Re: [PATCH] kvm: bios: end AP boot code execution in rombios

[Avi Kivity <avi@qumranet.com>]

Yang, Sheng wrote:
> On Monday 18 August 2008 10:33:11 Anthony Liguori wrote:
>   
>> Sebastian Herbszt wrote:
>>     
>>> Jump to rombios before executing the halt loop.
>>>       
>> Why?  More importantly, why is this specific to KVM?
>>     
>
> The bios copy AP boot up code to 0x10000 now in KVM, so it can be 
> overwrite by userspace program like grub. I found it caused 
> stop/cont/info cpus in grub corrupt AP. Please refer to "
> KVM: Fix exiting from HLT emulation with MP_STATE_HALTED"
> (well, I found the patch duplicated with Marcelo's patch later, then 
> drop it...)
>
> Yeah, I am agree this should not be a specific issue to KVM.
>   

It is actually kvm specific.  The SIPI code runs with interrupts 
disabled, so the 'hlt' instruction cannot be resumed due to an interrupt 
(and as the apic hasn't been configured yet, no nmis either).  But due 
to a bug in kvm, an exit to userspace (like the one caused by 'info 
cpus' or stop+cont) can cause kvm to resume executing after the hlt 
instruction, which is now arbitrary application code.

Marcelo has a patch that addresses this, unfortunately with a 
regression, so hopefully the kvm bug will be closed soon.  That's not to 
say running out of ROM space is a bad idea, so I'll apply this.

(would be even better not to copy the code at all)

-- 
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