Re: Cross vendor migration ideas

[Anthony Liguori <anthony@codemonkey.ws>]

Alexander Graf wrote:
>
> On 12.11.2008, at 16:45, Anthony Liguori wrote:
>
>> Alexander Graf wrote:
>>> Hi,
>>>
>>> I was thinking a bit about cross vendor migration recently and since 
>>> we're doing open source development, I figured it might be a good 
>>> idea to talk to everyone about this.
>>>
>>> So why are we having a problem?
>>>
>>> In normal operation we don't. If we're running a 32-bit kernel, we 
>>> can use SYSENTER to jump from kernel<->userspace. If we're on a 
>>> 64-bit kernel with 64-bit userspace, every CPU supports SYSCALL. At 
>>> least Linux is being smart on this and does use exactly these two 
>>> capabilities in these two cases.
>>> But if we're running in compat mode (64-bit kernel with 32-bit 
>>> userspace), things differ. Intel supports only SYSENTER here, while 
>>> AMD only supports SYSCALL. Both can still use int80.
>>
>> Obviously we can trap-and-emulate but that would be slow in a 
>> relatively fast past.
>
> If we can do it without emulation, I'd greatly prefer it, as 
> syscall/sysenter emulation in the hypervisor most probably isn't 
> exactly fast ;-). And you don't really want to degrade performance 
> just because you're migrating (think flashplayer here). I guess 
> Windows 64-bit contains even more 32-bit parts.
>
>> I wonder if patching is an option?
>
> Windows does have background daemons that check code in runtime and 
> compares that to checksums. So binary patching might break Windows 
> pretty easily. I'm really wondering why the CR8 patching still works - 
> maybe even that'll break with Windows 7.

The Hyper-V spec claims that the accesses to the TPR will always be in 5 
byte instructions.  Presumably this is to allow binary patching to be 
reliable.   I'm pretty sure newer versions of Windows don't access the 
TPR so frequently or at least access it through CR8.

With VT on modern processors, TPR patching is unnecessary so as long as 
they use CR8 on AMD processors, all should be okay.

Although here is obviously another spot where cross vendor migration 
could be problematic :-)

Regards,

Anthony Liguori

> Alex