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 18:06 -0700, Dave Hansen wrote:
> On 6/19/23 17:56, Huang, Kai wrote:
> > Any comments to below?
> 
> Nothing that I haven't already said in this thread:
> 
> > 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.
> 
> I kinda made a point of literally suggesting "atomic_t or
> set_bit()/test_bit()".  I even told you why: "built-in memory barriers".
> 
> Guess what READ/WRITE_ONCE() *don't* have.  Memory barriers.
> 

Hi Dave,

Sorry to bring this up again.  I thought more on this topic, and I think using
atotmic_t is only necessary if we add it right after setting up tdmr->pamt_* in
tdmr_set_up_pamt(), because there we need both compiler barrier and CPU memory
barrier to make sure memory order (as Kirill commented in the first reply).

However, if we add a new variable like below ...

+static bool tdx_private_mem_begin;
+
 /*
  * Wrapper of __seamcall() to convert SEAMCALL leaf function error code
  * to kernel error code.  @seamcall_ret and @out contain the SEAMCALL
@@ -1123,6 +1125,8 @@ static int init_tdx_module(void)
         */
        wbinvd_on_all_cpus();
 
+       tdx_private_mem_begin = true;


... then we don't need any more explicit barrier, because: 1) it's not possible
for compiler to optimize the order between setting tdmr->pamt_* and
tdx_private_mem_begin; 2) no CPU memory barrier is needed as WBINVD is a
serializing instruction so the wbinvd_on_all_cpus() above has already implied
memory barrier.

Does this make sense?