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

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

On Wed, Aug 06, 2014 at 04:00:12PM +0100, Jan Beulich wrote:
> >>> On 06.08.14 at 16:51, <konrad.wilk@xxxxxxxxxx> wrote:
> > 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.
> 
> But that's unsafe, as the hypervisor would continue to write fields
> in that structure if it finds a need to.

That is OK. The kdump kernel boots from a reserved region so the
area where the hypervisor keeps on writting is in the 'outside-kdump-kernel'
boot area.
> 
> Jan
> 

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