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 13:40:14 -0500
From:   Tom Lendacky <thomas.lendacky@....com>
To:     Borislav Petkov <bp@...en8.de>, Joerg Roedel <joro@...tes.org>
Cc:     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 6/21/2017 11:59 AM, Borislav Petkov wrote:
> 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.

Ok, so the (now) current version of the patch that doesn't change the
function signature is the right way to go.

Thanks,
Tom

> 
> Thanks.
>