[Xen-devel] non-emulated rdtsc: a smoking gun!

[Dan Magenheimer <dan.magenheimer@xxxxxxxxxx>]

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.)

I observed this use of rdtsc on a real running
released EL5U2-32b PV kernel, but the problem also
exists on 2.6.31 and probably on any currently
shipping PV kernel.  AND due to a bug(?) in HVM
management of TSC, I think it will occur in any
Linux HVM as well.  And, other than Xen/KVM
guaranteeing rdtsc is monotonically-increasing
(and tracks wallclock time across a migration
which Xen's emulated rdtsc doesn't yet do),
I don't think there is a solution.

The problem can occur if a migration or
save/restore results in the appearance that
the physical TSC went backwards.  For example:
1) A live migration occurs from machine A
   to machine B, and machine B was much more
   recently booted than machine A; or
2) A guest is saved on machine A, machine A
   has been running for a long time, machine
   A is rebooted, and the guest is restored
   on machine A shortly after it is booted.

If a delay() function is currently executing
in the guest kernel when the above occurs
and the rdtsc instruction is unemulated,
the delay() function will return immediately**
when the kernel vcpu regains control.
True, in many circumstances, the overhead
incurred by the migration or save/restore will
expire the intended delay, and so perhaps serve
the same purpose as the intended delay, but
there may also be circumstances where this is
not true.

** Note that some clever coding in the Linux
kernel sources averts a much worse disaster,
namely a very extended spinwait for hours or
days or more!  This cleverness may not exist
in all kernels -- or in applications that might
implement a similar rdtsc-based __udelay()-like
technique.

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