On Mon, Jul 30, 2018 at 10:42:05AM -0600, Alex Williamson wrote: > On Mon, 30 Jul 2018 18:49:58 +0300 > "Michael S. Tsirkin" <m...@redhat.com> wrote: > > > 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. > > I'm not sure the hardware supports atomic updates in these cases and > therefore I don't think the vIOMMU does either. Thanks, > > Alex
Interesting. What makes you think simply writing into PTE then flushing the cache isn't atomic? -- MST
