Re: [Xen-devel] [PATCH RFC 0/1] Introduce VCPUOP_reset_vcpu_info

[Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>]

On Wed, Aug 06, 2014 at 03:09:43PM +0100, Jan Beulich wrote:
> >>> On 06.08.14 at 15:08, <vkuznets@xxxxxxxxxx> wrote:
> > --- a/arch/x86/xen/enlighten.c
> > +++ b/arch/x86/xen/enlighten.c
> > @@ -183,8 +183,6 @@ static void xen_vcpu_setup(int cpu)
> >          * This path is called twice on PVHVM - first during bootup via
> >          * smp_init -> xen_hvm_cpu_notify, and then if the VCPU is being
> >          * hotplugged: cpu_up -> xen_hvm_cpu_notify.
> > -        * As we can only do the VCPUOP_register_vcpu_info once lets
> > -        * not over-write its result.
> >          *
> >          * For PV it is called during restore (xen_vcpu_restore) and bootup
> >          * (xen_setup_vcpu_info_placement). The hotplug mechanism does not
> > @@ -207,14 +205,23 @@ static void xen_vcpu_setup(int cpu)
> >         info.mfn = arbitrary_virt_to_mfn(vcpup);
> >         info.offset = offset_in_page(vcpup);
> >  
> > +       /*
> > +        * Call VCPUOP_reset_vcpu_info before VCPUOP_register_vcpu_info, 
> > this
> > +        * is required if we boot after kexec.
> > +        */
> > +
> > +       if (cpu != 0) {
> > +               err = HYPERVISOR_vcpu_op(VCPUOP_reset_vcpu_info, cpu, NULL);
> > +               if (err)
> > +                       pr_warn("VCPUOP_reset_vcpu_info for CPU%d failed: 
> > %d\n",
> > +                               cpu, err);
> > +       }
> 
> Just for my understanding of why exactly you need the new operation:
> Why is this being done here, when you already do the reset in the
> cpu-die/shutdown paths? And why not for CPU 0?

For CPU0 it will always error out (as we can't do the hypercall on
v != current as we are doing this on an BSP path) in the hypervisor.

This path could also been taken when doing a kdump instead of kexec
path - hence the reset hadn't happen in the cpu-die shutdown paths.

> 
> Furthermore, what is the state of vCPU-s beyond 31 going to be after
> they got their vCPU info reset? They won't have any other area as
> fallback. Yet I don't think you can now and forever guarantee that
> native_cpu_die() won't do anything requiring that structure.
> 
> Jan
> 
> 
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