Re: [PATCH 3/3] virtio_pci: Use the DMA API
On Tue, Oct 27, 2015 at 06:17:10PM -0700, Andy Lutomirski wrote: > From: Andy Lutomirski> > This fixes virtio-pci on platforms and busses that have IOMMUs. This > will break the experimental QEMU Q35 IOMMU support until QEMU is > fixed. In exchange, it fixes physical virtio hardware as well as > virtio-pci running under Xen. > > We should clean up the virtqueue API to do its own allocation and > teach virtqueue_get_avail and virtqueue_get_used to return DMA > addresses directly. > > Signed-off-by: Andy Lutomirski > --- > drivers/virtio/virtio_pci_common.h | 3 ++- > drivers/virtio/virtio_pci_legacy.c | 19 +++ > drivers/virtio/virtio_pci_modern.c | 34 -- > 3 files changed, 41 insertions(+), 15 deletions(-) Same here, you need to call the dma_sync* functions when passing data from/to the virtio-device. I think a good test for that is to boot a virtio kvm-guest with swiotlb=force and see if it still works. Joerg -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
Re: [PATCH 3/3] virtio_pci: Use the DMA API
On Wed, 2015-10-28 at 11:15 +0900, Joerg Roedel wrote: > On Tue, Oct 27, 2015 at 06:17:10PM -0700, Andy Lutomirski wrote: > > From: Andy Lutomirski> > > > This fixes virtio-pci on platforms and busses that have IOMMUs. > > This > > will break the experimental QEMU Q35 IOMMU support until QEMU is > > fixed. In exchange, it fixes physical virtio hardware as well as > > virtio-pci running under Xen. > > > > We should clean up the virtqueue API to do its own allocation and > > teach virtqueue_get_avail and virtqueue_get_used to return DMA > > addresses directly. > > > > Signed-off-by: Andy Lutomirski > > --- > > drivers/virtio/virtio_pci_common.h | 3 ++- > > drivers/virtio/virtio_pci_legacy.c | 19 +++ > > drivers/virtio/virtio_pci_modern.c | 34 -- > > > > 3 files changed, 41 insertions(+), 15 deletions(-) > > Same here, you need to call the dma_sync* functions when passing data > from/to the virtio-device. > > I think a good test for that is to boot a virtio kvm-guest with > swiotlb=force and see if it still works. That's useful but doesn't cover the cases where dma_wmb() is needed, right? We should make sure we're handling descriptors properly as we would for real hardware, and ensuring that addresses etc. are written to the descriptor (and a write barrier occurs) *before* the bit is set which tells the 'hardware' that it owns that descriptor. AFAICT we're currently setting the flags *first* in virtqueue_add(), let alone the missing write barrier between the two. -- dwmw2 smime.p7s Description: S/MIME cryptographic signature
Re: [PATCH 3/3] virtio_pci: Use the DMA API
On Wed, Oct 28, 2015 at 11:15:30AM +0900, Joerg Roedel wrote: > Same here, you need to call the dma_sync* functions when passing data > from/to the virtio-device. Okay, forget about this comment. This patch only converts to dma_coherent allocations, which don't need synchronization. > I think a good test for that is to boot a virtio kvm-guest with > swiotlb=force and see if it still works. But this holds, Its a good way to test if your changes work with bounce-buffering. Together with DMA_API_DEBUG you also see if your specified dma_directions are right. Joerg -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
Re: [PATCH 3/3] virtio_pci: Use the DMA API
On Wed, Oct 28, 2015 at 11:22:52AM +0900, David Woodhouse wrote: > On Wed, 2015-10-28 at 11:15 +0900, Joerg Roedel wrote: > > I think a good test for that is to boot a virtio kvm-guest with > > swiotlb=force and see if it still works. > > That's useful but doesn't cover the cases where dma_wmb() is needed, > right? > > We should make sure we're handling descriptors properly as we would for > real hardware, and ensuring that addresses etc. are written to the > descriptor (and a write barrier occurs) *before* the bit is set which > tells the 'hardware' that it owns that descriptor. Hmm, good question. The virtio code has virtio_rmb/wmb and should already call it in the right places. The virtio-barriers default to rmb()/wmb() unless weak_barriers is set, in which case it calls dma_rmb()/wmb(), just a compiler barrier on x86. And weak_barriers is set by the virtio drivers when creating the queue, and the drivers should know what they are doing. Saying this, I think the virtio-drivers should already get this right. Joerg -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html