Re: [PATCH] x86/hyper-v: guard against cpu mask changes in hyperv_flush_tlb_others()

[David Moses <mosesster@gmail.com>]

Sent from my iPhone

> On Aug 7, 2021, at 12:51 AM, Michael Kelley <mikelley@microsoft.com> wrote:
> 
> From: תומר אבוטבול <tomer432100@gmail.com>  Sent: Friday, August 6, 2021 11:03 AM
> 
>> Attaching the patches Michael asked for debugging 
>> 1) Print the cpumask when < num_possible_cpus():
>> diff --git a/arch/x86/hyperv/mmu.c b/arch/x86/hyperv/mmu.c
>> index e666f7eaf32d..620f656d6195 100644
>> --- a/arch/x86/hyperv/mmu.c
>> +++ b/arch/x86/hyperv/mmu.c
>> @@ -60,6 +60,7 @@ static void hyperv_flush_tlb_others(const struct cpumask *cpus,
>>         struct hv_tlb_flush *flush;
>>         u64 status = U64_MAX;
>>         unsigned long flags;
>> +       unsigned int cpu_last;
>> 
>>         trace_hyperv_mmu_flush_tlb_others(cpus, info);
>> 
>> @@ -68,6 +69,11 @@ static void hyperv_flush_tlb_others(const struct cpumask *cpus,
>> 
>>         local_irq_save(flags);
>> 
>> +       cpu_last = cpumask_last(cpus);
>> +       if (cpu_last > num_possible_cpus()) {
> 
> I think this should be ">=" since cpus are numbered starting at zero.
> In your VM with 64 CPUs, having CPU #64 in the list would be error.
> 
>> +               pr_emerg("ERROR_HYPERV: cpu_last=%*pbl", cpumask_pr_args(cpus));
>> +       }
>> +
>>         /*
>>          * Only check the mask _after_ interrupt has been disabled to avoid the
>>          * mask changing under our feet.
>> 
>> 2) disable the Hyper-V specific flush routines:
>> diff --git a/arch/x86/hyperv/mmu.c b/arch/x86/hyperv/mmu.c
>> index e666f7eaf32d..8e77cc84775a 100644
>> --- a/arch/x86/hyperv/mmu.c
>> +++ b/arch/x86/hyperv/mmu.c
>> @@ -235,6 +235,7 @@ static u64 hyperv_flush_tlb_others_ex(const struct cpumask *cpus,
>> 
>> void hyperv_setup_mmu_ops(void)
>>  {
>> +  return;
>>         if (!(ms_hyperv.hints & HV_X64_REMOTE_TLB_FLUSH_RECOMMENDED))
>>                 return;
> 
> Otherwise, this code looks good to me and matches what I had in mind.
> 
> Note that the function native_flush_tlb_others() is used when the Hyper-V specific
> flush function is disabled per patch #2 above, or when hv_cpu_to_vp_index() returns
> VP_INVALID.  In a quick glance through the code, it appears that native_flush_tlb_others()
> will work even if there's a non-existent CPU in the cpumask that is passed as an argument.
> So perhaps an immediate workaround is Patch #2 above.

The current code of hv_cpu_to_vp_index (where I generated the warning ) is returning VP_INVALID in this case (see previous mail) and look like it is not completely workaround the issue.
the cpu is hanging even not panic Will continue watching .
>   
> 
> Perhaps hyperv_flush_tlb_others() should be made equally tolerant of a non-existent
> CPU being in the list. But if you are willing, I'm still interested in the results of an
> experiment with just Patch #1.  I'm curious about what the CPU list looks like when
> it has a non-existent CPU.  Is it complete garbage, or is there just one non-existent
> CPU?
> 
 We will do my be not next week since vacation but the week after

> The other curiosity is that I haven't seen this Linux panic reported by other users,
> and I think it would have come to our attention if it were happening with any frequency.
> You see the problem fairly regularly.  So I'm wondering what the difference is.
> 
> Michael