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Re: [Xen-devel] Re: [PATCH 1/5] xen: events: use irq_alloc_desc(_at) ins

To: Sander Eikelenboom <linux@xxxxxxxxxxxxxx>
Subject: Re: [Xen-devel] Re: [PATCH 1/5] xen: events: use irq_alloc_desc(_at) instead of open-coding an IRQ allocator.
From: Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>
Date: Thu, 28 Oct 2010 14:12:39 +0100
Cc: Jeremy Fitzhardinge <jeremy@xxxxxxxx>, "xen-devel@xxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxx>, Wilk <konrad.wilk@xxxxxxxxxx>, Stefano Stabellini <Stefano.Stabellini@xxxxxxxxxxxxx>, Ian Campbell <Ian.Campbell@xxxxxxxxxxxxx>, Konrad
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On Thu, 28 Oct 2010, Sander Eikelenboom wrote:
> > - Xen won't allocate pirq numbers lower than 16 (probably because it
> > expects pirq == gsi for the first 16 gsi), so it might run out
> > of pirqs if we ask Xen to always choose the pirq number for us.  As a
> > consequence it is safer to keep using pirq == gsi, at least for the
> > first 16 gsis. This limitation should probably be fixed in Xen, but we
> > need to support older hypervisors so we cannot rely on the fix to be
> > present.
> I don't know if this discussion is for dom0 kernels only., if it is .. is 
> that support of older hypervisors necessarily true ?
> If i read the xen pvops wiki:
>       NOTE! xen/stable-2.6.32.x versions after June 2010 ( and 
> newer) require at least Xen 4.0.1-rc2 or newer to work properly!
>       xend and xenstored will fail to start if using those kernel versions 
> with for example Xen 4.0.0.
>       There's an issue with creating/using /dev/xen/ device nodes, which has 
> been fixed in Xen 4.0.1-rc2 and newer versions.
>       See this patch: 
> http://xenbits.xen.org/xen-4.0-testing.hg?rev/0e1521f654f2 and discussion at: 
> http://lists.xensource.com/archives/html/xen-devel/2010-06/msg01129.html for 
> more information.
> So a pvops dom0 kernel all ready seems to require a very recent hypervisor. 
> So for a dom0 pvops kernel there has to be much less worry about support for 
> older hypervisors for Xen guests it would only apply for pv domains ?
> So this could perhaps also be the opportunity to change things ?

That is true, however we should try to avoid introducing new
incompatibilities unless really necessary.
In this case it should be sufficient to keep using pirq == gsi for
gsi < 16.

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