Re: [Xen-devel] IOREQ server on Arm

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 26.09.18 at 23:32, <julien.grall@xxxxxxx> wrote:
> This brings me to the next question. Do you expect set_special_p2m_entry 
> to take a reference on the page?
> 
> If not, we may run into some troubles because AFAICT you can map twice 
> the ioreq page in a guest but reference will only be taken on the 
> allocation.
> 
> However, the unmap path will always drop a reference when removing the 
> page. This is because Xen at the moment, reference will not be taken on 
> mapping but allocation (we assume a page could not be mapped twice in a 
> guest).
> 
> Foreign mapping on Arm are a bit special because we get a reference on 
> mapping them and will drop it when the mapping disappear. So we would 
> not have any problem there.

Well, at least on x86 no refcounting happens at all in the p2m layer.
That's something we mean to fix eventually, but I'm unaware of any
work into that direction. If the situation is different on ARM, then
the behavior of set_special_p2m_entry() may need to differ between
it and x86 for the time being.

Jan



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