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

[Huang, Kai]

On Mon, 2023-06-19 at 17:46 +0300, kirill.shutemov@xxxxxxxxxxxxxxx wrote:
> On Mon, Jun 19, 2023 at 07:31:21AM -0700, Dave Hansen wrote:
> > On 6/19/23 04:43, Huang, Kai wrote:
> > > On Mon, 2023-06-12 at 06:47 -0700, Dave Hansen wrote:
> > > > On 6/12/23 03:27, Huang, Kai wrote:
> > > > > So I think a __mb() after setting tdmr->pamt_4k_base should be good enough, as
> > > > > it guarantees when setting to any pamt_*_size happens, the valid pamt_4k_base
> > > > > will be seen by other cpus.
> > > > > 
> > > > > Does it make sense?
> > > > Just use a normal old atomic_t or set_bit()/test_bit().  They have
> > > > built-in memory barriers are are less likely to get botched.
> > > Hi Dave,
> > > 
> > > 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.
> 

For this patch I think it's overkill to use spinlock because when the rebooting
cpu is reading this all other cpus have been stopped already, so there's no
concurrent thing here.

However I just recall that the next #MC handler patch can also take advantage of
this too because #MC handler can truly run concurrently with module
initialization.  Currently that one reads tdx_module_status first but again we
may have the same memory order issue.  So having a spinlock makes sense from #MC
handler patch's point of view.

I'll change to use spinlock if Dave is fine?

Thanks for feedback!