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

[Jason Wang <jasowang@redhat.com>]

On 2019/9/18 下午4:37, Tian, Kevin wrote:
>> From: Jason Wang [mailto:jasowang@redhat.com]
>> 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)?


I don't know, just want to say current API does not forbid this. So we 
probably need to take care it.


> Is Qemu doing its own same-content memory
> merging in GPA level, similar to KSM?


AFAIK, it doesn't.

Thanks


> Thanks
> Kevin