On Thu, May 23, 2024 at 04:47:53PM -0600, Alex Williamson wrote: > > > > I am suggesting to do both checks: > > > > - If the iommu domain indicates it has force coherency then leave PCI > > > > no-snoop alone and no flush > > > > - If the PCI NOSNOOP bit is or can be 0 then no flush > > > > - Otherwise flush > > > > > > How to judge whether PCI NOSNOOP can be 0? If following PCI spec > > > it can always be set to 0 but then we break the requirement for Intel > > > GPU. If we explicitly exempt Intel GPU in 2nd check then what'd be > > > the value of doing that generic check? > > > > Non-PCI environments still have this problem, and the first check does > > help them since we don't have PCI config space there. > > > > PCI can supply more information (no snoop impossible) and variant > > drivers can add in too (want no snoop) > > I'm not sure I follow either. Since i915 doesn't set or test no-snoop > enable, I think we need to assume drivers expect the reset value, so a > device that supports no-snoop expects to use it, ie. we can't trap on > no-snoop enable being set, the device is more likely to just operate > with reduced performance if we surreptitiously clear the bit. I'm not sure I understand this paragraph? > The current proposal is to enable flushing based only on the domain > enforcement of coherency. I think the augmentation is therefore that > if the device is PCI and the no-snoop enable bit is zero after reset > (indicating hardwired to zero), we also don't need to flush. Yes, that is a good additional starting point. > I'm not sure the polarity of the variant drive statement above is > correct. If the no-snoop enable bit is set after reset, we'd assume > no-snoop is possible, so the variant driver would only need a way to > indicate the device doesn't actually use no-snoop. For that it might > just virtualize the no-snoop enable setting to vfio-pci-core. Thanks, I wrote that with the idea that VFIO would always force no-snoop to 0. The variant driver could opt out of this. We could also do the reverse and leave no-snoop alone and have something force it off. The other issue to keep in mind is that if no-snoop is disabled when we attach the domains we shouldn't allow the VMM to turn it on later. Jason
