Re: [PATCH 0/4] KVM: arm64: Reduce overhead of full S2 teardown

[Shuai Xue <xueshuai@linux.alibaba.com>]

On 9/14/26 4:14 PM, Marc Zyngier wrote:
> On Mon, 14 Sep 2026 07:44:15 +0100,
> Shuai Xue <xueshuai@linux.alibaba.com> wrote:
>>
>>
>>
>> On 9/12/26 6:48 PM, Marc Zyngier wrote:
>>> Tearing down a full S2 is a pretty involved process, resulting in a
>>> lot of TLB invalidation. These TLBIs are either on a per leaf basis if
>>> the HW doesn't support range invalidation, or by top-level range if it
>>> does. Amusingly, the latter occurs even when nothing has been
>>> unmapped.
>>>
>>> Things are made worse with NV, as we have a bucket-load of shadow S2s,
>>> and the need to invalidate them all on the back of an MMU notifier.
>>> The latter will eventually be solved by the reverse-map tracking that
>>> Wei-Lin is working on, but we need to be better at full-S2 teardown.
>>>
>>> This small series adds a "no TLBI" unmapping primitive, which allows
>>> the caller to then whack the TLBs using a VMID-wide invalidation. This
>>> results in far fewer TLBIs, and a better recursive virtualisation as
>>> we get far fewer traps as a consequence.
>>>
>>> This applies on top of my shadow-s2 lifetime fixes, and is expected to
>>> be a prefix to Wei-Lin's series.
>>>
>>> Marc Zyngier (4):
>>>     KVM: arm64: pgtable: Add Stage-2 unmap without TLBI primitive
>>>     KVM: arm64: MMU: Add kvm_stage2_unmap_all() helper
>>>     KVM: arm64: nv: Move full s2_mmu unmap over to kvm_stage2_unmap_all()
>>>     KVM: arm64: nv: Move TLBI VMALLS12E1* emulation over to
>>>       kvm_stage2_unmap_all()
>>>
>>>    arch/arm64/include/asm/kvm_mmu.h     |  1 +
>>>    arch/arm64/include/asm/kvm_pgtable.h | 17 +++++++++++++
>>>    arch/arm64/include/asm/kvm_pkvm.h    |  1 +
>>>    arch/arm64/kvm/hyp/pgtable.c         | 38 +++++++++++++++++++++-------
>>>    arch/arm64/kvm/mmu.c                 | 15 +++++++++++
>>>    arch/arm64/kvm/nested.c              |  4 +--
>>>    arch/arm64/kvm/pkvm.c                |  2 ++
>>>    arch/arm64/kvm/sys_regs.c            | 18 ++++++-------
>>>    8 files changed, 75 insertions(+), 21 deletions(-)
>>>
>>
>>
>> Hi Marc,
>>
>> Thanks for putting this series together. I reviewed the four patches
>> and revisited the traces from my earlier Marc-only tests.
>>
>> I have a correctness concern about child page-table reclamation.
>>
> 
> You? Or your AI model? I'd really expect you to explain *your*
> perception of the problem rather than dumping the result of your AI in
> an email.

Hi Marc,

You're right. I used AI assistance (GPT-6 Astra) for the analysis.
Let me clarify the technical points.


> 
>> 1. Child page-table reclamation before the final TLBI
>>
>> In patch 1, SKIP_S2_TLBI suppresses invalidation for both leaf and table
>> descriptors, while stage2_unmap_walker() still immediately releases
>> empty child tables.
> 
> What is a "child" table?

I meant the next-level shadow S2 table obtained here in
stage2_unmap_walker():

         if (kvm_pte_table(ctx->old, ctx->level)) {
                 childp = kvm_pte_follow(ctx->old, mm_ops);

For example, if ctx->old is an L2 table descriptor, childp points
to the L3 table that it describes. This is a host-allocated shadow
page-table page, not a guest data page.

> 
>>
>> For a child table with page_count(childp) == 1, the sequence is:
>>
>>    stage2_unmap_walker()
>>      stage2_unmap_put_pte()
>>        clear the parent table descriptor
>>        skip its TLBI
>>      mm_ops->put_page(childp)
>>        kvm_s2_put_page()
>>          put_page()              /* drop the child's last reference */
>>
>>    ... process the remaining address ranges ...
>>
>>    __kvm_tlb_flush_vmid()         /* final invalidation in patch 2 */
>>
>> This path does not use the free_unlinked_table()/call_rcu() deferred
>> reclamation mechanism. The existing deferred-range-TLBI path still
>> invalidates table descriptors immediately; the new flag skips that
>> invalidation too.
> 
> And? What is the actual problem here?

The concern is that this page becomes available for reuse before
the corresponding invalidation completes.

With page_count(childp) == 1, the walker clears the parent descriptor
through stage2_unmap_put_pte(), then releases the next-level table
with mm_ops->put_page(childp). SKIP_S2_TLBI suppresses the table-
descriptor TLBI that previously happened before this release.
The final VMID-wide TLBI happens after the full walk.

If hardware can still walk through that page using old intermediate
translation state, it could interpret contents written by a new
owner as S2 descriptors. That is the failure scenario I intended
to describe.


> 
>>
>> Patch 4 provides a caller operating on active shadow MMUs:
>> kvm_s2_mmu_iterate_by_vmid() holds mmu_lock for write and visits valid
>> matching shadow MMUs, but does not require refcnt == 0 or wait for
>> other vCPUs using the MMU to exit.
> 
> Of course it doesn't, since this is simply emulating an instruction
> local to that vcpu. How would the actual HW "wait" for another CPU to
> stop using a set of translation?

Agreed. I should not have presented the absence of a vCPU-stop
mechanism as a problem. Stopping another vCPU and waiting for
invalidation to complete are different things.

My concern is the lifetime of the host shadow page-table memory:
whether it can be reclaimed before the final TLBI completes.

> 
>>
>> Another vCPU can therefore still use that shadow S2. The write lock
>> excludes software page-table updates, not hardware table walks.
>> The following interleaving is allowed:
>>
>>    vCPU B / hardware walker        vCPU A
>>    ------------------------        ----------------------------
>>    Holds an old reference to T
>>                                    Clears parent, skips TLBI
>>                                    Drops T's last reference
>>                                    T is reused by the allocator
>>    Accesses T via the old reference
>>                                    Performs final VMID-wide TLBI
>>
>> The final flush barriers cannot retroactively protect a table that
>> has already been freed and reused.
> 
> T is a shadow page-table page on the host. How can vcpu B hold a
> reference on that page? It isn't even in the same address space.

Sorry for the misleading. I meant a hardware table walk
on the PE running B going through T, not a software reference held
by B or a guest mapping of T.

> 
> Now, I can see that B could have a VA that *translate through* T, and
> that's rather annoying.
> 
> I'll have a think.
> 
> 	M.

Yes, translating through T is what I meant.

Would retaining the table-descriptor TLBI be a reasonable minimal
fix? That would preserve invalidation before releasing the next-level
table, while still avoiding the per-range flushes for empty chunks.

Thanks for looking into it.

Shuai