Re: [Xen-devel] query the page type of a hvm page from within a

To:	Jan Beulich <JBeulich@xxxxxxxxxx>
Subject:	Re: [Xen-devel] query the page type of a hvm page from within a hvm guest
From:	Olaf Hering <olaf@xxxxxxxxx>
Date:	Mon, 2 May 2011 11:56:48 +0200
Cc:	"xen-devel@xxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxx>, Tim Deegan <Tim.Deegan@xxxxxxxxxx>
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On Mon, May 02, Jan Beulich wrote:

> > +   if (get_mem_type_supported)
> 
> The name of the variable seems badly chosen, and the way you
> coded this the code wouldn't be able to use the new interface
> after migrating from an incapable hypervisor to a capable one.

For the plain kdump usage thats probably ok. I think ballooned out
oldmem pages will turn into ram pages during migration, but I'm not sure.

> > +           return -ENXIO;
> > +
> > +   if (pfn == prev_pfn)
> > +           return prev_mem_type == HVMMEM_ram_rw;
> 
> Did you in fact observe many immediately subsequent calls with
> the same input (i.e. is the caching really worthwhile)?

It depends on how expensive a hypercall is.
read_from_oldmem() did read full pages in my testing.
I think removing the caching and do the hypercall unconditional is ok.

Olaf

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