On Mon, Jul 30, 2018 at 09:01:37AM -0600, Alex Williamson wrote: > > > but I don't think it can be done > > > atomically with respect to inflight DMA of a physical device where we > > > cannot halt the device without interfering with its state. > > > > Guests never add pages to the balloon if they are under DMA, > > so that's fine - there's never an in-flight DMA, if > > there is guest is buggy and it's ok to crash it. > > It's not the ballooned page that I'm trying to note, it's the entire > remainder of the SubRegion which needs to be unmapped to remove that > one page. It's more compatible from an IOMMU perspective in that we're > only unmapping with the same granularity with which we mapped, but it's > incompatible with inflight DMA as we have no idea what DMA targets may > reside within the remainder of that mapping while it's temporarily > unmapped. I see. Yes you need to be careful to replace the host IOMMU PTE atomically. Same applies to vIOMMU though - if guest changes a PTE atomically host should do the same. -- MST
