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Subject: Re: [virtio-comment] Re: PCI cap for larger offsets/lengths
* Michael S. Tsirkin (mst@redhat.com) wrote: > 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. I thought it was defined as 8 bits by the capability-linked list structure in PCI? Dave > -- > 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/ > -- Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK
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