Re: [Qemu-devel] [virtio] virtqueue allocation and thread-safety

[Vasile Catalin-B50542 <catalin.vasile@freescale.com>]

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Does the guest memory allocation include the descriptor tables allocation?
Which part of the structures is the descriptor tables?

On 15.01.2015 03:41, Fam Zheng wrote:
> On Wed, 01/14 14:01, Vasile Catalin-B50542 wrote:
>> Hi,
>>
>> I'm trying to make a new virtio device.
>> I got it running (I made a functional dummy device & guest driver).
>> Now I'm trying to build some communication between the device and guest
>> driver.
>> I can't seem to find where the actual allocation of virtqueues are made.
>> I've looked inside virtio_init(), but it just allocates the vq structures
>> and I don't see
>> any pointers inside that structure that might give and idea of something
>> dynamically
>> allocated. There is a member of that structure named "vector", but it's type
>> is uint16_t.
>> I've also looked inside the virtio_add_queue(), and it just makes some
>> constant assignments.
>> Where are the vqs buffer space actually allocated?
> The guest memory is prepared by guest. See the virtio spec "Virtual I/O Device
> (VIRTIO) Version 1.0" section "3.2.1 Supplying Buffers to The Device":
>
> <quote>
>
> The driver offers buffers to one of the device’s virtqueues as follows:
>
> 1. The driver places the buffer into free descriptor(s) in the descriptor
> table, chaining as necessary (see 2.4.5 The Virtqueue Descriptor Table).
>
> 2. The driver places the index of the head of the descriptor chain into the
> next ring entry of the available ring.
>
> 3. Steps 1 and 2 MAY be performed repeatedly if batching is possible.
>
> 4. The driver performs suitable a memory barrier to ensure the device sees the
> updated descriptor table and available ring before the next step.
>
> 5. The available idx is increased by the number of descriptor chain heads added
> to the available ring.
>
> 6. The driver performs a suitable memory barrier to ensure that it updates the
> idx field before checking for notification suppression.
>
> 7. If notifications are not suppressed, the driver notifies the device of the
> new available buffers.
>
> </unquote>
>
> Fam
>
>> One more thing. Are the virtqueue functions thread safe, from both point of
>> views
>> (qemu and guest driver API's view)?
>> Also I don't see any dynamic allocations/freeing when pushing and popping,
>> either.
>>
>> Cătă
>>

-- 

CatalinVasile

Intern, DN-Software

FreescaleSemiconductor, Inc.

www.freescale.com

phone: 073-021-1938

e-mail: catalin.vasile@freescale.com <mailto:your.name@freescale.com>

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