Re: [PATCH] discard MSR writes

["Markus Rechberger" <markus.rechberger-5C7GfCeVMHo@public.gmane.org>]

Avi Kivity wrote:
> Amit Shah wrote:
>> On Tuesday 20 November 2007 15:42:35 Avi Kivity wrote:
>>  
>>> Amit Shah wrote:
>>>    
>>>> On Tuesday 20 November 2007 15:17:54 Avi Kivity wrote:
>>>>      
>>>>> Amit Shah wrote:
>>>>>        
>>>>>> On Tuesday 20 November 2007 00:38:05 Markus Rechberger wrote:
>>>>>>          
>>>>>>> this patch discards MSR writes to the Performance Event-Select
>>>>>>> Registers, this is the first issue why vista seems to fail although
>>>>>>> now vista ends up in an endless loop a bit later.
>>>>>>> Qemu currently also discards those writes.
>>>>>>>             
>>>>>> Won't this make the corresponding rdmsrs fail? What happens when the
>>>>>> rdmsr returns an error, but windows then uses some garbage value 
>>>>>> (as it
>>>>>> thinks the wrmsr succeeded, so the rdmsr also should)?
>>>>>>           
>>>>> rdmsr will inject #GP for these msrs.  Implementing set_msr() doesn't
>>>>> affect rdmsr.
>>>>>
>>>>>         
>>>> >From the AMD programming manual, vol 2:
>>>>
>>>> The performance event-select registers can be read and written only by
>>>> system software running at CPL = 0 using the RDMSR and WRMSR
>>>> instructions, respectively. Any attempt to read or write these 
>>>> registers
>>>> at CPL > 0 causes a general-protection exception to occur.
>>>>       
>>> Look through the code that implements rdmsr, it doesn't care about the
>>> manuals and will happily inject a #GP for rdmsr of any unimplemented 
>>> msr
>>> (like PerfEvtSel)  wrmsr and rdmsr implementations are not linked.
>>>     
>>
>> That's right; but isn't that wrong if we cause it? I mean if we just 
>> allow the wrmsr access to go through (and if they're actually used, 
>> not disabled as you mentioned separately), then there'll be no 
>> interrupts when the guest expects them to occur, or the rdmsr will 
>> fail, when the guest thinks it shouldn't have.
>>
>>   
>
> It is wrong; but at least it fails loudly.  We can't implement all 
> msrs (Intel and AMD are adding them faster than we can code), so we 
> must make sure that where we don't implement things, at least we have 
> visibility if the guest tries to use them.
>
>> I guess we're putting forth the same point: if the wrmsr is not for 
>> disabling interrupts, we shouldn't let it go through, or just 
>> implement the required emulation.
>>   
>
> Yes.  Ignoring an msr will "fix" one guest but kill another.
>
I also discussed this with Joerg, since Qemu doesn't handle those MSR 
writes at the moment we think it's ok temporary. Lateron it should be 
emulated (but we're hunting a different issue at the moment). Our 
perfmon guys would also prefer a proper emulation.

Markus



-------------------------------------------------------------------------
This SF.net email is sponsored by: Microsoft
Defy all challenges. Microsoft(R) Visual Studio 2005.
http://clk.atdmt.com/MRT/go/vse0120000070mrt/direct/01/