Re: [PATCH] KVM: PPC: Book3S HV: Disable preemption around host-side HPT lock users

[Ritesh Harjani (IBM) <ritesh.list@gmail.com>]

Amit Machhiwal <amachhiw@linux.ibm.com> writes:

> A previous change ("KVM: PPC: Book3S HV: Add preempt_disable() around
> virtual-mode HPTE bit-lock users") at [1] added preempt_disable() around
> guest vCPU HPT hcalls and page fault handlers executed in virtual mode.
> This ensures vCPU threads cannot be preempted while holding
> HPTE_V_HVLOCK or the reverse-map lock.
>
> However, several host-side virtual-mode paths in book3s_64_mmu_hv.c also
> acquire HPTE_V_HVLOCK and lock_rmap() in process context with preemption
> enabled.  Specifically, kvm_unmap_rmapp() and kvm_age_rmapp() take both
> locks during MMU notifier callbacks and memslot flushes, while
> resize_hpt_rehash_hpte() acquires HPTE_V_HVLOCK during HPT resizing.  If
> a host thread is preempted while holding either bit-lock, any guest vCPU
> thread scheduled on the same CPU spinning for the same lock with
> preemption disabled will never yield, resulting in a permanent CPU hang.
>
> Fix this by disabling preemption across these host-side critical
> sections.
>
> For kvm_unmap_rmapp() and kvm_age_rmapp(), place preempt_disable()
> before lock_rmap() so that both the rmap chain lock and the subsequent
> HPTE_V_HVLOCK bit-lock are held under a single non-preemptible window.
> To preserve the ABBA lock ordering between the two locks, re-enable
> preemption after dropping the rmap lock on any retry or early-exit path
> before spinning on cpu_relax(), allowing the lock holder to be
> scheduled.
>
> For kvm_htab_write(), the HPT migration restore path, replace the direct
> calls to kvmppc_do_h_remove() with kvmppc_virtmode_do_h_remove(), a thin
> wrapper that brackets kvmppc_do_h_remove() with preempt_disable() and
> preempt_enable(), mirroring the existing kvmppc_virtmode_do_h_enter()
> helper.
>
> For resize_hpt_rehash_hpte(), place preempt_disable() before the
> try_lock_hpte() spin loop and preempt_enable() after unlock_hpte() at
> the function exit.
>
> [1] https://git.kernel.org/powerpc/c/abbbe5ff83e7c9ed3a4d61b73011f1df7bf87907
>
> Fixes: 6165d5dd99db ("KVM: PPC: Book3S HV: add virtual mode handlers for HPT hcalls and page faults")
> Cc: stable@vger.kernel.org # v5.14+
> Signed-off-by: Amit Machhiwal <amachhiw@linux.ibm.com>
> ---
> Changes in this version:
> - Separated host-side fixes in book3s_64_mmu_hv.c into a standalone patch
>   (guest-side paths were merged from v1 resend).
> - Previous discussion: https://lore.kernel.org/all/20261007182116.12479-3-amachhiw@linux.ibm.com/
>

Thanks. Looks good to me. Please feel free to add:
Reviewed-by: Ritesh Harjani (IBM) <ritesh.list@gmail.com>