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
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