linux-kernel - Re: [Xen-devel] [PATCH v11 05/12] xen/pvh: Update E820 to work with PVH

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Date:	Wed, 18 Dec 2013 15:30:37 -0500
From:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
To:	Stefano Stabellini <stefano.stabellini@...citrix.com>
Cc:	xen-devel@...ts.xenproject.org, linux-kernel@...r.kernel.org,
	boris.ostrovsky@...cle.com, david.vrabel@...rix.com,
	mukesh.rathor@...cle.com, jbeulich@...e.com
Subject: Re: [Xen-devel] [PATCH v11 05/12] xen/pvh: Update E820 to work with
 PVH

On Wed, Dec 18, 2013 at 06:25:15PM +0000, Stefano Stabellini wrote:
> On Tue, 17 Dec 2013, Konrad Rzeszutek Wilk wrote:
> > From: Mukesh Rathor <mukesh.rathor@...cle.com>
> > 
> > In xen_add_extra_mem() we can skip updating P2M as it's managed
> > by Xen. PVH maps the entire IO space, but only RAM pages need
> > to be repopulated.
> > 
> > Signed-off-by: Mukesh Rathor <mukesh.rathor@...cle.com>
> > Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
> > ---
> >  arch/x86/xen/setup.c | 19 +++++++++++++++++--
> >  1 file changed, 17 insertions(+), 2 deletions(-)
> > 
> > diff --git a/arch/x86/xen/setup.c b/arch/x86/xen/setup.c
> > index 2137c51..f93bca1 100644
> > --- a/arch/x86/xen/setup.c
> > +++ b/arch/x86/xen/setup.c
> > @@ -27,6 +27,7 @@
> >  #include <xen/interface/memory.h>
> >  #include <xen/interface/physdev.h>
> >  #include <xen/features.h>
> > +#include "mmu.h"
> >  #include "xen-ops.h"
> >  #include "vdso.h"
> >  
> > @@ -81,6 +82,9 @@ static void __init xen_add_extra_mem(u64 start, u64 size)
> >  
> >  	memblock_reserve(start, size);
> >  
> > +	if (xen_feature(XENFEAT_auto_translated_physmap))
> > +		return;
> > +
> >  	xen_max_p2m_pfn = PFN_DOWN(start + size);
> >  	for (pfn = PFN_DOWN(start); pfn < xen_max_p2m_pfn; pfn++) {
> >  		unsigned long mfn = pfn_to_mfn(pfn);
> > @@ -103,6 +107,7 @@ static unsigned long __init xen_do_chunk(unsigned long start,
> >  		.domid        = DOMID_SELF
> >  	};
> >  	unsigned long len = 0;
> > +	int xlated_phys = xen_feature(XENFEAT_auto_translated_physmap);
> >  	unsigned long pfn;
> >  	int ret;
> >  
> > @@ -116,7 +121,7 @@ static unsigned long __init xen_do_chunk(unsigned long start,
> >  				continue;
> >  			frame = mfn;
> >  		} else {
> > -			if (mfn != INVALID_P2M_ENTRY)
> > +			if (!xlated_phys && mfn != INVALID_P2M_ENTRY)
> >  				continue;
> >  			frame = pfn;
> >  		}
> > @@ -154,6 +159,13 @@ static unsigned long __init xen_do_chunk(unsigned long start,
> >  static unsigned long __init xen_release_chunk(unsigned long start,
> >  					      unsigned long end)
> >  {
> > +	/*
> > +	 * Xen already ballooned out the E820 non RAM regions for us
> > +	 * and set them up properly in EPT.
> > +	 */
> > +	if (xen_feature(XENFEAT_auto_translated_physmap))
> > +		return end - start;
> > +
> >  	return xen_do_chunk(start, end, true);
> >  }
> >  
> > @@ -222,6 +234,9 @@ static void __init xen_set_identity_and_release_chunk(
> >  	 * (except for the ISA region which must be 1:1 mapped) to
> >  	 * release the refcounts (in Xen) on the original frames.
> >  	 */
> > +	if (xen_feature(XENFEAT_auto_translated_physmap))
> > +		goto skip;
> > +
> >  	for (pfn = start_pfn; pfn <= max_pfn_mapped && pfn < end_pfn; pfn++) {
> >  		pte_t pte = __pte_ma(0);
> >  
> > @@ -231,7 +246,7 @@ static void __init xen_set_identity_and_release_chunk(
> >  		(void)HYPERVISOR_update_va_mapping(
> >  			(unsigned long)__va(pfn << PAGE_SHIFT), pte, 0);
> >  	}
> > -
> > +skip:
> >  	if (start_pfn < nr_pages)
> >  		*released += xen_release_chunk(
> >  			start_pfn, min(end_pfn, nr_pages));
> 
> A goto? Really? What's wrong with an if?

Moves too much code.
> Also considering that you are turning xen_release_chunk into a nop, the
> only purpose of this function on PVH is to call set_phys_range_identity.
> Can't we just do that?

And set_phys_range_identity ends up just doing:

 769         if (unlikely(xen_feature(XENFEAT_auto_translated_physmap)))             
 770                 return pfn_e - pfn_s;   

So what we really could do (which is what Mukesh's patch had):

if (xen_feature(XENFEAT..) {
	if (start_pfn < nr_pages)
		*released += min(end_pfn, nr_pages) - start_pfn;
	*identity += end_pfn - start_pfn;
}

in its own function. We could do that.
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