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Subject: Re: [PATCH] xen/x86: add diagnostic printout to xen_mc_flush() in
 case of error
To:     Boris Ostrovsky <boris.ostrovsky@oracle.com>,
        linux-kernel@vger.kernel.org, xen-devel@lists.xenproject.org,
        x86@kernel.org
Cc:     sstabellini@kernel.org, hpa@zytor.com, tglx@linutronix.de,
        mingo@redhat.com, bp@alien8.de
References: <20181123162451.3351-1-jgross@suse.com>
 <0727da38-bfe8-f484-6b06-7205c1d953c1@oracle.com>
From:   Juergen Gross <jgross@suse.com>
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Date:   Tue, 27 Nov 2018 08:46:38 +0100
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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@suse.com>
>> ---
>>  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