Re: VMI Interface Proposal Documentation for I386, Part 4

[Zachary Amsden]

Alan Cox wrote:
> On Iau, 2006-03-16 at 00:37 +0100, Pavel Machek wrote:
>   
> > This code used to work when ran as root:
> >     
>
> Unless it page faulted, or was on SMP, or ....
>   

Actually, quite interestingly, I believe you can take page faults in 
this scenario - you might end up getting rescheduled and lose the effect 
disabling interrupts, but I think the kernel lives on just fine - as 
long as it doesn't BUG_ON about this.  On SMP, clearly you can't 
disabled IRQs on all processors with it.  But I really think the point 
is to try to eliminate IRQs on a single processor during some critical 
timing sensitive region.  One thing you definitely can't do safely is 
make sysenter based syscalls off the vsyscall page - you will notice 
that you always come back with interrupts enabled.

I just really don't think that is a good idea to do in userspace, when 
writing a kernel module to accomplish this safely is actually really 
quite easy.  I would argue that the various CMOS timer update utilities 
in userspace that do this same thing, really should be moved into the 
kernel as fast as possible - they could race against other CPUs in 
kernel mode that are doing the same thing, and there is no locking 
discipline here whatsoever.

>   
> > I'm not sure how will X like this.
> >     
>
> X has not used this ability for many years.
>   

Good to know.  I thought some piece of xinit still used it to do 
dot-clock probing - but I could be wrong.  We really don't care about 
getting accurate information here, since the dot-clocks don't actually 
exist in a VM.  We simulate virtual SVGA hardware instead of passing 
through any installed card.

Zach
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