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Subject: Re: [PATCH] xen/x86: add diagnostic printout to xen_mc_flush() in
 case of error
To:     Juergen Gross <jgross@suse.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>
 <04dc8206-d7ad-9438-2f68-c25bf5a10b96@suse.com>
From:   Boris Ostrovsky <boris.ostrovsky@oracle.com>
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Message-ID: <a379cd7c-2da3-930f-606f-9d567db77c51@oracle.com>
Date:   Tue, 27 Nov 2018 08:57:18 -0500
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On 11/27/18 2:46 AM, Juergen Gross wrote:
> 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

Yes, I saw that later, after I responded to the patch.


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

I did.

Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>

(with commit message fixed)


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