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

[Ian Campbell <ian.campbell@xxxxxxxxxx>]

On Tue, 2015-07-14 at 18:41 +0300, Oleksandr Dmytryshyn wrote:
> 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?

Look at how the balloon driver does it, the hypercalls you want are
XENMEM_(increase|decrease)_reservation.

Ian.


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