Re: [Qemu-devel] [virtio-dev] [PATCH v3 0/7] Vhost-pci for inter-VM communication

[Stefan Hajnoczi <stefanha@gmail.com>]

On Wed, Dec 20, 2017 at 4:06 AM, Michael S. Tsirkin <mst@redhat.com> wrote:
> On Tue, Dec 19, 2017 at 05:05:59PM +0000, Stefan Hajnoczi wrote:
>> On Tue, Dec 19, 2017 at 2:56 PM, Michael S. Tsirkin <mst@redhat.com> wrote:
>> >>  * Please handle short reads/writes and EAGAIN with the UNIX domain socket.  Do
>> >>    not use read/write_all() functions because they hang QEMU until I/O
>> >>    completes.
>> >
>> > I'm not sure I agree with this one. vhost-user uses this extensively
>> > right now. It might be a worth-while goal to drop this limitation
>> > but I don't see why we should start with vhost-pci.
>> >
>> > And in particular, VCPU can't make progress unless a slave is present.
>>
>> Hang on, we're talking about different things:
>>
>> The QEMU vhost-user master blocks because vhost_*() functions are
>> synchronous (they don't use callbacks or yield).  Fixing that is
>> beyond the scope of this patch series and I'm not asking for it.
>>
>> This patch series adds a from-scratch vhost-user slave implementation
>> which has no good reason to be blocking.  A single malicious or broken
>> guest must not be able to hang a vhost-pci network switch.
>
> Hmm that's not an easy change. But I agree, it's more important for
> the switch.

It's easy,  See "[PATCH v3 4/7] vhost-pci-slave: add vhost-pci slave
implementation".  There is only one qemu_chr_fe_read_all() call and
one qemu_chr_fe_write_all() call!

The read is the first thing that happens when the socket becomes
readable.  The write is the last thing that happens when a vhost-user
protocol message has been handled.

Moreover, the socket is only used in half-duplex with 1
request/response at any given time.

This means most of the code will be unchanged.  Only the beginning of
vp_slave_read() and vp_slave_write() need to be adapted.  I'm not
asking for a lot.

Stefan