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

[Alex Williamson <alex.williamson@redhat.com>]

On Mon, 30 Jul 2018 17:51:28 +0300
"Michael S. Tsirkin" <mst@redhat.com> wrote:

> On Mon, Jul 30, 2018 at 08:39:39AM -0600, Alex Williamson wrote:
> > This is more
> > compatible with the IOMMU mappings,  
> 
> Precisely. These are at page granularity.

(This/these being memory API mappings for context)

SubRegions are not page granule, the entire previous SubRegion needs to
be unmapped and any remaining SubRegions re-mapped.
 
> > 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.  Thanks,

Alex