linux-kernel - Re: [PATCH RESEND v2 2/2] xen: enable vnuma for PV guest

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Date:	Tue, 19 Nov 2013 09:46:30 -0500
From:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
To:	David Vrabel <david.vrabel@...rix.com>
Cc:	Elena Ufimtseva <ufimtseva@...il.com>,
	xen-devel@...ts.xenproject.org, boris.ostrovsky@...cle.com,
	tglx@...utronix.de, mingo@...hat.com, hpa@...or.com,
	x86@...nel.org, akpm@...ux-foundation.org, tangchen@...fujitsu.com,
	wency@...fujitsu.com, ian.campbell@...rix.com,
	stefano.stabellini@...citrix.com, mukesh.rathor@...cle.com,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH RESEND v2 2/2] xen: enable vnuma for PV guest

On Tue, Nov 19, 2013 at 02:35:59PM +0000, David Vrabel wrote:
> On 19/11/13 14:16, Konrad Rzeszutek Wilk wrote:
> > On Tue, Nov 19, 2013 at 11:54:08AM +0000, David Vrabel wrote:
> >> On 18/11/13 21:58, Elena Ufimtseva wrote:
> >>> Enables numa if vnuma topology hypercall is supported and it is domU.
> >> [...]
> >>> --- a/arch/x86/xen/setup.c
> >>> +++ b/arch/x86/xen/setup.c
> >>> @@ -20,6 +20,7 @@
> >>>  #include <asm/numa.h>
> >>>  #include <asm/xen/hypervisor.h>
> >>>  #include <asm/xen/hypercall.h>
> >>> +#include <asm/xen/vnuma.h>
> >>>  
> >>>  #include <xen/xen.h>
> >>>  #include <xen/page.h>
> >>> @@ -598,6 +599,9 @@ void __init xen_arch_setup(void)
> >>>  	WARN_ON(xen_set_default_idle());
> >>>  	fiddle_vdso();
> >>>  #ifdef CONFIG_NUMA
> >>> -	numa_off = 1;
> >>> +	if (!xen_initial_domain() && xen_vnuma_supported())
> >>> +		numa_off = 0;
> >>> +	else
> >>> +		numa_off = 1;
> >>>  #endif
> >>>  }
> >>
> >> I think this whole #ifdef CONFIG_NUMA can be removed and hence
> >> xen_vnuma_supported() can be removed as well.
> >>
> >> For any PV guest we can call the xen_numa_init() and it will do the
> >> right thing.
> >>
> >> For dom0, the hypercall will either: return something sensible (if in
> >> the future Xen sets something up), or it will error.
> >>
> >> If Xen does not have vnuma support, the hypercall will error.
> >>
> >> In both error cases, the dummy numa node is setup as required.
> > 
> > Incorrect. It will end up calling:
> > 
> >                  if (!numa_init(amd_numa_init))                                  
> > 
> > which will crash dom0 (see 8d54db795 "xen/boot: Disable NUMA for PV guests.")
> > as that amd_numa_init is called before the dummy node init.
> 
> No it won't.  Any error path after the check for a PV guest will add the
> dummy node and return success, skipping any of the hardware-specific setup.

Duh! I totally missed 'return' at the end of the check!

However, even with that (so the return), that means
this part won't be called:

649         numa_init(dummy_numa_init);                                             

Which means there won't be any dummy numa setup?

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