On Mon, May 20, 2024 at 02:52:43AM +0000, Tian, Kevin wrote: > +Daniel > > > From: Jason Gunthorpe <jgg@xxxxxxxxxx> > > Sent: Saturday, May 18, 2024 1:11 AM > > > > On Thu, May 16, 2024 at 02:31:59PM -0600, Alex Williamson wrote: > > > > > Yes, exactly. Zero'ing the page would obviously reestablish the > > > coherency, but the page could be reallocated without being zero'd and as > > > you describe the owner of that page could then get inconsistent > > > results. > > > > I think if we care about the performance of this stuff enough to try > > and remove flushes we'd be better off figuring out how to disable no > > snoop in PCI config space and trust the device not to use it and avoid > > these flushes. > > > > iommu enforcement is nice, but at least ARM has been assuming that the > > PCI config space bit is sufficient. > > > > Intel/AMD are probably fine here as they will only flush for weird GPU > > cases, but I expect ARM is going to be unhappy. > > > > My impression was that Intel GPU is not usable w/o non-coherent DMA, > but I don't remember whether it's unusable being a functional breakage > or a user experience breakage. e.g. I vaguely recalled that the display > engine cannot afford high resolution/high refresh rate using the snoop > way so the IOMMU dedicated for the GPU doesn't implement the force > snoop capability. > > Daniel, can you help explain the behavior of Intel GPU in case nosnoop > is disabled in the PCI config space? > > Overall it sounds that we are talking about different requirements. For > Intel GPU nosnoop is a must but it is not currently done securely so we > need add proper flush to fix it, while for ARM looks you don't have a > case which relies on nosnoop so finding a way to disable it is more > straightforward? Intel GPU weirdness should not leak into making other devices insecure/slow. If necessary Intel GPU only should get some variant override to keep no snoop working. It would make alot of good sense if VFIO made the default to disable no-snoop via the config space. Jason
