Re: RE: rdtsc: correctness vs performance on Xen (and KVM?)

[Jeremy Fitzhardinge <jeremy@goop.org>]

On 09/02/09 00:20, Keir Fraser wrote:
> The problem is a bit easier with vsyscall potentially. For example, give
> each thread its own vsyscall clock data area (easy?), updated by kernel
> whenever the thread is scheduled, and increment a version counter, checked
> before and after by the vsyscall operation.
>   

Yes.  Perhaps the very simplest way would be to make the kernel update
the pvclock version counter on context switch, the same way Xen does;
that would allow the usermode vsyscall code to use exactly the same
algorithm as the kernel code.  Would Xen cope with that?

> Well, I don't know how easy or fast that could actually be implemented, but
> I'm at least confident it could work. But it does need kernel assistance.
>   

Yes.  I'm very uneasy about letting usermode have direct access to bits
of Xen without the kernel's knowledge anyway.  It suddenly means we need
to not only maintain a Xen<->kernel ABI, but a Xen<->usermode ABI as well.

    J