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Re: [Xen-devel] Question about mapping between domains

Hi, Ian. Thank You for the responce.

Currently have 3 kernels: Thin Dom0 (privileged), DomD (privileged
driver domain),
DomU (not privileged)

On Mon, Jul 13, 2015 at 12:04 PM, Ian Campbell <ian.campbell@xxxxxxxxxx> wrote:
> The way we deal with this elsewhere in the kernel is that we only ever
> do grant mappings over ballooned out pages, which are allocated via
> gnttab_alloc_pages. That way when they are unmapped the page is expected
> to be entry and no backing mfn is lost. The page can then subsequently
> be ballooned back in as normal.
We can not use this case because our DRM driver has already allocated memory
which will be mapped later.

> There is an additional quirk for a 1:1 mapped dom0 which is that we
> don't actually decrease reservation when ballooning, but keep the 1:1
> mfn in anticipation of ballooning it back in later.
Could You please tell me a bit more information about this quirk. How this quirk
can be enabled?

> If you can't arrange to use already ballooned buffers for your DMA
> buffer then you will need to manually balloon it out before and balloon
> it back in later.
This is my case. I'll try to to this.

> You may also want to extend the dom0 1:1 quirk described above to your
> 1:1 mapped domD.
Necessarily I will do this.

> If you have sufficient control over/knowledge of the domD IPA space then
> you could also try and arrange that the region used for these mappings
> does not correspond to any real RAM in the guest (i.e. stick it in an
> MMIO hole). That depends on you never needing to find an associated
> struct page though, which will depend on your use case.
Necessarily I will do this.

> Ian.

Oleksandr Dmytryshyn | Product Engineering and Development
M +38.067.382.2525


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