Re: [PATCH] kvm: bios: Put AP boot up code to 0x1000

["Sebastian Herbszt" <herbszt@gmx.de>]

Yang, Sheng wrote:

>> > Please refer to my another patch comment. Basicly, AP is running
>> > HLT in 0x10038, but it can exit due to some reason, here QEmu
>> > want to raise it up . But after AP exit from HLT, it can't return
>> > to it because the code was overrided by grub and no HLT loop
>> > there. Then AP had gone away...
>>
>> Thank you for pointing me to "[PATCH] KVM: Fix QEmu interrupted HLT
>> emulation". You wrote there:
>> "Though I also sent a patch for BIOS, it's necessary to get correct
>> behavior here."
>>
>> Would i be correct saying the problem is kvm/qemu only and with
>> that patch applied the bios fix is not needed?
> 
> No... I think more important is get bios fixed. Anyway, copy BIOS part 
> of running code to a place that may overwritten by userspace program 
> is wrong, if you expect AP still running when userspace program is 
> running.
> 
> In fact, if BIOS code in right place, we won't need fix HLT, for HLT 
> have a assumption that it might be interrupt by some event, so 
> usually it is wrapped by a loop, as current BIOS did. 

The AP enters HALT state with IF=0. What can make it resume at the
following "jmp 1b"? NMI? SMI?

>> If you still want to change the bios code i would suggest jumping
>> to the original smp_ap_boot_code location in rombios32.bin which
>> should not get overwritten. The following patch is based on the
>> bochs version.
>>
> 
> Yeah, I just curious why we need to copy BIOS here and there in the 
> beginning... 
> 
> Thanks for your patch, but is it based on bochs upstream?

Yes it's based on bochs upstream rombios32start.S r1.5.

> I found the 
> patch don't apply...

This one should apply to current kvm version.

> I also want to know why we need copy AP boot 
> code, if anyone know the detail... 

I think i read something about a need for 4kb alignment for the
startup vector in the mp specs. AP_BOOT_ADDR is currently
at 0x10000 (16 x 4kb).

- Sebastian

--- rombios32start.S.orig Wed Aug  6 21:37:22 2008
+++ rombios32start.S Wed Aug  6 21:39:17 2008
@@ -40,6 +40,7 @@ _start:
 
   .code16
 smp_ap_boot_code_start:
+  cli
   xor %ax, %ax
   mov %ax, %ds
   xor %eax, %eax
@@ -60,6 +61,7 @@ smp_ap_boot_code_start:
 12:
 
   incw CPU_COUNT_ADDR
+  ljmp $0xe000, $(1f-_start)
 1:
   hlt
   jmp 1b