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Mon, 4 Nov 2019 16:35:59 +0000 (UTC) Date: Mon, 4 Nov 2019 16:35:56 +0000 From: "Dr. David Alan Gilbert" To: geoff@hostfission.com Subject: Re: RFC: New device for zero-copy VM memory access Message-ID: <20191104163556.GD3420@work-vm> References: <88f1c3701740665b0ebe2f24c8ce7ade@hostfission.com> <20191031132443.GB3128@work-vm> <20191031155204.GD3128@work-vm> <20191104115546.GB3420@work-vm> <9b49de1379825ac1445766f4a8d198dc@hostfission.com> MIME-Version: 1.0 In-Reply-To: <9b49de1379825ac1445766f4a8d198dc@hostfission.com> User-Agent: Mutt/1.12.1 (2019-06-15) X-Scanned-By: MIMEDefang 2.79 on 10.5.11.12 X-MC-Unique: o_UwBjCWMgqYUbQh_Ma1EQ-1 X-Mimecast-Spam-Score: 0 Content-Type: text/plain; charset=WINDOWS-1252 Content-Transfer-Encoding: quoted-printable Content-Disposition: inline X-detected-operating-system: by eggs.gnu.org: GNU/Linux 2.2.x-3.x [generic] [fuzzy] X-Received-From: 205.139.110.120 X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.23 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: marcandre.lureau@redhat.com, maxime.coquelin@redhat.com, QEMU Developers , Peter Maydell Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" * geoff@hostfission.com (geoff@hostfission.com) wrote: >=20 >=20 > On 2019-11-04 22:55, Dr. David Alan Gilbert wrote: > > * geoff@hostfission.com (geoff@hostfission.com) wrote: > > >=20 > > >=20 > > > On 2019-11-03 21:10, geoff@hostfission.com wrote: > > > > On 2019-11-01 02:52, Dr. David Alan Gilbert wrote: > > > > > * geoff@hostfission.com (geoff@hostfission.com) wrote: > > > > > > > > > > > > > > > > > > On 2019-11-01 01:52, Peter Maydell wrote: > > > > > > > On Thu, 31 Oct 2019 at 14:26, wrote: > > > > > > > > As the author of Looking Glass, I also have to consider the > > > > > > > > maintenance > > > > > > > > and the complexity of implementing the vhost protocol into = the > > > > > > > > project. > > > > > > > > At this time a complete Porthole client can be implemented = in 150 > > > > > > > > lines > > > > > > > > of C without external dependencies, and most of that is boi= lerplate > > > > > > > > socket code. This IMO is a major factor in deciding to avoi= d > > > > > > > > vhost-user. > > > > > > > > > > > > > > This is essentially a proposal that we should make our projec= t and > > > > > > > code more complicated so that your project and code can be si= mpler. > > > > > > > I hope you can see why this isn't necessarily an argument tha= t will hold > > > > > > > very much weight for us :-) > > > > > > > > > > > > Certainly, I do which is why I am still going to see about usin= g > > > > > > vhost, > > > > > > however, a device that uses vhost is likely more complex then > > > > > > the device > > > > > > as it stands right now and as such more maintenance would be > > > > > > involved on > > > > > > your end also. Or have I missed something in that vhost-user ca= n > > > > > > be used > > > > > > directly as a device? > > > > > > > > > > The basic vhost-user stuff isn't actually that hard; if you aren= 't > > > > > actually shuffling commands over the queues you should find it pr= etty > > > > > simple - so I think your assumption about it being simpler if you > > > > > avoid > > > > > it might be wrong. It might be easier if you use it! > > > > > > > > I have been looking into this and I am yet to find some decent > > > > documentation or a simple device example I can use to understand ho= w to > > > > create such a device. Do you know of any reading or examples I can > > > > obtain > > > > on how to get an initial do nothing device up and running? > > > > > > > > -Geoff > > >=20 > > > Scratch that, the design just solidified for me and I am now making > > > progress, however it seems that vhost-user can't do what we need here= : > > >=20 > > > 1) I dont see any way to recieve notification of socket > > > disconnection, in > > > our use case the client app needs to be able to be (re)connected > > > dynamically. It might be possible to get this event by registering > > > it on > > > the chardev manually but this seems like it would be a kludge. > >=20 > > My understanding was that someone added support for reconnection of > > vhost-user; I'm not sure of the detail - cc'ing in Maxime and > > Marc-Andre. > >=20 > > > 2) I don't see any method of notifying the vhost-user client of the > > > removal of a shared memory mapping. Again, these may not be > > > persistently > > > mapped in the guest as we have no control over the buffer > > > allocation, and > > > as such, we need a method to notify the client that the mapping has > > > become > > > invalid. > > >=20 > > > 3) VHOST_USER_SET_MEM_TABLE is a one time request, again this breaks > > > our > > > usage as we need to change this dynamically at runtime. > >=20 > > I've seen (3) being sent multiple times (It's messy but it happens); so > > I think that fixes (2) as well for you. >=20 > Yes, but it's ignored. >=20 > /* > * For non-vring specific requests, like VHOST_USER_SET_MEM_TABLE, > * we just need send it once in the first time. For later such > * request, we just ignore it. > */ > if (vhost_user_one_time_request(msg->hdr.request) && dev->vq_index != =3D 0) > { > msg->hdr.flags &=3D ~VHOST_USER_NEED_REPLY_MASK; > return 0; > } Curious. I could swear I'd already dealt with multiple copies of this message coming over vhost-user and having to deal with it when it did. But now I'm confused, isn't vq_index a unique number per queue, so is this really stopping it happening multiple times, or just making sure it only happens for the first queue? Dave > >=20 > > Dave > >=20 > > > Unless there are viable solutions to these problems there is no way > > > that > > > vhost-user can be used for this kind of a device. > > >=20 > > > -Geoff > > >=20 > > > > > > > > > > > > > > Dave > > > > > > > > > > > > > > > > > > > thanks > > > > > > > -- PMM > > > > > -- > > > > > Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK > > -- > > Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK -- Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK