Re: [RFC PATCH 00/20] mshv: enable kexec with Hyper-V donated pages and partitions

[Mike Rapoport]

Hi Jork,

Only had time to skim through the patches.
I have a couple of high level questions for now.

On Wed, May 27, 2026 at 05:41:42PM -0700, Jork Loeser wrote:
> When Linux runs as an L1 Virtual Host (L1VH) under Hyper-V, the MSHV
> root partition driver deposits pages to the hypervisor and creates
> partitions for guest VMs. Prior patches enabled kexec for L1VH, but
> only when no partitions had been created and no memory had been donated.
> 
> This series lifts that limitation. It uses KHO (Kexec Handover) to:
> 
>  - Track all pages deposited to the hypervisor in a KHO radix tree
>    and preserve them across kexec so the new kernel knows which pages
>    are owned by the hypervisor.
> 
>  - Freeze running partitions before kexec, record their IDs in the
>    KHO FDT, and vacuum (tear down + reclaim memory) stale partitions
>    after kexec.
> 
>  - In case of a crash, exclude hypervisor-owned pages from crash
>    dump collection by passing the radix tree root PA via Hyper-V
>    crash MSR P2 to the crash kernel.
> 
> Dependency on Pratyush's KHO series
> ===================================
> 
> Patches 1-12 are cherry-picked from Pratyush Yadav's v1 series
> "kho: make boot time huge page allocation work nicely with KHO" [1],
> which is still under discussion. This series uses functionality from
> those patches -- specifically the meta-data page enumeration via table
> callbacks and the restructured radix tree API. It also extends the
> KHO radix tree with:
> 
>  - A freeze mechanism to lock the tree before serializing for kexec
>    (patch 13).

There were a lot of effort to make KHO stateless and drop the requirement
for finalization/freeze.

Why is this necessary to add a freeze mechanism to kho_radix_tree?
If it's a hard requirement of mshv maybe the freeze part should be handled
there?
 
>  - A crash-kernel-safe variant that memremaps radix nodes for use
>    outside the direct map (patch 14).
> 
> Patch overview
> ==============
> 
> Patches 1-12:  KHO radix tree and memblock changes (from [1])
> Patch 13:      Radix tree freeze and del_key() error reporting

del_key() error reporting sounds like something we'd want to avoid.
del_key() is called on "freeing" path and during error handling, it would
be hard if at all possible to deal with errors from del_key().

> Patch 14:      Crash-kernel-safe radix tree presence check
> Patch 15:      Page tracker using KHO radix tree for deposited pages
> Patch 16:      Debugfs interface for page tracker
> Patches 17-18: Crash MSR reshuffling + crash dump page exclusion
> Patch 19:      Export kexec_in_progress for modules

Isn't there another way to differentiate kexec reboot?

> Patch 20:      Freeze and vacuum partitions across kexec
> 
> Feedback
> ========
> 
> This is an RFC. I am looking for feedback on the overall approach as
> well as the KHO changes (patches 13-14).
> 
> [1] https://lore.kernel.org/linux-mm/20260429133928.850721-1-pratyush@xxxxxxxxxx/
> 
> Based-on: linux-next/master (next-20260527)

-- 
Sincerely yours,
Mike.