[Xen-devel] RE: rdtsc: correctness vs performance on Xen (and KVM?)

[Dan Magenheimer <dan.magenheimer@xxxxxxxxxx>]

> >> Making vsyscall work...
> > 
> > While I highly respect your opinion, and while vsyscall
> > may be a fine choice in the future, it just doesn't
> > solve the problem today and won't solve it ever for
> > currently shipping PV OS's.  If you can figure out a
> > way to allow vsyscall to be installed as a module and
> > still achieve its performance, it
> > might be a possible solution, but otherwise we have
> > to go around the OS to solve this problem.
> 
> Do you believe there's a solution which doesn't involve PV kernel
> modifications? I think the suggestions you've made so far 
> would require such modifications.

That is certainly my goal.  I *think* the proposal
does NOT require PV OS mods as the communication
is strictly between an app and Xen.  However, I'm
really not familiar with all the subtleties of the
x86 architecture so could be missing something.
I think these are the two key architectural dependencies
that I'm not certain of:

1) fake rdmsr (or hypercall if it works) returns a virtual
   address within a range of addresses that is not "owned by"
   the OS (e.g. maybe in Xen address space?).  The page is
   only readable outside of ring 0, but writeable in ring 0
   (by Xen).
2) All TLB misses on this page are handled directly by Xen
   so the OS never sees the address/page.

If these are OK, and you see other parts of the proposal
that require PV kernel mods, please point them out.

Thanks,
Dan

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