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Subject: Re: PCI cap for larger offsets/lengths
* Gerd Hoffmann (kraxel@redhat.com) wrote: > On Mon, Nov 26, 2018 at 11:16:12AM +0000, Stefan Hajnoczi wrote: > > On Fri, Sep 21, 2018 at 10:54:59AM +0100, Dr. David Alan Gilbert wrote: > > > Hi, > > > We've got an experimental virtio device (using vhost-user) we're playing with > > > that would like to share multiple large mappings from the client back to qemu. > > > > CCing Michael Tsirkin and Gerd Hoffman. Gerd could use this for > > virtio-gpu where some memory must be owned by the host. > > Yep. For virtio-gpu I want be able to map host gpu resources (which > must be allocated by the host gpu driver) into the guest address space. > > > > 'virtio_pci_cap' only has 32bit offset and length fields and so > > > I've got a different capability to express larger regions: > > > > > > > > > /* Additional shared memory capability */ > > > #define VIRTIO_PCI_CAP_SHARED_MEMORY_CFG 8 > > > > > > struct virtio_pci_shm_cap { > > > struct virtio_pci_cap cap; > > > le32 offset_hi; /* Most sig 32 bits of offset */ > > > le32 length_hi; /* Most sig 32 bits of length */ > > > u8 id; /* To distinguish shm chunks */ > > > }; > > > > > > One oddity is that I'm allowing multiple instances of this capability > > > on one device, distinguished by their 'id' field which I've made device > > > type specific, e.g.: > > > > > > #define VIRTIO_MYDEV_PCI_SHMCAP_ID_CACHE 0 > > > #define VIRTIO_MYDEV_PCI_SHMCAP_ID_JOURNAL 1 > > For my experimental virtio-gpu code I use one pci bar to reserve address > space. It is a separate pci bar. First, because it is a 64bit bar. > Second, because it is declared as prefetchable (unlike the mmio bar > which is not). I also simply use the whole bar, so no offset/length is > needed. > > gpu resources are sub-regions within that pci bar, and they are managed > using device-specific commands. > > 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. > 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? Dave > cheers, > Gerd > -- Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK
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