linux-kernel - Re: [PATCH 1/3] Implement emulator_write

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Date:	Mon, 27 Aug 2007 20:26:50 +0300
From:	Avi Kivity <avi@...ranet.com>
To:	Anthony Liguori <aliguori@...ibm.com>
CC:	kvm-devel@...ts.sourceforge.net, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/3] Implement emulator_write_phys()

Anthony Liguori wrote:
> On Mon, 2007-08-27 at 18:45 +0300, Avi Kivity wrote:
>   
>> Anthony Liguori wrote:
>>     
>>> Since a hypercall may span two pages and is a gva, we need a function to write
>>> to a gva that may span multiple pages.  emulator_write_phys() seems like the
>>> logical choice for this.
>>>
>>> @@ -962,8 +962,35 @@ static int emulator_write_std(unsigned long addr,
>>>  			      unsigned int bytes,
>>>  			      struct kvm_vcpu *vcpu
>>>       
>> I think that emulator_write_emulated(), except for being awkwardly 
>> named, should do the job.  We have enough APIs.
>>
>> But!  We may not overwrite the hypercall instruction while a vcpu may be 
>> executing, since there's no atomicity guarantee for code fetch.  We have 
>> to to be out of guest mode while writing that insn.
>>     
>
>
> Hrm, good catch.
>
> How can we get out of guest mode given SMP guest support?
>
>   

kvm_flush_remote_tlbs() is something that can be generalized.  
Basically, you set a bit in each vcpu and send an IPI to take them out.

But that's deadlock prone and complex.  Maybe you can just take 
kvm->lock, zap the mmu and the flush tlbs, and patch the instruction at 
your leisure, as no vcpu will be able to map memory until the lock is 
released.

-- 
Any sufficiently difficult bug is indistinguishable from a feature.

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