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dkim=none (message not signed) header.d=none;lists.linux-foundation.org; dmarc=none action=none header.from=oracle.com; Received: from BYAPR10MB3573.namprd10.prod.outlook.com (2603:10b6:a03:11e::32) by SJ0PR10MB4448.namprd10.prod.outlook.com (2603:10b6:a03:2ad::24) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.20.4173.22; Thu, 3 Jun 2021 22:16:45 +0000 Received: from BYAPR10MB3573.namprd10.prod.outlook.com ([fe80::b09d:e36a:4258:d3d0]) by BYAPR10MB3573.namprd10.prod.outlook.com ([fe80::b09d:e36a:4258:d3d0%7]) with mapi id 15.20.4173.030; Thu, 3 Jun 2021 22:16:45 +0000 Subject: Re: vhost: multiple worker support To: Stefan Hajnoczi Cc: target-devel@vger.kernel.org, linux-scsi@vger.kernel.org, pbonzini@redhat.com, jasowang@redhat.com, mst@redhat.com, sgarzare@redhat.com, virtualization@lists.linux-foundation.org References: <20210525180600.6349-1-michael.christie@oracle.com> From: Mike Christie Message-ID: <523e6207-c380-9a9d-7a5d-7b7ee554d7f2@oracle.com> Date: Thu, 3 Jun 2021 17:16:42 -0500 User-Agent: Mozilla/5.0 (X11; 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I >> ended up just doing Stefan's original idea where userspace has the >> kernel create a worker and we pass back the pid. This has the benefit >> over the workqueue and userspace thread approach where we only have >> one'ish code path in the kernel during setup to detect old tools. The >> main IO paths and device/vq setup/teardown paths all use common code. >> >> The kernel patches here allow us to then do N workers device and also >> share workers across devices. >> >> I've also included a patch for qemu so you can get an idea of how it >> works. If we are ok with the kernel code then I'll break that up into >> a patchset and send to qemu-devel. > > It seems risky to allow userspace process A to "share" a vhost worker > thread with userspace process B based on a matching pid alone. Should > they have ptrace_may_access() or similar? > I'm not sure. I already made it a little restrictive in this posting, but it may not be enough depending on what's possible and what we want to allow. Right now to share a worker the userspace process doing the VHOST_SET_VRING_WORKER ioctl has to be the owner. Before we do a VHOST_SET_VRING_WORKER, vhost_dev_ioctl calls vhost_dev_check_owner, so we will fail if 2 random processes try to share. So we can share a worker across a vhost-dev's N virtqueues or share a worker with multiple vhost devs and their virtqueues, but the devs have to be managed by the same VM/qemu process. If that's all we want to support, then is the owner check enough? If we want to share workers across VMs then I think we definitely want something like ptrace_may_access.