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

Sent from my iPhone

On Aug 7, 2021, at 12:51 AM, Michael Kelley [off-list ref] wrote:

From: תומר אבוטבול [off-list ref]  Sent: Friday, August 6, 2021 11:03 AM

quoted

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.

quoted

+               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