Re: [PATCH v3 3/6] x86/hyperv: Skip LP/VP creation on kexec

[Wei Liu <wei.liu@kernel.org>]

On Mon, May 04, 2026 at 03:09:21PM +0000, Michael Kelley wrote:
> From: Jork Loeser <jloeser@linux.microsoft.com> Sent: Tuesday, April 7, 2026 6:37 PM
> > 
> > After a kexec the logical processors and virtual processors already
> > exist in the hypervisor because they were created by the previous
> > kernel. Attempting to add them again causes either a BUG_ON or
> > corrupted VP state leading to MCEs in the new kernel.
> > 
> > Add hv_lp_exists() to probe whether an LP is already present by
> > calling HVCALL_GET_LOGICAL_PROCESSOR_RUN_TIME. When it succeeds the
> > LP exists and we skip the add-LP and create-VP loops entirely.
> > 
> > Also add hv_call_notify_all_processors_started() which informs the
> > hypervisor that all processors are online. This is required after
> > adding LPs (fresh boot) and is a no-op on kexec since we skip that
> > path.
> 
> Adding hv_call_notify_all_processors_started() seems like it should be
> a separate patch. And this paragraph in the commit message leaves me
> with questions:  Is it really "required"?  If it is, how does the existing
> upstream code ever work? Does the change need to be backported
> to stable kernels? If it isn't *really* required, what are the implications
> of not doing it?

It is complicated. If I remember correctly, we realized this call was
absolutely needed if SEV-SNP host side support is enabled. If that
support is not enabled, then things continue to work. I think it is the
right thing to do to always make this call.

We don't need to backport this yet.

Wei