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 15:01:03 +0100
From:	Julien Grall <julien.grall@....com>
To:	Vitaly Kuznetsov <vkuznets@...hat.com>
Cc:	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

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.

> 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".

Regards,

-- 
Julien Grall