From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: virtio-comment-return-570-cohuck=redhat.com@lists.oasis-open.org Sender: List-Post: List-Help: List-Unsubscribe: List-Subscribe: Received: from lists.oasis-open.org (oasis-open.org [10.110.1.242]) by lists.oasis-open.org (Postfix) with ESMTP id A424F985B94 for ; Tue, 27 Nov 2018 15:11:09 +0000 (UTC) Date: Tue, 27 Nov 2018 10:10:55 -0500 From: "Michael S. Tsirkin" Message-ID: <20181127100904-mutt-send-email-mst@kernel.org> References: <20180921095459.GA2842@work-vm> <20181126111612.GG15869@stefanha-x1.localdomain> <20181126135245.v4lklw6qwa2ggv4t@sirius.home.kraxel.org> <20181126140338.GC2547@work-vm> <20181126145145.cbuzccf3qo4z2nau@sirius.home.kraxel.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20181126145145.cbuzccf3qo4z2nau@sirius.home.kraxel.org> Subject: [virtio-comment] Re: PCI cap for larger offsets/lengths To: Gerd Hoffmann Cc: "Dr. David Alan Gilbert" , Stefan Hajnoczi , virtio-comment@lists.oasis-open.org List-ID: On Mon, Nov 26, 2018 at 03:51:45PM +0100, Gerd Hoffmann wrote: > Hi, > > > > So, I'm wondering whenever it makes sense to just do the same for your > > > device. Just use one pci bar as shared memory umbrella, specify that > > > one using the virtio vendor cap, then have sub-regions within that bar > > > for the various regions you have. Manage them dynamically (using > > > device-specific virtio commands) or just have a static configuration (in > > > device-specific config space). > > > > Ours are static subdivisions; so it felt easier to declare them; it's a > > shame to make that device specific. > > Shared memory handling is device specific anyway, so I fail to see why > this is a problem. Or do you want place virtio queues there (which > could be common ground for multiple device types) ? > > > > That avoids the problem with multiple capabilities of the same kind, and > > > it also avoids exhausting the cap IDs quicky if every device defines > > > their own VIRTIO_FOO_DEVICE_PCI_SHMCAP_ID_BAR_REGION. > > > > Is having multiple capabilities of the same type actually a problem, or > > is it just historical in the defitinition of virtio? > > I think the reason is that you can in theory have the same region twice, > once in an IO bar and once in an MMIO bar, and then the guest could > prefer the IO bar if possible and use the MMIO bar otherwise (PCIe slot > without IO address window for example). I think that was never actually > done in practice, There's an option to enable that for AMD CPUs where MMIO faults are slower than on intel CPUs. > and for (prefetchable) memory bars it doesn't make > sense at all. So that would unlikely be a problem in practice. > > Running out of capability IDs could become a real problem though. > > cheers, > Gerd We can always add more bits if we run out of these. there's no real limit on capability size. -- MST This publicly archived list offers a means to provide input to the OASIS Virtual I/O Device (VIRTIO) TC. In order to verify user consent to the Feedback License terms and to minimize spam in the list archive, subscription is required before posting. Subscribe: virtio-comment-subscribe@lists.oasis-open.org Unsubscribe: virtio-comment-unsubscribe@lists.oasis-open.org List help: virtio-comment-help@lists.oasis-open.org List archive: https://lists.oasis-open.org/archives/virtio-comment/ Feedback License: https://www.oasis-open.org/who/ipr/feedback_license.pdf List Guidelines: https://www.oasis-open.org/policies-guidelines/mailing-lists Committee: https://www.oasis-open.org/committees/virtio/ Join OASIS: https://www.oasis-open.org/join/