Re: [PATCH v7 11/11] x86/virt/tdx: Optimize tdx_pamt_get/put()

[Nikolay Borisov]

On 7/18/26 04:45, Rick Edgecombe wrote:
> From: "Kirill A. Shutemov" <kirill.shutemov@xxxxxxxxxxxxxxx>
>
> The Dynamic PAMT get/put helpers use a global spinlock to serialize all
> refcount updates and SEAMCALL invocations. This gives correct behavior for
> concurrent callers, but leads to contention. It is especially bad from the
> KVM side, which is designed to allow faulting in EPT under a shared lock.
> With the global spinlock, not only is the lock an exclusive one, but it is
> for all TDs instead of just a single one.
>
> But taking the global lock each time is actually unnecessary. Only the 0->1
> and 1->0 refcount transitions actually need the lock (to pair with
> SEAMCALLs that actually add and remove with the Dynamic PAMT pages). The
> common case of incrementing or decrementing a non-zero refcount can be
> done locklessly.
>
> So create a fast and slow path. Check the refcount outside the lock and
> only take it for the slow path (0->1 and 1->0 transitions).
>
> On the put side make the refcount adjustment and lock taking atomic so if
> a 'get' happens between them, it doesn't cause the Dynamic PAMT to be
> freed incorrectly. On the get side there is no technique for doing the
> refcount adjustment and lock atomically, so check the refcount again
> inside the lock.
>
> AI was used under supervision to collect/apply feedback, review code and
> workshop logs. It assisted in identifying/evaluating the stale
> conditionals for the races resolved from the atomic_dec_and_lock() change.
> Separate from atomic_dec_and_lock() fallout, it suggested to change
> atomic_inc() to atomic_set(pamt_refcount, 1) in the put error path for the
> sake of being more precise, which Kiryl had also suggested in the past.
> The model also suggested updated comments following the
> atomic_dec_and_lock() change based on some directed prompting. The
> comments were subsequently edited or further prompted for fine tuning.
>
> Signed-off-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
> Co-developed-by: Rick Edgecombe <rick.p.edgecombe@xxxxxxxxx>
> Signed-off-by: Rick Edgecombe <rick.p.edgecombe@xxxxxxxxx>
> Reviewed-by: Chao Gao <chao.gao@xxxxxxxxx>
> Reviewed-by: Tony Lindgren <tony.lindgren@xxxxxxxxxxxxxxx>

Reviewed-by: Nikolay Borisov <nik.borisov@xxxxxxxx>