Re: [Xen-devel] [PATCH] xen/x86: add diagnostic printout to xen_mc_flush() in case of error

[Juergen Gross <jgross@xxxxxxxx>]

On 26/11/2018 21:11, Boris Ostrovsky wrote:
> On 11/23/18 11:24 AM, Juergen Gross wrote:
>> Failure of an element of a Xen multicall is signalled via a WARN()
>> only unless the kernel is compiled with MC_DEBUG. It is impossible to
> 
> s/unless/if
> 
> 
>> know which element failed and why it did so.
>>
>> Change that by printing the related information even without MC_DEBUG,
>> even if maybe in some limited form (e.g. without information which
>> caller produced the failing element).
>>
>> Move the printing out of the switch statement in order to have the
>> same information for a single call.
>>
>> Signed-off-by: Juergen Gross <jgross@xxxxxxxx>
>> ---
>>  arch/x86/xen/multicalls.c | 35 ++++++++++++++++++++---------------
>>  1 file changed, 20 insertions(+), 15 deletions(-)
>>
>> diff --git a/arch/x86/xen/multicalls.c b/arch/x86/xen/multicalls.c
>> index 2bce7958ce8b..0766a08bdf45 100644
>> --- a/arch/x86/xen/multicalls.c
>> +++ b/arch/x86/xen/multicalls.c
>> @@ -69,6 +69,11 @@ void xen_mc_flush(void)
>>  
>>      trace_xen_mc_flush(b->mcidx, b->argidx, b->cbidx);
>>  
>> +#if MC_DEBUG
>> +    memcpy(b->debug, b->entries,
>> +           b->mcidx * sizeof(struct multicall_entry));
>> +#endif
>> +
>>      switch (b->mcidx) {
>>      case 0:
>>              /* no-op */
>> @@ -87,32 +92,34 @@ void xen_mc_flush(void)
>>              break;
>>  
>>      default:
>> -#if MC_DEBUG
>> -            memcpy(b->debug, b->entries,
>> -                   b->mcidx * sizeof(struct multicall_entry));
>> -#endif
>> -
>>              if (HYPERVISOR_multicall(b->entries, b->mcidx) != 0)
>>                      BUG();
>>              for (i = 0; i < b->mcidx; i++)
>>                      if (b->entries[i].result < 0)
>>                              ret++;
>> +    }
>>  
>> +    if (WARN_ON(ret)) {
>> +            pr_err("%d of %d multicall(s) failed: cpu %d\n",
>> +                   ret, b->mcidx, smp_processor_id());
>> +            for (i = 0; i < b->mcidx; i++) {
>> +                    if (b->entries[i].result < 0) {
>>  #if MC_DEBUG
>> -            if (ret) {
>> -                    printk(KERN_ERR "%d multicall(s) failed: cpu %d\n",
>> -                           ret, smp_processor_id());
>> -                    dump_stack();
>> -                    for (i = 0; i < b->mcidx; i++) {
>> -                            printk(KERN_DEBUG "  call %2d/%d: op=%lu 
>> arg=[%lx] result=%ld\t%pF\n",
>> -                                   i+1, b->mcidx,
>> +                            pr_err("  call %2d: op=%lu arg=[%lx] 
>> result=%ld\t%pF\n",
>> +                                   i + 1,
>>                                     b->debug[i].op,
>>                                     b->debug[i].args[0],
>>                                     b->entries[i].result,
>>                                     b->caller[i]);
>> +#else
>> +                            pr_err("  call %2d: op=%lu arg=[%lx] 
>> result=%ld\n",
>> +                                   i + 1,
>> +                                   b->entries[i].op,
>> +                                   b->entries[i].args[0],
>> +                                   b->entries[i].result);
>> +#endif
> 
> Doesn't (non-debug) hypervisor corrupt op and args?

No. Only debug hypervisor does so.

See my patch (and rather lengthy discussion) on xen-devel:

https://lists.xen.org/archives/html/xen-devel/2018-11/msg02714.html

> 
> (Also, we don't really need to print anything when b->entries[i].result
> == 0)

Right. Did you miss the:

+                       if (b->entries[i].result < 0) {

above?


Juergen

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