Re: [PATCH 4/9] Implement hsave v5

[Anthony Liguori <anthony@codemonkey.ws>]

Alexander Graf wrote:
>
> On 30.10.2008, at 21:44, Anthony Liguori wrote:
>
>> Alexander Graf wrote:
>>>
>>> On 30.10.2008, at 20:29, Avi Kivity wrote:
>>>
>>>> Alexander Graf wrote:
>>>>>
>>>>> I don't see any benefit from actually exporting that MSR atm. KVM 
>>>>> does never read it and it'd just add additional overhead on every 
>>>>> userspace transition, as it'd need to be synced every time.
>>>>
>>>> We only sync MSRs on save/restore.
>>>
>>> Oh, really? Looks like I interpreted that code wrong :-). 
>>> Nevertheless, we don't use the hsave given by the guest OS, because 
>>> we can't trust it. So saving and restoring a non-used value sounds 
>>> pretty useless - unless you're migrating from KVM to Qemu. Is this 
>>> kind of migration (going to be) possible?
>>
>> But what if the guest tries to reread the hsave value it wrote?  
>> Couldn't that break a guest?
>
> Yes, it could. I just haven't seen any VMM do that yet. But I agree - 
> it could be implemented, I just didn't deem it necessary for now.

It's always easier to do it right the first time then to wait until 
someone has to debug a weird issue :-)

Regards,

Anthony Liguori

> Alex