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

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Message-ID: <87bn1lx2fd.fsf@vitty.brq.redhat.com>
Date:	Mon, 25 Jul 2016 15:39:02 +0200
From:	Vitaly Kuznetsov <vkuznets@...hat.com>
To:	Julien Grall <julien.grall@....com>
Cc:	David Vrabel <david.vrabel@...rix.com>,
	xen-devel@...ts.xenproject.org, 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>,
	Jan Beulich <JBeulich@...e.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	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

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. 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...

-- 
  Vitaly