On Tue, Aug 07, 2018 at 08:13:56AM +1000, Benjamin Herrenschmidt wrote: > On Tue, 2018-08-07 at 00:46 +0300, Michael S. Tsirkin wrote: > > On Tue, Aug 07, 2018 at 07:26:35AM +1000, Benjamin Herrenschmidt wrote: > > > On Mon, 2018-08-06 at 23:35 +0300, Michael S. Tsirkin wrote: > > > > > As I said replying to Christoph, we are "leaking" into the interface > > > > > something here that is really what's the VM is doing to itself, which > > > > > is to stash its memory away in an inaccessible place. > > > > > > > > > > Cheers, > > > > > Ben. > > > > > > > > I think Christoph merely objects to the specific implementation. If > > > > instead you do something like tweak dev->bus_dma_mask for the virtio > > > > device I think he won't object. > > > > > > Well, we don't have "bus_dma_mask" yet ..or you mean dma_mask ? > > > > > > So, something like that would be a possibility, but the problem is that > > > the current virtio (guest side) implementation doesn't honor this when > > > not using dma ops and will not use dma ops if not using iommu, so back > > > to square one. > > > > Well we have the RFC for that - the switch to using DMA ops unconditionally > > isn't > > problematic itself IMHO, for now that RFC is blocked > > by its perfromance overhead for now but Christoph says > > he's trying to remove that for direct mappings, > > so we should hopefully be able to get there in X weeks. > > That would be good yes. > > ../.. > > > > --- a/drivers/virtio/virtio_ring.c > > > +++ b/drivers/virtio/virtio_ring.c > > > @@ -155,7 +155,7 @@ static bool vring_use_dma_api(struct virtio_device > > > *vdev) > > > * the DMA API if we're a Xen guest, which at least allows > > > * all of the sensible Xen configurations to work correctly. > > > */ > > > - if (xen_domain()) > > > + if (xen_domain() || arch_virtio_direct_dma_ops(&vdev->dev)) > > > return true; > > > > > > return false; > > > > Right but can't we fix the retpoline overhead such that > > vring_use_dma_api will not be called on data path any longer, making > > this a setup time check? > > Yes it needs to be a setup time check regardless actually ! > > The above is broken, sorry I was a bit quick here (too early in the > morning... ugh). We don't want the arch to go override the dma ops > every time that is callled. > > But yes, if we can fix the overhead, it becomes just a matter of > setting up the "right" ops automatically. > > > > (Passing the dev allows the arch to know this is a virtio device in > > > "direct" mode or whatever we want to call the !iommu case, and > > > construct appropriate DMA ops for it, which aren't the same as the DMA > > > ops of any other PCI device who *do* use the iommu). > > > > I think that's where Christoph might have specific ideas about it. > > OK well, assuming Christoph can solve the direct case in a way that > also work for the virtio !iommu case, we still want some bit of logic > somewhere that will "switch" to swiotlb based ops if the DMA mask is > limited. > > You mentioned an RFC for that ? Do you happen to have a link ?
No but Christoph did I think. > It would be indeed ideal if all we had to do was setup some kind of > bus_dma_mask on all PCI devices and have virtio automagically insert > swiotlb when necessary. > > Cheers, > Ben. >
