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

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> 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.

Jan


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