Re: [Xen-devel] [Ques]:xen_ident_map_ISA ant it's mfns...

[Mukesh Rathor <mukesh.rathor@xxxxxxxxxx>]

On Mon, 23 Jan 2012 12:43:17 -0400
Konrad Rzeszutek Wilk <konrad@xxxxxxxxxx> wrote:

> On Fri, Jan 20, 2012 at 05:29:32PM -0800, Mukesh Rathor wrote:
> > Hi,
> > 
> > I am bit confused about the do_update_va_mapping call that dom0
> > makes in xen_ident_map_ISA() to map ffff8800000a0000 to mfn a0. The
> > mfn belongs to DOMID_IO. Before the mfn is mapped, the l1 entry is
> > not empty:
> > 
> > 0000000139d08500:  00100001380a0027
> > 
> > After the mapping:
> > 
> > 0000000139d08500:  00100000000a0467  as expected.
> > 
> > However, the mfn 1380a0 still seems to belong to dom0. Shouldn't
> > that have been returned back to the xen heap? 
> 
> It should if dom0 had returned it back. The setup.c code you where
> it returns the swatches of memory has a check where it decides to
> return only memory above 1M. The <1MB is left unused.
> 
> In other words, we make all the PTE's to the look as if PFN are
> 1:1 to MFN. But if you do a MFN lookup to PFN (mfn_to_pfn) for the
> regions below 1MB, you end up getting PFNs that are _not_ in the 1MB
> region. Instead they are just normal memory.
> 
> There is a good reason for this, which if I remeber right is to allow
> fixmap to work correctly, but if you are using ioremap it would return
> empty RAM so that most of the PnP devices (like RTC) would not load.
> 
> hmm, I think so . I remember thinking to remove that once (so allow
> the <1MB region to be freed), and I hit some pretty big problems.
> 
> Is there a particular reason you want to "Free" that memory?

I am not trying to "free" it. It just occured to me the old frames are
never used, 1380a0 in the above case.

thanks,
Mukesh



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