Re: non-emulated rdtsc: a smoking gun!

[Jeremy Fitzhardinge <jeremy@goop.org>]

On 10/22/09 13:53, Dan Magenheimer wrote:
> I just found a Linux kernel use of rdtsc that
> MAY cause a significant failure if rdtsc is
> unemulated and a poorly timed migration (or
> save/restore) occurs under Xen or KVM.
>
> The problem is that a call to __udelay() --
> or any member of the delay() family -- may
> return prematurely**.  Since these functions
> "must guarantee that we wait at least the
> amount of time" specified, there are likely
> unknown kernel circumstances where a
> premature return will cause problems.
> (Disclaimer: I haven't gone through every use
> of every call site of every member of the delay
> function family to prove this.)
>   

It won't matter, as they're only used to control timing to external IO
devices.  If a domain has a passthrough device, it can't be migrated or
save/restored.  Nobody should be using them for software timing.

    J