Re: [Qemu-devel] [RFC PATCH 0/3] Balloon inhibit enhancements

[Michael S. Tsirkin]

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.

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MST