linux-kernel - Re: [Xen-devel] [PATCH] xen pv-on-hvm: add pfn_is

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Date:	Thu, 12 Jul 2012 16:22:34 +0200
From:	Olaf Hering <olaf@...fle.de>
To:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
Cc:	Jeremy Fitzhardinge <jeremy@...p.org>,
	xen-devel@...ts.xensource.com, linux-kernel@...r.kernel.org
Subject: Re: [Xen-devel] [PATCH] xen pv-on-hvm: add pfn_is_ram helper for
 kdump

On Thu, Jul 12, Konrad Rzeszutek Wilk wrote:

> On Thu, Jul 12, 2012 at 11:00:05AM +0200, Olaf Hering wrote:
> > Register pfn_is_ram helper speed up reading /proc/vmcore in the kdump
> > kernel. See commit message of 997c136f518c ("fs/proc/vmcore.c: add hook
> > to read_from_oldmem() to check for non-ram pages") for details.
> 
> What about the 'unregister_oldmem_pfn_is_ram' call? Should this
> be implemented here as well?

There is no need to unregister because PVonHVM is not going away during
the lifetime of the guest.

> > +#include <linux/crash_dump.h>
> 
> Should this also have an #idef CONFIG_PROC_VMCORE?
> Or is it going to compile OK without CONFIG_PROC_VMCORE defined?

All includes are supposed to work without ifdefs in the code, which is
the case also for crash_dump.h

> > + *   previous kernel. If the pfn is ballooned, handle it properly.
> 
> . and by handle it properly you mean return -ENXIO?

Return code 0 means its a ballooned page and needs special care,
non-null means it has to be handled as a RAM page.

If the hypervisor is too old (4.1 and earlier) it just means that in
case of kdump it will just cause high load in dom0. 
In case of kexec it will mean that the new kernel will crash because it
can not know which pfn is actually there.

> > + * Returns 0 if the pfn is not backed by a RAM page.
> > + */
> > +static int xen_oldmem_pfn_is_ram(unsigned long pfn)
> > +{
> > +	struct xen_hvm_get_mem_type a;
> 
> Can you make this have already initialized values, like:
> 
> 	struct xen_hvm_get_mem_type a = {
> 		.domid = DOMID_SELF;
> 		.pfn = pfn
> 	};

Yes.

> Also is this hypercall new? Or has it been in existence for some time?

Its new in 4.2, and was backported to 4.1.1 as well.


Olaf
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