Re: [Xen-devel] [PATCH] xen/pvhvm: Support more than 32 VCPUs when migrating (v3).

[Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>]

On 07/21/2016 10:14 AM, Konrad Rzeszutek Wilk wrote:
> On Fri, Jul 10, 2015 at 02:57:51PM -0400, Konrad Rzeszutek Wilk wrote:
>> On Fri, Jul 10, 2015 at 02:37:46PM -0400, Konrad Rzeszutek Wilk wrote:
>>> When Xen migrates an HVM guest, by default its shared_info can
>>> only hold up to 32 CPUs. As such the hypercall
>>> VCPUOP_register_vcpu_info was introduced which allowed us to
>>> setup per-page areas for VCPUs. This means we can boot PVHVM
>>> guest with more than 32 VCPUs. During migration the per-cpu
>>> structure is allocated freshly by the hypervisor (vcpu_info_mfn
>>> is set to INVALID_MFN) so that the newly migrated guest
>>> can make an VCPUOP_register_vcpu_info hypercall.
>>>
>>> Unfortunatly we end up triggering this condition in Xen:
>>> /* Run this command on yourself or on other offline VCPUS. */
>>>  if ( (v != current) && !test_bit(_VPF_down, &v->pause_flags) )
>>>
>>> which means we are unable to setup the per-cpu VCPU structures
>>> for running vCPUS. The Linux PV code paths make this work by
>>> iterating over every vCPU with:
>>>
>>>  1) is target CPU up (VCPUOP_is_up hypercall?)
>>>  2) if yes, then VCPUOP_down to pause it.
>>>  3) VCPUOP_register_vcpu_info
>>>  4) if it was down, then VCPUOP_up to bring it back up
>>>
>>> But since VCPUOP_down, VCPUOP_is_up, and VCPUOP_up are
>>> not allowed on HVM guests we can't do this. However with the
>>> Xen git commit f80c5623a126afc31e6bb9382268d579f0324a7a
>>> ("xen/x86: allow HVM guests to use hypercalls to bring up vCPUs"")
>> <sigh> I was in my local tree which was Roger's 'hvm_without_dm_v3'
>> looking at patches and spotted this - and thought it was already in!
>>
>> Sorry about this patch - and please ignore it until the VCPU_op*
>> can be used by HVM guests.
> The corresponding patch is in Xen tree 
> (192df6f9122ddebc21d0a632c10da3453aeee1c2)
>
> Could folks take a look at the patch pls?
>
> Without it you can't migrate an Linux guest with more than 32 vCPUs.
>
>>> we can do this. As such first check if VCPUOP_is_up is actually
>>> possible before trying this dance.
>>>
>>> As most of this dance code is done already in 'xen_setup_vcpu'
>>> lets make it callable on both PV and HVM. This means moving one
>>> of the checks out to 'xen_setup_runstate_info'.
>>>
>>> Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>
>>> ---
>>>  arch/x86/xen/enlighten.c | 23 +++++++++++++++++------
>>>  arch/x86/xen/suspend.c   |  7 +------
>>>  arch/x86/xen/time.c      |  3 +++
>>>  3 files changed, 21 insertions(+), 12 deletions(-)
>>>
>>> diff --git a/arch/x86/xen/enlighten.c b/arch/x86/xen/enlighten.c
>>> index 46957ea..187dec6 100644
>>> --- a/arch/x86/xen/enlighten.c
>>> +++ b/arch/x86/xen/enlighten.c
>>> @@ -238,12 +238,23 @@ static void xen_vcpu_setup(int cpu)
>>>  void xen_vcpu_restore(void)
>>>  {
>>>     int cpu;
>>> +   bool vcpuops = true;
>>> +   const struct cpumask *mask;
>>>  
>>> -   for_each_possible_cpu(cpu) {
>>> +   mask = xen_pv_domain() ? cpu_possible_mask : cpu_online_mask;
>>> +
>>> +   /* Only Xen 4.5 and higher supports this. */
>>> +   if (HYPERVISOR_vcpu_op(VCPUOP_is_up, smp_processor_id(), NULL) == 
>>> -ENOSYS)
>>> +           vcpuops = false;
>>> +
>>> +   for_each_cpu(cpu, mask) {
>>>             bool other_cpu = (cpu != smp_processor_id());
>>> -           bool is_up = HYPERVISOR_vcpu_op(VCPUOP_is_up, cpu, NULL);
>>> +           bool is_up = false;
>>>  
>>> -           if (other_cpu && is_up &&
>>> +           if (vcpuops)
>>> +                   is_up = HYPERVISOR_vcpu_op(VCPUOP_is_up, cpu, NULL);


You can just say
   is_up = (HYPERVISOR_vcpu_op(VCPUOP_is_up, cpu, NULL) > 0);

and then you won't need vcpuops bool.




>>> +
>>> +           if (vcpuops && other_cpu && is_up &&
>>>                 HYPERVISOR_vcpu_op(VCPUOP_down, cpu, NULL))
>>>                     BUG();
>>>  
>>> @@ -252,7 +263,7 @@ void xen_vcpu_restore(void)
>>>             if (have_vcpu_info_placement)
>>>                     xen_vcpu_setup(cpu);
>>>  
>>> -           if (other_cpu && is_up &&
>>> +           if (vcpuops && other_cpu && is_up &&
>>>                 HYPERVISOR_vcpu_op(VCPUOP_up, cpu, NULL))
>>>                     BUG();
>>>     }
>>> @@ -1704,8 +1715,8 @@ void __ref xen_hvm_init_shared_info(void)
>>>      * in that case multiple vcpus might be online. */
>>>     for_each_online_cpu(cpu) {
>>>             /* Leave it to be NULL. */
>>> -           if (cpu >= MAX_VIRT_CPUS)
>>> -                   continue;
>>> +           if (cpu >= MAX_VIRT_CPUS && cpu <= NR_CPUS)
>>> +                   per_cpu(xen_vcpu, cpu) = NULL; /* Triggers 
>>> xen_vcpu_setup.*/
>>>             per_cpu(xen_vcpu, cpu) = 
>>> &HYPERVISOR_shared_info->vcpu_info[cpu];

I don't think I understand this change.

Can you have cpu > NR_CPUS? And isn't per_cpu(xen_vcpu, cpu) NULL
already (as the comment at the top suggests)?


-boris


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