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 12:18:24 -0700 (PDT)
From:	Stefano Stabellini <sstabellini@...nel.org>
To:	Vitaly Kuznetsov <vkuznets@...hat.com>
cc:	Julien Grall <julien.grall@....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, Vitaly Kuznetsov wrote:
> Julien Grall <julien.grall@....com> writes:
> 
> > Hello,
> >
> > 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.
> >
> 
> I probably know too little about ARM but it seems that unlike x86 we
> don't need the knowledge of _other_ vCPU ids before we start them so
> MPIDR looks very suitable.
> 
> >> It is a bit more convenient in the mapping I
> >> introduce as we can set it to a negative value to indicate there is no
> >> mapping available. I can definitely change that and use something like
> >> U32_MAX-1 to instead but I'm not sure it is worth it...
> >
> > I looked at the definition of cpu_acpi_id on x86 which return
> > x86_cpu_to_acpiid that has been defined to an uint32_t.
> >
> > So you are assuming that it will never be possible to have an ID >
> > 0x80000000.
> >
> > Also, this may not be true on ARM depending how we define the VCPU
> > mapping. We could decide to use the MPIDR which is in this case may be
> > considered as "negative".
> 
> While we're not obliged to have the same type for xen_vcpu_id on all
> arches I see no point in diverging without a reason. I can do v3 making
> the mapping uint32 

I agree that making the mapping uint32_t would be desirable. It would
even make sense from the int types point of view in Linux.


>and indicating the missing value as U32_MAX-1 if nobody is against the
>idea.

Why U32_MAX-1? (int)-1 is (unsigned)U32_MAX. Even
XEN_INVALID_MAX_VCPU_ID is defined as (~0U).