Re: [RFC PATCH v5 20/45] KVM: x86/mmu: Allocate/free S-EPT pages using tdx_{alloc,free}_control_page()

[Sean Christopherson]

On Mon, Feb 09, 2026, Dave Hansen wrote:
> On 2/6/26 07:01, Sean Christopherson wrote:
> >  /* Bump PAMT refcount for the given page and allocate PAMT memory if needed */
> >  int __tdx_pamt_get(u64 pfn, struct tdx_pamt_cache *cache)
> > @@ -2272,7 +2272,7 @@ int __tdx_pamt_get(u64 pfn, struct tdx_pamt_cache *cache)
> >         if (ret)
> >                 goto out_free;
> >  
> > -       scoped_guard(spinlock, &pamt_lock) {
> > +       scoped_guard(raw_spinlock_irqsave, &pamt_lock) {
> >                 /*
> >                  * Lost race to other tdx_pamt_add(). Other task has already allocated
> >                  * PAMT memory for the HPA.
> > @@ -2348,7 +2348,7 @@ void __tdx_pamt_put(u64 pfn)
> 
> Why does this need to be a raw spinlock? irqsave, sure, but raw?

Huh, TIL.  (And just when I thought I finally had my head wrapped around RT "spinlocks"):

  The hard interrupt related suffixes for spin_lock / spin_unlock operations
  (_irq, _irqsave / _irqrestore) do not affect the CPU’s interrupt disabled state.

Ah, and running RCU callbacks from soft IRQ context is straight up disallowed for
PREEMPT_RT.

  /* By default, use RCU_SOFTIRQ instead of rcuc kthreads. */
  static bool use_softirq = !IS_ENABLED(CONFIG_PREEMPT_RT);
  #ifndef CONFIG_PREEMPT_RT
  module_param(use_softirq, bool, 0444);
  #endif

So yeah, just spinlock_irqsave should be fine.  Though the way things are trending,
it'll be a moot point if KVM ends up freeing S-EPT page tables from task context.

> The page allocator locks are used in this context and aren't raw.