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

[Boris Ostrovsky]

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@xxxxxxxxxx>
> > > ---
> > >    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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