Re: [Qemu-devel] add 'monitor' and 'mwait' instruction (update)

[Joachim Henke]

R. Armiento wrote:
> Is this hack really 'safe'? I don't claim to know much about modern  
> x86 instructions, but some googling tells me that mwait is supposed  
> to wake on a monitored memory write (but is allowed to wake up  
> earlier, hence it is acceptable but CPU consuming to emulate it  
> with a nop). Couldn't there be situations where someone depends on  
> mwait waking up without there being an event that wakes hlt? Or are  
> we sure qemu's hlt will happen to wake up anyway?

Currently the Linux kernel simply uses monitor/mwait as a faster  
'hlt' replacement, so it should be "safe" there. I don't know about  
other guest OSs. Anyway, I proposed this hack only as a quick  
"solution" for local usage.

> Again, excuse my lack of knowledge of the internals of qemu and  
> kqemu. If 'hlt' can be emulated to give CPU time back to the host  
> OS until an interrupt occurs in the guest; then it is not obvious  
> why mwait couldn't be simulated in a similar way, only with the  
> addition that qemu also restarts guest CPU execution should there  
> be writes in monitored memory. While I have no idea of how much  
> work it would be, it would much surprise me if this is truly un- 
> doable, at least for non-kqemu emulation.

Problem is, at the moment I've no idea, how we could achieve this  
memory monitoring in a safe and simple way in user space. But, as you  
already told, we only need monitor/mwait emulation, when using kernel- 
kqemu _and_ having the MONITOR flag set in the host cpuid. So I'm  
sure, Fabrice would be able to do the trick - maybe by using the  
hosts monitor/mwait instructions.

--
Joachim Henke
http://he-jo.net/