Re: [RFC PATCH 08/12] KVM: TDX: Use atomic64_dec_return() instead of a poor equivalent

[Ira Weiny <ira.weiny@intel.com>]

Sean Christopherson wrote:
> Use atomic64_dec_return() when decrementing the number of "pre-mapped"
> S-EPT pages to ensure that the count can't go negative without KVM
> noticing.  In theory, checking for '0' and then decrementing in a separate
> operation could miss a 0=>-1 transition.  In practice, such a condition is
> impossible because nr_premapped is protected by slots_lock, i.e. doesn't
> actually need to be an atomic (that wart will be addressed shortly).
> 
> Don't bother trying to keep the count non-negative, as the KVM_BUG_ON()
> ensures the VM is dead, i.e. there's no point in trying to limp along.
> 
> Signed-off-by: Sean Christopherson <seanjc@google.com>

Reviewed-by: Ira Weiny <ira.weiny@intel.com>

[snip]