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

[Pasha Tatashin <pasha.tatashin@soleen.com>]

On 05-31 20:10, Mike Rapoport wrote:
> 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.

Yes, using KHO directly here is incorrect. The state machine is provided 
by LUO, so we should use LUO here. MSHV should provide a file that 
userspace adds to LUO, and all state machine management would be the 
same as for all other clients participating in LU.

> 
> 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?
j  
> >  - 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@kernel.org/
> > 
> > Based-on: linux-next/master (next-20260527)
> 
> -- 
> Sincerely yours,
> Mike.