Re: [PATCH v2] KVM: VMX: Conditionally reload debug register 6

[Avi Kivity <avi@redhat.com>]

On 09/01/2009 04:47 PM, Jan Kiszka wrote:
>> btw, something else I've considered was not to do any emulation for 'mov
>> dr' but instead load the debug registers and disable the intercept.  So
>> far the only issue I've seen is that we lose support for real mode guest
>> self-debug on intel (pre-unrestricted guest).  What do you think of this?
>>      
> I think you can't have both: optimized dr save/restore on vmentry/exit
> and optimized dr access. If you drop on-demand dr register
> readout/update, you need to deal with this on every vmentry/exit. My
> feeling is that this would be awfully slower, even slower than what we
> currently have without your patches.
>    

I'll clarify:

In the normal case, we'll have #DB and MOV DR intercepted.

On #DB, update vcpu->dr6.
On MOV DR, disable MOV DR interception, enable dr swap, and let the 
guest execute; on the next exit, reenable MOV DR interception and 
disable dr swap.

It's similar to the fpu code where we don't emulate fpu instructions.  
It doesn't really buy us a lot.

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