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

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Message-ID: <528B774F.7090902@citrix.com>
Date:	Tue, 19 Nov 2013 14:35:59 +0000
From:	David Vrabel <david.vrabel@...rix.com>
To:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.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 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.

David
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