Re: [PATCH v11 18/20] x86: Handle TDX erratum to reset TDX private memory during kexec() and reboot

[Peter Zijlstra]

On Mon, Jun 19, 2023 at 04:41:13PM -0700, Dave Hansen wrote:
> On 6/19/23 07:46, kirill.shutemov@xxxxxxxxxxxxxxx wrote:
> >>>
> >>> Using atomic_set() requires changing tdmr->pamt_4k_base to atomic_t, which is a
> >>> little bit silly or overkill IMHO.  Looking at the code, it seems
> >>> arch_atomic_set() simply uses __WRITE_ONCE():
> >> How about _adding_ a variable that protects tdmr->pamt_4k_base?
> >> Wouldn't that be more straightforward than mucking around with existing
> >> types?
> > What's wrong with simple global spinlock that protects all tdmr->pamt_*?
> > It is much easier to follow than a custom serialization scheme.
> 
> Quick, what prevents a:
> 
> 	spin_lock() => #MC => spin_lock()
> 
> deadlock?
> 
> Plain old test/sets don't deadlock ever.

Depends on what you mean; anything that spin-waits will deadlock,
doesn't matter if its a test-and-set or not.

The thing with these non-maskable exceptions/interrupts is that they
must be wait-free. If serialization is required it needs to be try based
and accept failure without waiting.