Re: [Qemu-devel] vhost, iova, and dirty page tracking

[Tian, Kevin]

> From: Jason Wang [mailto:address@hidden]
> Sent: Wednesday, September 18, 2019 2:10 PM
> 
> >>
> >> Note that the HVA to GPA mapping is not an 1:1 mapping. One HVA
> range
> >> could be mapped to several GPA ranges.
> > This is fine. Currently vfio_dma maintains IOVA->HVA mapping.
> >
> > btw under what condition HVA->GPA is not 1:1 mapping? I didn't realize it.
> 
> 
> I don't remember the details e.g memory region alias? And neither kvm
> nor kvm API does forbid this if my memory is correct.
> 

I checked https://qemu.weilnetz.de/doc/devel/memory.html, which
provides an example of aliased layout. However, its aliasing is all
1:1, instead of N:1. From guest p.o.v every writable GPA implies an
unique location. Why would we hit the situation where multiple
write-able GPAs are mapped to the same HVA (i.e. same physical
memory location)? Is Qemu doing its own same-content memory
merging in GPA level, similar to KSM?

Thanks
Kevin