Date: Wed, 21 Jun 2017 18:59:22 +0200
From: Borislav Petkov <bp@alien8.de>
To: Joerg Roedel <joro@8bytes.org>
Cc: Tom Lendacky <thomas.lendacky@amd.com>, linux-arch@vger.kernel.org,
        linux-efi@vger.kernel.org, kvm@vger.kernel.org,
        linux-doc@vger.kernel.org, x86@kernel.org, kexec@lists.infradead.org,
        linux-kernel@vger.kernel.org, kasan-dev@googlegroups.com,
        linux-mm@kvack.org, iommu@lists.linux-foundation.org,
        Rik van Riel <riel@redhat.com>,
        Radim =?utf-8?B?S3LEjW3DocWZ?= <rkrcmar@redhat.com>,
        Toshimitsu Kani <toshi.kani@hpe.com>, Arnd Bergmann <arnd@arndb.de>,
        Jonathan Corbet <corbet@lwn.net>,
        Matt Fleming <matt@codeblueprint.co.uk>,
        "Michael S. Tsirkin" <mst@redhat.com>,
        Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>,
        Paolo Bonzini <pbonzini@redhat.com>,
        Larry Woodman <lwoodman@redhat.com>,
        Brijesh Singh <brijesh.singh@amd.com>, Ingo Molnar <mingo@redhat.com>,
        Andy Lutomirski <luto@kernel.org>, "H. Peter Anvin" <hpa@zytor.com>,
        Andrey Ryabinin <aryabinin@virtuozzo.com>,
        Alexander Potapenko <glider@google.com>,
        Dave Young <dyoung@redhat.com>, Thomas Gleixner <tglx@linutronix.de>,
        Dmitry Vyukov <dvyukov@google.com>
Subject: Re: [PATCH v6 26/34] iommu/amd: Allow the AMD IOMMU to work with
 memory encryption
Message-ID: <20170621165921.tv2jfhf5dz7hsjsy@pd.tnic>
References: <20170607191309.28645.15241.stgit@tlendack-t1.amdoffice.net>
 <20170607191745.28645.81756.stgit@tlendack-t1.amdoffice.net>
 <20170614174208.p2yr5exs4b6pjxhf@pd.tnic>
 <0611d01a-19f8-d6ae-2682-932789855518@amd.com>
 <20170615094111.wga334kg2bhxqib3@pd.tnic>
 <20170621153721.GP30388@8bytes.org>
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On Wed, Jun 21, 2017 at 05:37:22PM +0200, Joerg Roedel wrote:
> > 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.

Right, the reason above - memory modifiable by an IO device. You could
add a comment there explaining the need for the volatile.

> 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.

Ok.

Thanks.

-- 
Regards/Gruss,
    Boris.

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