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

[Huang, Kai]

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():

static __always_inline void arch_atomic_set(atomic_t *v, int i)                
{
        __WRITE_ONCE(v->counter, i);                                           
}

Is it better to just use __WRITE_ONCE() or WRITE_ONCE() here?

-       tdmr->pamt_4k_base = pamt_base[TDX_PS_4K];
+       WRITE_ONCE(tdmr->pamt_4k_base, pamt_base[TDX_PS_4K]);