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auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=stefanha@redhat.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: multipart/signed; micalg=pgp-sha256; protocol="application/pgp-signature"; boundary="i0OVKvlOGSTYaquq" Content-Disposition: inline Received-SPF: pass client-ip=170.10.133.124; envelope-from=stefanha@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -31 X-Spam_score: -3.2 X-Spam_bar: --- X-Spam_report: (-3.2 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.398, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_LOW=-0.7, RCVD_IN_MSPIKE_H2=-0.001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 autolearn=ham autolearn_force=no X-Spam_action: no action 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: Elena Ufimtseva , Jag Raman , Swapnil Ingle , John Levon , QEMU Devel Mailing List , Alex Williamson , "thanos.makatos@nutanix.com" Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" --i0OVKvlOGSTYaquq Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: quoted-printable On Mon, Sep 13, 2021 at 05:23:33PM +0000, John Johnson wrote: > >> On Sep 9, 2021, at 10:25 PM, John Johnson = wrote: > >>> On Sep 8, 2021, at 11:29 PM, Stefan Hajnoczi wr= ote: > >>> On Thu, Sep 09, 2021 at 05:11:49AM +0000, John Johnson wrote: > >>>> =09I did look at coroutines, but they seemed to work when the sender > >>>> is triggering the coroutine on send, not when request packets are ar= riving > >>>> asynchronously to the sends. > >>>=20 > >>> This can be done with a receiver coroutine. Its job is to be the only > >>> thing that reads vfio-user messages from the socket. A receiver > >>> coroutine reads messages from the socket and wakes up the waiting > >>> coroutine that yielded from vfio_user_send_recv() or > >>> vfio_user_pci_process_req(). > >>>=20 > >>> (Although vfio_user_pci_process_req() could be called directly from t= he > >>> receiver coroutine, it seems safer to have a separate coroutine that > >>> processes requests so that the receiver isn't blocked in case > >>> vfio_user_pci_process_req() yields while processing a request.) > >>>=20 > >>> Going back to what you mentioned above, the receiver coroutine does > >>> something like this: > >>>=20 > >>> if it's a reply > >>> reply =3D find_reply(...) > >>> qemu_coroutine_enter(reply->co) // instead of signalling reply->c= v > >>> else > >>> QSIMPLEQ_INSERT_TAIL(&pending_reqs, request, next); > >>> if (pending_reqs_was_empty) { > >>> qemu_coroutine_enter(process_request_co); > >>> } > >>>=20 > >>> The pending_reqs queue holds incoming requests that the > >>> process_request_co coroutine processes. > >>>=20 > >>=20 > >>=20 > >> =09How do coroutines work across threads? There can be multiple vCPU > >> threads waiting for replies, and I think the receiver coroutine will b= e > >> running in the main loop thread. Where would a vCPU block waiting for > >> a reply? I think coroutine_yield() returns to its coroutine_enter() c= aller > >=20 > >=20 > >=20 > > A vCPU thread holding the BQL can iterate the event loop if it has > > reached a synchronous point that needs to wait for a reply before > > returning. I think we have this situation when a MemoryRegion is > > accessed on the proxy device. > >=20 > > For example, block/block-backend.c:blk_prw() kicks off a coroutine and > > then runs the event loop until the coroutine finishes: > >=20 > > Coroutine *co =3D qemu_coroutine_create(co_entry, &rwco); > > bdrv_coroutine_enter(blk_bs(blk), co); > > BDRV_POLL_WHILE(blk_bs(blk), rwco.ret =3D=3D NOT_DONE); > >=20 > > BDRV_POLL_WHILE() boils down to a loop like this: > >=20 > > while ((cond)) { > > aio_poll(ctx, true); > > } > >=20 >=20 > =09I think that would make vCPUs sending requests and the > receiver coroutine all poll on the same socket. If the =E2=80=9Cwrong=E2= =80=9D > routine reads the message, I=E2=80=99d need a second level of synchroniza= tion > to pass the message to the =E2=80=9Cright=E2=80=9D one. e.g., if the vCP= U coroutine > reads a request, it needs to pass it to the receiver; if the receiver > coroutine reads a reply, it needs to pass it to a vCPU. >=20 > =09Avoiding this complexity is one of the reasons I went with > a separate thread that only reads the socket over the mp-qemu model, > which does have the sender poll, but doesn=E2=80=99t need to handle incom= ing > requests. Only one coroutine reads from the socket, the "receiver" coroutine. In a previous reply I sketched what the receiver does: if it's a reply reply =3D find_reply(...) qemu_coroutine_enter(reply->co) // instead of signalling reply->cv else QSIMPLEQ_INSERT_TAIL(&pending_reqs, request, next); if (pending_reqs_was_empty) { qemu_coroutine_enter(process_request_co); } The qemu_coroutine_enter(reply->co) call re-enters the coroutine that was created by the vCPU thread. Is this the "second level of synchronization" that you described? It's very similar to signalling reply->cv in the existing patch. Now I'm actually thinking about whether this can be improved by keeping the condvar so that the vCPU thread doesn't need to call aio_poll() (which is awkward because it doesn't drop the BQL and therefore blocks other vCPUs from making progress). That approach wouldn't require a dedicated thread for vfio-user. > > I also want to check that I understand the scenarios in which the > > vfio-user communication code is used: > >=20 > > 1. vhost-user-server > >=20 > > The vfio-user communication code should run in a given AioContext (it > > will be the main loop by default but maybe the user will be able to > > configure a specific IOThread in the future). > >=20 >=20 > =09Jag would know more, but I believe it runs off the main loop. > Running it in an iothread doesn=E2=80=99t gain much, since it needs BQL t= o > run the device emulation code. >=20 >=20 > > 2. vCPU thread vfio-user clients > >=20 > > The vfio-user communication code is called from the vCPU thread where > > the proxy device executes. The MemoryRegion->read()/write() callbacks > > are synchronous, so the thread needs to wait for a vfio-user reply > > before it can return. > >=20 > > Is this what you had in mind? >=20 > =09The client is also called from the main thread - the GET_* > messages from vfio_user_pci_realize() as well as MAP/DEMAP messages > from guest address space change transactions. It is also called by > the migration thread, which is a separate thread that does not run > holding BQL. Thanks for mentioning those additional cases. 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