprint.co.uk>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Larry Woodman <lwoodman@...hat.com>,
        Brijesh Singh <brijesh.singh@....com>,
        Ingo Molnar <mingo@...hat.com>,
        Andy Lutomirski <luto@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        Andrey Ryabinin <aryabinin@...tuozzo.com>,
        Alexander Potapenko <glider@...gle.com>,
        Dave Young <dyoung@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Dmitry Vyukov <dvyukov@...gle.com>
Subject: Re: [PATCH v6 26/34] iommu/amd: Allow the AMD IOMMU to work with
 memory encryption

On Thu, Jun 15, 2017 at 11:41:12AM +0200, Borislav Petkov wrote:
> On Wed, Jun 14, 2017 at 03:40:28PM -0500, Tom Lendacky wrote:
> > > WARNING: Use of volatile is usually wrong: see Documentation/process/volatile-considered-harmful.rst
> > > #134: FILE: drivers/iommu/amd_iommu.c:866:
> > > +static void build_completion_wait(struct iommu_cmd *cmd, volatile u64 *sem)
> > > 
> > 
> > The semaphore area is written to by the device so the use of volatile is
> > appropriate in this case.
> 
> Do you mean this is like the last exception case in that document above:
> 
> "
>   - Pointers to data structures in coherent memory which might be modified
>     by I/O devices can, sometimes, legitimately be volatile.  A ring buffer
>     used by a network adapter, where that adapter changes pointers to
>     indicate which descriptors have been processed, is an example of this
>     type of situation."
> 
> ?

So currently (without this patch) the build_completion_wait function
does not take a volatile parameter, only wait_on_sem() does.

Wait_on_sem() needs it because its purpose is to poll a memory location
which is changed by the iommu-hardware when its done with command
processing.

But the 'volatile' in build_completion_wait() looks unnecessary, because
the function does not poll the memory location. It only uses the
pointer, converts it to a physical address and writes it to the command
to be queued.


Regards,

	Joerg