On Wed, Oct 30, 2013 at 05:10:37PM +0100, Paolo Bonzini wrote: > Il 30/10/2013 16:56, Alex Williamson ha scritto: > > On Wed, 2013-10-30 at 16:42 +0100, Paolo Bonzini wrote: > >> Il 30/10/2013 16:33, Gleb Natapov ha scritto: > >>>> Hmm, ok. In that case I can drop this patch and I think the rest just > >>>> boils down to userspace use of the device. I had been close()'ing the > >>>> kvm device fd when all QEMU vfio devices are detached, but I can just as > >>>> easily leave it open in case a new device is added later. I'll send out > >>>> a new series after doing some more review and testing. Do you have any > >>>> comments on the rest of the series? Thanks, > >>> > >>> If I understand 4/4 correctly if there is VFIO device connected we > >>> assume non coherent domain. How hard it would be to do proper checking > >>> in this path series? > >> > >> Yes, that's my understanding as well. Is the performance impact measurable? > > > > I have additional patches to do this, the blocker is that intel-iommu > > strips IOMMU_CACHE from the flags I provide if the IOMMU domain as a > > whole (ie. all of the hardware units involved in the domain) do not all > > support Snoop Control (SC). Thus I can't rely on simply tracking the > > hardware capabilities of the domain because some IOMMU PTEs will have > > SNP set, others will not. It depends on the state of the IOMMU domain > > at the time of the mapping. Therefore the only way to switch back to > > coherent from non-coherent is to unmap and remap everything. This is > > what legacy KVM device assignment does, but I can't see how that's not > > racy wrt inflight DMA. > > > > The patch approach I was taking is: > > > > 1) Enable KVM to handle the VM as non-coherent (which is trued, VFIO > > never sets IOMMU_CACHE currently due to the above issues). > > > > 2) Get QEMU to enable the KVM device, fixing the coherency issue. > > > > 3) Fix Intel IOMMU to honor IOMMU_CACHE regardless of hw capabilities > > (patch posted). > > > > 4) Make VFIO always map w/ IOMMU_CACHE > > > > 5) Create VFIO external user interface to query capabilities. > > > > 6) Update KVM device to use it. > > > > As to performance, for graphics I can't tell a difference whether > > NoSnoop is prevented or KVM does WBINVD. I'm hoping though that we can > > consider the mode enabled by this patch as a temporary step in the > > process and we'll eventually get to a state where we only emulate WBINVD > > when necessary. Correctness trumps performance in the current round. > > Thanks, > > Thanks for the explanation. Gleb, this looks fine to me. WDYT? > To me too. Alex please resend with first patch dropped (kvm_vfio_destroy() will have to free dev). -- Gleb. -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
