Re: [PATCH v2 2/2] turn off kvmclock when resetting cpu

[Zachary Amsden <zamsden@redhat.com>]

On 05/05/2010 09:19 AM, Glauber Costa wrote:
> Currently, in the linux kernel, we reset kvmclock if we are rebooting
> into a crash kernel through kexec. The rationale, is that a new kernel
> won't follow the same memory addresses, and the memory where kvmclock is
> located in the first kernel, will be something else in the second one.
>
> We don't do it in normal reboots, because the second kernel ends up
> registering kvmclock again, which has the effect of turning off the
> first instance.
>
> This is, however, totally wrong. This assumes we're booting into
> a kernel that also has kvmclock enabled. If by some reason we reboot
> into something that doesn't do kvmclock including but not limited to:
>   * rebooting into an older kernel without kvmclock support,
>   * rebooting with no-kvmclock,
>   * rebootint into another O.S,
>
> we'll simply have the hypervisor writting into a random memory position
> into the guest. Neat, uh?
>
> Moreover, I believe the fix belongs in qemu, since it is the entity
> more prepared to detect all kinds of reboots (by means of a cpu_reset),
> not to mention the presence of misbehaving guests, that can forget
> to turn kvmclock off.
>
> It is also necessary to reset other msrs, so this patch resets
> everything that kvm exports through its MSR list.
>    

Does that include the TSC?

Zach