linux-kernel - Re: [patch 1/3] x86, intr-remap: set redirection hint in the IRTE

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Date:	Tue, 31 Aug 2010 11:29:30 +0800
From:	Weidong Han <weidong.han@...el.com>
To:	"Siddha, Suresh B" <suresh.b.siddha@...el.com>
CC:	"H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...e.hu>,
	Thomas Gleixner <tglx@...utronix.de>,
	LKML <linux-kernel@...r.kernel.org>,
	"stable@...nel.org" <stable@...nel.org>
Subject: Re: [patch 1/3] x86, intr-remap: set redirection hint in the IRTE

Siddha, Suresh B wrote:
> On Sun, 2010-08-29 at 18:47 -0700, Han, Weidong wrote:
>   
>> Siddha, Suresh B wrote:
>>     
>>> Currently the redirection hint in the interrupt-remapping table entry
>>> is set to 0, which means the remapped interrupt is directed to the
>>> processors listed in the destination. So in logical flat mode
>>> in the presence of intr-remapping, this results in a single
>>> interrupt multi-casted to multiple cpu's as specified by the destination
>>> bit mask. But what we really want is to send that interrupt to one of the cpus
>>> based on the lowest priority delivery mode.
>>>
>>> Set the redirection hint in the IRTE to '1' to indicate that we want
>>> the remapped interrupt to be directed to only one of the processors
>>> listed in the destination.
>>>
>>> This fixes the issue of same interrupt getting delivered to multiple cpu's
>>> in the logical flat mode in the presence of interrupt-remapping. While
>>> there is no functional issue observed with this behavior, this will
>>> impact performance of such configurations (<=8 cpu's using logical flat
>>> mode in the presence of interrupt-remapping)
>>>
>>> Signed-off-by: Suresh Siddha <suresh.b.siddha@...el.com>
>>> Cc: Weidong Han <weidong.han@...el.com>
>>> Cc: <stable@...nel.org> # [v2.6.32+]
>>> ---
>>>
>>> diff --git a/arch/x86/kernel/apic/io_apic.c b/arch/x86/kernel/apic/io_apic.c
>>> index f1efeba..90f8a75 100644
>>> --- a/arch/x86/kernel/apic/io_apic.c
>>> +++ b/arch/x86/kernel/apic/io_apic.c
>>> @@ -1392,6 +1392,7 @@ int setup_ioapic_entry(int apic_id, int irq,
>>>  		irte.dlvry_mode = apic->irq_delivery_mode;
>>>  		irte.vector = vector;
>>>  		irte.dest_id = IRTE_DEST(destination);
>>> +		irte.redir_hint = 1;
>>>  
>>>  		/* Set source-id of interrupt request */
>>>  		set_ioapic_sid(&irte, apic_id);
>>> @@ -3343,6 +3344,7 @@ static int msi_compose_msg(struct pci_dev *pdev, unsigned int irq,
>>>  		irte.dlvry_mode = apic->irq_delivery_mode;
>>>  		irte.vector = cfg->vector;
>>>  		irte.dest_id = IRTE_DEST(dest);
>>> +		irte.redir_hint = 1;
>>>  
>>>  		/* Set source-id of interrupt request */
>>>  		if (pdev)
>>>
>>>
>>>   
>>>       
>> hi Suresh,
>>
>> I think it needs to set redir_hint only for lowest priority delivery 
>> mode. Why set it for all delivery modes?
>>     
>
> Weidong, For all the IRQ types (irrespective of lowest priority/fixed
> mode etc) x86 linux kernel doesn't use irq multi-cast model (which is
> what irte.redir_hint == 0 means). So as it is not particularly tied with
> lowest priority mode (irte.redir_hint is equally applicable for fixed
> mode though we don't use it currently), it makes sense to set it
> unconditionally for the current usages.
>
>
>   
Thanks for your explanation. It's ok.

Regards,
Weidong

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