Re: [PATCH v7 5/5] x86/sgx: Enable automatic SVN updates for SGX enclaves

[Dave Hansen <dave.hansen@intel.com>]

On 7/14/25 06:54, Dave Hansen wrote:
> On 7/14/25 00:35, Reshetova, Elena wrote:
>> I think you put it: this would require a spinlock in the fast path and
>> *in theory* if we are running many many concurrent enclaves can create 
>> contention
> FWIW, my mental model is that spinlocks that are held for short periods
> of time are pretty much the same cost as an atomic under contention.
> 
> If there are lots of users, the cost of moving the cacheline for the
> atomic or the spinlock dominates everything else. It doesn't matter
> whether that cacheline is an atomic_t or spinlock_t.
> 
> The only difference is that there is _visible_ spinning for a spinlock.

Oh, and I had a brain fart on this one. You've got a mutex, not a spinlock.

But the concept still applies: for small critical sections, the cost of
moving the cacheline dominates the cost of everything else, no matter if
it's a mutex, spinlock or atomic.

Never add complexity unless you're getting actual, real-world
performance out of it. In this case, the only thing you'd _maybe_
improve with added complexity is an open()/close() loop on /dev/sgx,
which is completely unrealistic.