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Wed, 8 Sep 2021 13:37:54 +0000 From: John Levon To: Stefan Hajnoczi Subject: Re: [PATCH RFC server v2 05/11] vfio-user: run vfio-user context Thread-Topic: [PATCH RFC server v2 05/11] vfio-user: run vfio-user context Thread-Index: AQHXm2yIPjWEzOwcWUWSWCWzNjp1MquaK6sAgAAK7oA= Date: Wed, 8 Sep 2021 13:37:53 +0000 Message-ID: References: <3c043becf2b6e820f5392e0cadb465d5d9b9e6f8.1630084211.git.jag.raman@oracle.com> In-Reply-To: Accept-Language: en-GB, en-US Content-Language: en-US X-MS-Has-Attach: X-MS-TNEF-Correlator: authentication-results: redhat.com; dkim=none (message not signed) header.d=none;redhat.com; dmarc=none action=none header.from=nutanix.com; x-ms-publictraffictype: Email x-ms-office365-filtering-correlation-id: d58028c6-1fa8-4e0f-81b5-08d972cddc72 x-ms-traffictypediagnostic: BL0PR02MB4883: x-ms-exchange-transport-forked: True x-microsoft-antispam-prvs: x-proofpoint-crosstenant: true x-ms-oob-tlc-oobclassifiers: OLM:8882; x-ms-exchange-senderadcheck: 1 x-ms-exchange-antispam-relay: 0 x-microsoft-antispam: BCL:0; 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envelope-from=john.levon@nutanix.com; helo=mx0b-002c1b01.pphosted.com X-Spam_score_int: -24 X-Spam_score: -2.5 X-Spam_bar: -- X-Spam_report: (-2.5 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.393, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, 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@oracle.com" , "john.g.johnson@oracle.com" , "thuth@redhat.com" , Jagannathan Raman , Swapnil Ingle , "philmd@redhat.com" , "qemu-devel@nongnu.org" , "alex.williamson@redhat.com" , "marcandre.lureau@gmail.com" , Thanos Makatos , "alex.bennee@linaro.org" Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On Wed, Sep 08, 2021 at 01:58:46PM +0100, Stefan Hajnoczi wrote: > > +static void *vfu_object_attach_ctx(void *opaque) > > +{ > > + VfuObject *o =3D opaque; > > + int ret; > > + > > +retry_attach: > > + ret =3D vfu_attach_ctx(o->vfu_ctx); > > + if (ret < 0 && (errno =3D=3D EAGAIN || errno =3D=3D EWOULDBLOCK)) = { >=20 > Does this loop consume 100% CPU since this is non-blocking? Looks like it. Instead after vfu_create_ctx, there should be a vfu_get_poll= _fd() to get the listen socket, then a qemu_set_fd_handler(vfu_object_attach_ctx) to handle the attach when the listen socket is ready, modulo the below part= . > libvfio-user has non-blocking listen_fd but conn_fd is always blocking. It is, but in vfu_run_ctx(), we poll on it: ``` 790 if (vfu_ctx->flags & LIBVFIO_USER_FLAG_ATTACH_NB) { = =20 791 sock_flags =3D MSG_DONTWAIT | MSG_WAITALL; = =20 792 } = =20 793 return get_msg(hdr, sizeof(*hdr), fds, nr_fds, ts->conn_fd, sock_fl= ags); =20 ``` > This means ATTACH_NB is not useful because vfu_attach_ctx() is actually > blocking. You're correct that vfu_attach_ctx is in fact partially blocking: after accepting the connection, we call negotiate(), which can indeed block waiti= ng if the client hasn't sent anything. > I think this means vfu_run_ctx() is also blocking in some places Correct. There's a presumption that if a message is ready, we can read it a= ll without blocking, and equally that we can write to the socket without block= ing. The library docs are not at all clear on this point. > and QEMU's event loop might hang :( >=20 > Can you make libvfio-user non-blocking in order to solve these issues? I presume you're concerned about the security aspect: a malicious client co= uld withhold a write, and hence hang the device server. Problem is the libvfio-user API is synchronous: there's no way to return half-way through a vfu_attach_ctx() (or a vfu_run_ctx() after we read the header) then resume. We'd have to have a whole separate API to do that, so a separate thread see= ms a better approach? regards john=