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Subject: Re: [Patch v4 0/3] Introduce a driver to support host accelerated
 access to Microsoft Azure Blob
To:     Long Li <longli@microsoft.com>,
        Christoph Hellwig <hch@infradead.org>
Cc:     "longli@linuxonhyperv.com" <longli@linuxonhyperv.com>,
        "linux-fs@vger.kernel.org" <linux-fs@vger.kernel.org>,
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        "linux-hyperv@vger.kernel.org" <linux-hyperv@vger.kernel.org>,
        "gregkh@linuxfoundation.org" <gregkh@linuxfoundation.org>
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From:   Bart Van Assche <bvanassche@acm.org>
Message-ID: <115d864c-46c2-2bc8-c392-fd63d34c9ed0@acm.org>
Date:   Tue, 20 Jul 2021 08:15:30 -0700
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On 7/20/21 12:05 AM, Long Li wrote:
>> Subject: Re: [Patch v4 0/3] Introduce a driver to support host accelerated
>> access to Microsoft Azure Blob
>>
>> On Mon, Jul 19, 2021 at 09:37:56PM -0700, Bart Van Assche wrote:
>>> such that this object storage driver can be implemented as a
>>> user-space library instead of as a kernel driver? As you may know vfio
>>> users can either use eventfds for completion notifications or polling.
>>> An interface like io_uring can be built easily on top of vfio.
>>
>> Yes.  Similar to say the NVMe K/V command set this does not look like a
>> candidate for a kernel driver.
> 
> The driver is modeled to support multiple processes/users over a VMBUS
> channel. I don't see a way that this can be implemented through VFIO? 
> 
> Even if it can be done, this exposes a security risk as the same VMBUS
> channel is shared by multiple processes in user-mode.

Sharing a VMBUS channel among processes is not necessary. I propose to
assign one VMBUS channel to each process and to multiplex I/O submitted
to channels associated with the same blob storage object inside e.g. the
hypervisor. This is not a new idea. In the NVMe specification there is a
diagram that shows that multiple NVMe controllers can provide access to
the same NVMe namespace. See also diagram "Figure 416: NVM Subsystem
with Three I/O Controllers" in version 1.4 of the NVMe specification.

Bart.