Re: [PATCH v2] x86/tsx: fix KVM guest live migration for tsx=on

[Pawan Gupta]

On Mon, Apr 11, 2022 at 04:07:01PM -0400, Jon Kohler wrote:
> Move automatic disablement for TSX microcode deprecation from tsx_init() to
> x86_get_tsx_auto_mode(), such that systems with tsx=on will continue to
> see the TSX CPU features (HLE, RTM) even on updated microcode.
>
> KVM live migration could be possibly be broken in 5.14+ commit 293649307ef9
> ("x86/tsx: Clear CPUID bits when TSX always force aborts"). Consider the
> following scenario:
>
> 1. KVM hosts clustered in a live migration capable setup.
> 2. KVM guests have TSX CPU features HLE and/or RTM presented.
> 3. One of the three maintenance events occur:
> 3a. An existing host running kernel >= 5.14 in the pool updated with the
>    new microcode.
> 3b. A new host running kernel >= 5.14 is commissioned that already has the
>    microcode update preloaded.
> 3c. All hosts are running kernel < 5.14 with microcode update already
>    loaded and one existing host gets updated to kernel >= 5.14.
> 4. After maintenance event, the impacted host will not have HLE and RTM
>   exposed, and live migrations with guests with TSX features might not
>   migrate.

Which part was this reproduced on? AFAIK server parts(except for some
Intel Xeon E3s) did not get such microcode update.

Thanks,
Pawan