Re: [PATCH 2/2] KVM: x86 emulator: Discard CR2 in x86 emulator

[Avi Kivity <avi-atKUWr5tajBWk0Htik3J/w@public.gmane.org>]

Sheng Yang wrote:
> On Thursday 15 November 2007 18:15:20 Avi Kivity wrote:
>   
>> Sheng Yang wrote:
>>     
>>> From 9cd9d5cde7341d5e9de41b1070cea7a98e7d8cc9 Mon Sep 17 00:00:00 2001
>>> From: Sheng Yang <sheng.yang-ral2JQCrhuEAvxtiuMwx3w@public.gmane.org>
>>> Date: Thu, 15 Nov 2007 15:11:58 +0800
>>> Subject: [PATCH 2/2] KVM: x86 emulator: Discard CR2 in x86 emulator
>>>
>>> For CR2 is unreliable and unavailable in many condition, this patch
>>> completely decode memory operand instead of using CR2 in x86 emulator.
>>>       
>> One of my innermost wishes...
>>
>>     
>>> diff --git a/drivers/kvm/x86.c b/drivers/kvm/x86.c
>>> index aa6c3d8..85a0776 100644
>>> --- a/drivers/kvm/x86.c
>>> +++ b/drivers/kvm/x86.c
>>> @@ -1293,7 +1293,7 @@ int emulate_instruction(struct kvm_vcpu *vcpu,
>>>
>>>  		vcpu->emulate_ctxt.vcpu = vcpu;
>>>  		vcpu->emulate_ctxt.eflags = kvm_x86_ops->get_rflags(vcpu);
>>> -		vcpu->emulate_ctxt.cr2 = cr2;
>>> +		vcpu->emulate_ctxt.memop = 0;
>>>       
>> We have c->modrm_ea which can be used for the memory operand.
>>     
>
> I don't think using the name modrm_ea is good for explicit encoding, so I add 
> this. 

I agree the name isn't good (we already use it for MemAbs decoding, 
too).  We can rename it later.
> thBut I am think of is it better to be in decode_cache?
>
>   

c-> is the decode cache.  Maybe I misunderstood you?

>>> diff --git a/drivers/kvm/x86_emulate.c b/drivers/kvm/x86_emulate.c
>>> index c020010..95536a8 100644
>>> --- a/drivers/kvm/x86_emulate.c
>>> +++ b/drivers/kvm/x86_emulate.c
>>> @@ -880,6 +880,8 @@ done_prefixes:
>>>  			break;
>>>  		}
>>>  		c->src.type = OP_MEM;
>>> +		ctxt->memop = insn_fetch(u32, c->src.bytes, c->eip);
>>> +		c->eip -= c->src.bytes; /* keep the page fault ip */
>>>       
>> I don't understand this.  In the cases where the memory operand address
>> is encoded in the instruction, we fetch it explicity.  When it isn't,
>> this is broken.
>>     
>
> But we mark implicit encoding instructions as "ImplicitOps", so only explicit 
> ones should get here. And my former patch deal with the implicit ones, and 
> modrm_ea has priority to memop, so I think it's OK.
>   

I still don't understand.  Which instruction benefits from this change?  
And shouldn't the be marked MemAbs instead?



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


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