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Re: [Xen-devel] RT Xen on ARM - R-Car series

On 04.02.19 13:21, Julien Grall wrote:
What I meant is the virtual address stays the same but the guest physical 
address may change. I don't see how this could be broken today, can you explain 

I suppose guest's mapping change is not quite atomic from the hypervisor point 
of view, so domain could be caught in the middle.

Moreover, having that buffer mapped to XEN will reduce context switch time as a 
side effect.

I am still unsure whether we really want to keep that always mapped.

Each guest can support up to 128 vCPUs. So we would have 128 runstates mapped. 
Each runstate would take up to 2 pages. This means that each guest would 
require up to 1MB of vmap.

Here buffer allocation on XEN side might benefit, even aligning/fitting the 
runstate into one page might work. But I understand it is undesirable and 
requires lot of changes

I thought more about it during the week-end. I would actually not implement 
get_gfn but implement a function similar to get_page_from_gva on x86. The 
reason behind this is the function on Arm is quite complex as it caters many 
different use case.

I'll look into this. But I have to massage my yocto first.

Andrii Anisov.

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