Re: [PATCH v12 19/22] x86/kexec(): Reset TDX private memory on platforms with TDX erratum

[Huang, Kai]

On Thu, 2023-06-29 at 03:19 +0000, Huang, Kai wrote:
> On Wed, 2023-06-28 at 12:20 +0300, Nikolay Borisov wrote:
> > > +	atomic_inc_return(&tdx_may_has_private_mem);
> > > +
> > >    	/* Config the key of global KeyID on all packages */
> > >    	ret = config_global_keyid();
> > >    	if (ret)
> > > @@ -1154,6 +1167,15 @@ static int init_tdx_module(void)
> > >    	 * as suggested by the TDX spec.
> > >    	 */
> > >    	tdmrs_reset_pamt_all(&tdx_tdmr_list);
> > > +	/*
> > > +	 * No more TDX private pages now, and PAMTs/TDMRs are
> > > +	 * going to be freed.  Make this globally visible so
> > > +	 * tdx_reset_memory() can read stable TDMRs/PAMTs.
> > > +	 *
> > > +	 * Note atomic_dec_return(), which is an atomic RMW with
> > > +	 * return value, always enforces the memory barrier.
> > > +	 */
> > > +	atomic_dec_return(&tdx_may_has_private_mem);
> > 
> > Make a comment here which either refers to the comment at the increment 
> > site.
> 
> I guess I got your point.  Will try to make better comments.
> 
> > 
> > >    out_free_pamts:
> > >    	tdmrs_free_pamt_all(&tdx_tdmr_list);
> > >    out_free_tdmrs:
> > > @@ -1229,6 +1251,63 @@ int tdx_enable(void)
> > >    }
> > >    EXPORT_SYMBOL_GPL(tdx_enable);
> > >    
> > > +/*
> > > + * Convert TDX private pages back to normal on platforms with
> > > + * "partial write machine check" erratum.
> > > + *
> > > + * Called from machine_kexec() before booting to the new kernel.
> > > + */
> > > +void tdx_reset_memory(void)
> > > +{
> > > +	if (!platform_tdx_enabled())
> > > +		return;
> > > +
> > > +	/*
> > > +	 * Kernel read/write to TDX private memory doesn't
> > > +	 * cause machine check on hardware w/o this erratum.
> > > +	 */
> > > +	if (!boot_cpu_has_bug(X86_BUG_TDX_PW_MCE))
> > > +		return;
> > > +
> > > +	/* Called from kexec() when only rebooting cpu is alive */
> > > +	WARN_ON_ONCE(num_online_cpus() != 1);
> > > +
> > > +	if (!atomic_read(&tdx_may_has_private_mem))
> > > +		return;
> > 
> > I think a comment is warranted here explicitly calling our the ordering 
> > requirement/guarantees. Actually this is a non-rmw operation so it 
> > doesn't have any bearing on the ordering/implicit mb's achieved at the 
> > "increment" site.
> 
> We don't need explicit ordering/barrier here, if I am not missing something. 
> The atomic_{inc/dec}_return() already made sure the memory ordering -- which
> guarantees when @tdx_may_has_private_mem reads true _here_, the TDMRs/PAMTs must
> be stable.
> 
> Quoted from Documentation/atomic_t.txt:
> 
> "
>  - RMW operations that have a return value are fully ordered;   
> 
>  ...
> 
> Fully ordered primitives are ordered against everything prior and everything   
> subsequent. Therefore a fully ordered primitive is like having an smp_mb()     
> before and an smp_mb() after the primitive.
> "
> 
> 
> Am I missing anything? 

OK I guess I figured out by myself after more thinking.  Although the
atomic_{inc|dec}_return() code path has guaranteed when @tdx_may_has_private_mem
is true, TDMRs/PAMTs are stable, but here in the reading path, the code below

	tdmrs_reset_pamt_all(&tdx_tdmr_list);

may still be executed speculatively before the if () statement completes

	if (!atomic_read(&tdx_may_has_private_mem))
		return;

So we need CPU memory barrier instead of compiler barrier.

I'll change to use "RMW with return value" to provide the CPU barrier.

Thanks for the feedback!