linux-kernel - RE: [PATCH] fix x2apic defect that Linux kernel doesn't mask 8259A interrupt during the time window between changing VT-d table base address and initializing these VT-d entries(smpboot.c and apic.c )

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Date:	Wed, 10 Oct 2012 23:02:29 +0000
From:	"Zhang, Lin-Bao (Linux Kernel R&D)" <linbao.zhang@...com>
To:	Suresh Siddha <suresh.b.siddha@...el.com>
CC:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"alan@...rguk.ukuu.org.uk" <alan@...rguk.ukuu.org.uk>,
	"mingo@...hat.com" <mingo@...hat.com>,
	"Croxon, Nigel" <nigel.croxon@...com>,
	"tglx@...utronix.de" <tglx@...utronix.de>,
	"hpa@...or.com" <hpa@...or.com>, "x86@...nel.org" <x86@...nel.org>,
	"a.p.zijlstra@...llo.nl" <a.p.zijlstra@...llo.nl>,
	"Sakkinen, Jarkko" <jarkko.sakkinen@...el.com>,
	"joerg.roedel@....com" <joerg.roedel@....com>,
	"agordeev@...hat.com" <agordeev@...hat.com>,
	"yinghai@...nel.org" <yinghai@...nel.org>,
	"Zhang, Lin-Bao (Linux Kernel R&D)" <linbao.zhang@...com>
Subject: RE: [PATCH] fix x2apic defect that Linux kernel doesn't mask 8259A
 interrupt during the time window between changing VT-d table base address
 and initializing these VT-d entries(smpboot.c and apic.c )


Hi Suresh, 

Now , I want to clarify 2 type of errors :
a) some interrupt doesn't work through Interrupt -remapped IO-APIC. 
b) an interrupt is coming , there is an IO-APIC entry to know how to route it to which CPU, 
but VT-d entry is empty, ITP will report Present bit is clear. 

BIOS should provide simple IO-APIC entries and VT-d entries ,although we don't trust them. 
in the issue that we have happened to is of the second case. 

1 , 
> As I mentioned earlier, the current design already ensures that all the IO-APIC
> RTE's are masked between the time we enable interrupt-remapping to the time
> when the IO-APIC RTE's are configured correctly.
> 
> So I looked at why you are seeing the problem with v2.6.32 but not with the
> recent kernels. And I think I found out the reason.

I want to know what masking IO-APIC means? 
I know from BIOS point of view ,they can disable IO-APIC, just like no IO-APIC equipment. 
So masking IO-APIC entries equals to disable IO-APIC ?  when a interrupt is coming ,  
System doesn't make it through IO-APIC ,and directly to Local APIC ? 
If yes , I have a question : 
I first draw a time line :
    a)                          Clear IO-APIC table                    c)  
    |-------------------------------------------------|------------------------------------------------------|
Change VT-d table                  b)                   create IO-APIC table and VT-d table and initialize them
Empty table                       empty IO-APIC table   
When interrupt is coming between a) and b) ?   I suppose the error : "VT-d entry 's Present bit is clear ).
When interrupt is coming between b) and c) ?   ??
When interrupt is coming after c) point ? 		I suppose everything will work fine.





2,  
We know maybe bios will provide a simple IO-APIC table for us, although we OS doesn't believe it. 
I also did a test : 

 	-  disable x2apic in BIOS. 
find kernel source 3.3.8 , and delete that patch (comment that key line) .
I found that it would display :
     Kernel panic - not syncing: timer doesn't work through Interrupt-remapped IO-APIC.   
But if we disable x2apic in BIOS , rhel6.2 doesn't need any patch, it can run. Very strange. 

	- enable x2apic in BIOS 
	delete your patch from 3.3.8 , same issue with us. VT-d entry error (Present bit0 is clear ).
But it doesn't display the error above(don't tell me it doesn't work through IO-APIC ,  
it represents io-apic entry exists) .


-- Bob(LinBao Zhang)
HP linux kernel enginner