linux-kernel - Re: [PATCH] xen: vcpu_info reinit error after 'xl save -c' & 'xl restore' on PVOPS VM which has multi-cpu

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Date:	Tue, 28 Apr 2015 17:31:05 -0400
From:	Boris Ostrovsky <boris.ostrovsky@...cle.com>
To:	"Ouyang Zhaowei (Charles)" <ouyangzhaowei@...wei.com>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	David Vrabel <david.vrabel@...rix.com>
CC:	linux-kernel@...r.kernel.org,
	Dingweiping <weiping.ding@...wei.com>,
	Yanqiangjun <yanqiangjun@...wei.com>, jinjian@...wei.com
Subject: Re: [PATCH] xen: vcpu_info reinit error after 'xl save -c' & 'xl
 restore' on PVOPS VM which has multi-cpu

On 04/28/2015 08:30 AM, Ouyang Zhaowei (Charles) wrote:
>
> On 2015.4.26 7:31, Boris Ostrovsky wrote:
>> On 04/24/2015 05:30 AM, Ouyang Zhaowei (Charles) wrote:
>>> If a PVOPS VM has multi-cpu the vcpu_info of cpu0 is the member of the structure HYPERVISOR_shared_info,
>>> and the others is not, but after 'xl save -c/restore' the vcpu_info will be reinitialized,
>>> the vcpu_info of all the vcpus will be considered as the member of HYPERVISOR_shared_info.
>>> This will cause the cpu1 and other cpu keep receiving interrupts, and the cpu0 is waiting them to
>>> finish the job.
>>> So we do not reinit the vcpu_info when PVOPS vm is doing 'xl save -c/restore'.
>>>
>>> Signed-off-by: Charles Ouyang <ouyangzhaowei@...wei.com>
>>> ---
>>>    arch/x86/xen/suspend.c | 3 ++-
>>>    1 file changed, 2 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/arch/x86/xen/suspend.c b/arch/x86/xen/suspend.c
>>> index d949769..b2bed45 100644
>>> --- a/arch/x86/xen/suspend.c
>>> +++ b/arch/x86/xen/suspend.c
>>> @@ -32,7 +32,8 @@ static void xen_hvm_post_suspend(int suspend_cancelled)
>>>    {
>>>    #ifdef CONFIG_XEN_PVHVM
>>>           int cpu;
>>> -       xen_hvm_init_shared_info();
>>> +       if (!suspend_cancelled)
>>> +               xen_hvm_init_shared_info();
>>>           xen_callback_vector();
>>>           xen_unplug_emulated_devices();
>>>           if (xen_feature(XENFEAT_hvm_safe_pvclock)) {
>> Do we need to call other routines if suspend is canceled?
>>
>> Also, if suspend is canceled then we don't do xen_irq_resume() if that's what you meant by "vcpu_info will be reinitialized". Were you referring some other re-initialization?
>>
> Hi Boris,
>
> Sorry I didn't make myself clear.
>
> About the "vcpu_info reinitialize", I mean in the function "xen_hvm_init_shared_info()" the pointer "xen_vcpu" will be reset and all
> point to HYPERVISOR_shared_info->vcpu_info[cpu].
>
> void __ref xen_hvm_init_shared_info(void)
> ----
> 1702          * When xen_hvm_init_shared_info is run at boot time only vcpu 0 is
> 1703          * online but xen_hvm_init_shared_info is run at resume time too and
> 1704          * in that case multiple vcpus might be online. */
> 1705         for_each_online_cpu(cpu) {
> 1706                 /* Leave it to be NULL. */
> 1707                 if (cpu >= MAX_VIRT_CPUS)
> 1708                         continue;
> 1709                 per_cpu(xen_vcpu, cpu) = &HYPERVISOR_shared_info->vcpu_info[cpu];
> 1710         }
> 1711 }
>
>
> But on Xen boot the init function "xen_start_kernel" only set the cpu0 to point to  HYPERVISOR_shared_info->vcpu_info[0]
>
> asmlinkage __visible void __init xen_start_kernel(void)


We are talking about HVM guests here and xen_start_kernel is only called 
for PV.  But even if it was, xen_vcpu pointers for other VCPUs are set 
in xen_vcpu_setup(), which is called when non-boot VCPUs are coming up.

And I wonder whether the actual problem is that we don't call 
xen_vcpu_setup() on canceled suspend (as we don't need to, really) and 
therefore if we call xen_hvm_init_shared_info() then 
per_cpu(xen_vcpu,cpu) for *non-boot* cpus is will become wrong.

-boris

> ----
> 1563         /* Don't do the full vcpu_info placement stuff until we have a
> 1564            possible map and a non-dummy shared_info. */
> 1565         per_cpu(xen_vcpu, 0) = &HYPERVISOR_shared_info->vcpu_info[0];
> 1566
> 1567         local_irq_disable();
>
> Other cpus are set to point to "xen_vcpu_info" in function xen_vcpu_setup().
>
> So after xl save -c/restore, the pointer xen_vcpu will be reset in function "xen_hvm_init_shared_info" and point to a wrong place.
> This may cause all the cpus cannot handle irqs except cpu0, so IMHO it's not necessary to call xen_hvm_init_shared_info again if
> suspend is canceled.
>
>> (The patch itself looks like the right thing to do though).
>>
>> -boris
>>
>> .
>>

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