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

[Oleksandr Dmytryshyn <oleksandr.dmytryshyn@xxxxxxxxxxxxxxx>]

On Tue, Jul 14, 2015 at 6:31 PM, Oleksandr Dmytryshyn
<oleksandr.dmytryshyn@xxxxxxxxxxxxxxx> wrote:
>
> 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.
Here is one question.
Could anybody tell me how to manually balloon a page in/out?

> > 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
> GlobalLogic
> M +38.067.382.2525
> www.globallogic.com
>
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