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

[Zhang, Lin-Bao (Linux Kernel R&D)]

Hi Suresh, 

	Thanks very much for your reply!
I tested this patch in 2.6.32 with our machine , it can indeed can resolve current issue. that 's good. 
I also check 3.x version of kernel source , for example , 3.3.8 and 3.0.0 , they indeed include similar patch , 
I think the key action is this line :
+	/*
+	 * The number of IO-APIC IRQ registers (== #pins):
+	 */
+	nr_ioapic_registers[idx] = entries;

In 3.3.8 , it is like this:
			/*
4011         * The number of IO-APIC IRQ registers (== #pins):
4012         */
4013        ioapics[idx].nr_registers = entries;
I also use 3.3.8 to test , no modification don't reproduce our issue. 
If I comment this line (ioapics[idx].nr_registers = entries;) , it will reproduce the problem that occurs in 2.6.32. 
so this can prove that your patch should work for 2.6.x kernel. 

But I am not sure why it can work. Let's discuss again. I am researching the whole source again. 
It seems that we have 2 directions to fix current problem :
a) your patch : 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.
b) my patch mask interrupt during VT-d table base address changing and VT-d entries initialized successfully. 
I suppose during the time window , we should disable 8259A interrupt. 

> 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 , we can think ,as your patch , during the window , IO-apic is useless or we can think IO-APIC doesn't exist ? 
Could you mind please sharing your design details ? thanks very much! 

> 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.
> 
> 2.6.32 kernel is missing this fix,
> 
Yes, 2.6.32 doesn't have this patch. 3.x.x indeed have it. 

> Because of this, in v2.6.32, mask_IO_APIC_setup() is not working as expected
> as nr_ioapic_registers[] are not yet initialized and thus the io-apic RTE's are not
> masked as expected.
> 
> We just need the last hunk of that patch, I think.
> 
> Can you please apply the appended patch to 2.6.32 kernel and see if the issue
> you mentioned gets fixed? If so, we can ask the -stable and OSV's teams to
> pick up this fix.
Yes , it can resolve current issue. 
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