Re: Broken udelay() on KVM host with a vcpu loaded

Marc Zyngier
2026-02-10T19:54:39Z

On Tue, 10 Feb 2026 15:58:14 +0000,
Quentin Perret <qperret@google.com> wrote:
>
> Ouch, it does seem that the SET_ONE_REG stuff allows to mess with that
> value _out of vcpu context_, so yeah userspace could change the value
> while a vcpu thread is preempted in the middle of a udelay loop ...

I don't think so. You can only do that on the vcpu fd, and if the vcpu
is loaded, it means that you are already holding the vcpu mutex. And
if you do that from the vcpu thread, then you already have done a
vcpu_put().

M.

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