linux-kernel - Re: [Xen-devel] [PATCH linux v2 0/9] xen: pvhvm: support bootup on secondary vCPUs

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Date:	Mon, 25 Jul 2016 14:37:50 -0700 (PDT)
From:	Stefano Stabellini <sstabellini@...nel.org>
To:	Julien Grall <julien.grall@....com>
cc:	Vitaly Kuznetsov <vkuznets@...hat.com>,
	Juergen Gross <jgross@...e.com>,
	Stefano Stabellini <sstabellini@...nel.org>,
	Andrew Cooper <andrew.cooper3@...rix.com>, x86@...nel.org,
	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...hat.com>,
	David Vrabel <david.vrabel@...rix.com>,
	Jan Beulich <JBeulich@...e.com>,
	"H. Peter Anvin" <hpa@...or.com>, xen-devel@...ts.xenproject.org,
	Boris Ostrovsky <boris.ostrovsky@...cle.com>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [Xen-devel] [PATCH linux v2 0/9] xen: pvhvm: support bootup on
 secondary vCPUs

On Mon, 25 Jul 2016, Julien Grall wrote:
> On 25/07/16 14:39, Vitaly Kuznetsov wrote:
> > Julien Grall <julien.grall@....com> writes:
> > 
> > > Hi David,
> > > 
> > > On 25/07/16 13:38, David Vrabel wrote:
> > > > On 30/06/16 16:56, Vitaly Kuznetsov wrote:
> > > > > It may happen that Xen's and Linux's ideas of vCPU id diverge. In
> > > > > particular, when we crash on a secondary vCPU we may want to do kdump
> > > > > and unlike plain kexec where we do migrate_to_reboot_cpu() we try
> > > > > booting
> > > > > on the vCPU which crashed. This doesn't work very well for PVHVM
> > > > > guests as
> > > > > we have a number of hypercalls where we pass vCPU id as a parameter.
> > > > > These
> > > > > hypercalls either fail or do something unexpected. To solve the issue
> > > > > we
> > > > > need to have a mapping between Linux's and Xen's vCPU ids.
> > > > > 
> > > > > This series solves the issue for x86 PVHVM guests. PV guests don't
> > > > > (and
> > > > > probably won't) support kdump so I always assume Xen's vCPU id ==
> > > > > Linux's
> > > > > vCPU id. ARM guests will probably need to get proper mapping once we
> > > > > start
> > > > > supporting kexec/kdump there.
> > > > 
> > > > Applied to for-linus-4.8, thanks.
> > > 
> > > It would have been nice to send a ping before applying. This patch
> > > series is containing Xen ARM code which has not been acked by Stefano,
> > > nor had feedback from ARM side.
> > > 
> > > For instance given that all the hypercalls are representing a "vcpu
> > > id" using "uint32_t" it is a bit weird to use "int" to define
> > > xen_vcpu_id (see patch #3).
> > 
> > CPU id is usually 'int' in linux and now we pass it to all
> > hypercalls as it is.
> 
> Well, we need to differentiate between the internal representation of the CPU
> which is based on the boot order and the logical CPU ID. For instance on ARM,
> the logical CPU ID may not be contiguous nor 0 for the first CPU.
> 
> From my understanding, the macros in patch #3 will be used at the last minute
> when prepare the hypercall data. IHMO this is very similar to a logical ID and
> defined as uint32_t by the hypercall ABI.
> 
> Although, I agree that currently we use the internal CPU id on ARM which is
> very unfortunate because this value is based on the order of the nodes in the
> device tree.
> 
> One way to abolish it on ARM would be to use the MPIDR (or at least a part)
> for the VCPU ID.

That's a good idea -- much more reliable than the device tree cpu order.