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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>
From:   Boris Ostrovsky <boris.ostrovsky@oracle.com>
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Message-ID: <0727da38-bfe8-f484-6b06-7205c1d953c1@oracle.com>
Date:   Mon, 26 Nov 2018 15:11:05 -0500
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Sender: linux-kernel-owner@vger.kernel.org
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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?

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


-boris


>  			}
>  		}
> -#endif
>  	}
>  
>  	b->mcidx = 0;
> @@ -126,8 +133,6 @@ void xen_mc_flush(void)
>  	b->cbidx = 0;
>  
>  	local_irq_restore(flags);
> -
> -	WARN_ON(ret);
>  }
>  
>  struct multicall_space __xen_mc_entry(size_t args)