Re: [PATCH v4 1/3] mshv: limit SynIC management to MSHV-owned resources

[Anirudh Rayabharam <anirudh@anirudhrb.com>]

On Mon, Apr 27, 2026 at 02:38:52PM -0700, Jork Loeser wrote:
> The SynIC is shared between VMBus and MSHV. VMBus owns the message
> page (SIMP), event flags page (SIEFP), global enable (SCONTROL),
> and SINT2. MSHV adds SINT0, SINT5, and the event ring page (SIRBP).
> 
> Currently mshv_synic_cpu_init() redundantly enables SIMP, SIEFP, and
> SCONTROL that VMBus already configured, and mshv_synic_cpu_exit()
> disables all of them. This is wrong because MSHV can be torn down
> while VMBus is still active. In particular, a kexec reboot notifier
> tears down MSHV first. Disabling SCONTROL, SIMP, and SIEFP out
> from under VMBus causes its later cleanup to write SynIC MSRs while
> SynIC is disabled, which the hypervisor does not tolerate.
> 
> Restrict MSHV to managing only the resources it owns:
> - SINT0, SINT5: mask on cleanup, unmask on init
> - SIRBP: enable/disable as before
> - SIMP, SIEFP, SCONTROL: leave to VMBus when it is active (L1VH
>   and nested root partition); on a non-nested root partition VMBus
>   does not run, so MSHV must enable/disable them
> 
> While here, fix the SIEFP and SIRBP memremap() and virt_to_phys()
> calls to use HV_HYP_PAGE_SHIFT/HV_HYP_PAGE_SIZE instead of
> PAGE_SHIFT/PAGE_SIZE. The hypervisor always uses 4K pages for SynIC
> register GPAs regardless of the kernel page size, so using PAGE_SHIFT
> produces wrong addresses on ARM64 with 64K pages.
> 
> Note that initialization order matters - VMBUS first, MSHV second,
> and the reverse on de-init. Ideally, we would want a dedicated SYNIC
> driver that replaces the cross-dependencies with a clear API and
> dynamic tracking. Such refactor should go into its own dedicated
> series, outside of this kexec fix series.
> 
> Signed-off-by: Jork Loeser <jloeser@linux.microsoft.com>
> ---
>  drivers/hv/hv.c         |   3 +
>  drivers/hv/mshv_synic.c | 150 ++++++++++++++++++++++++++--------------
>  2 files changed, 103 insertions(+), 50 deletions(-)

Reviewed-by: Anirudh Rayabharam (Microsoft) <anirudh@anirudhrb.com>