Re: [PATCHv5 10/16] x86/tdx: Convert shared memory back to private on kexec

[Kirill A. Shutemov]

On Tue, Jan 16, 2024 at 10:01:47AM +0200, Nikolay Borisov wrote:
> 
> 
> On 16.01.24 г. 9:28 ч., Kirill A. Shutemov wrote:
> 
> <snip>
> 
> > > > @@ -41,6 +44,9 @@
> > > >    static atomic_long_t nr_shared;
> > > > +static atomic_t conversions_in_progress;
> > > > +static bool conversion_allowed = true;
> > > 
> > > Given the usage model of this variable, shouldn't it be simply accessed via
> > > READ/WRITE_ONCE macros?
> > 
> > What do you see it changing?
> 
> 
> Serving as documentation that you are accessing a shared variable without an
> explicit lock (unless I'm missing something). conversion_allowed can be read
> by multiple threads, no ? And it's written by a single thread?

I don't think READ_ONCE()/WRITE_ONCE() have documentation sense you imply.

I would argue adding them will add more confusion as they serve no purpose
in this context: issuing multiple loads/stores for the variable have no
impact on outcome.

> > kexec on AMD will not work without them, I think. But noops makes sense
> > anyway. Will fix.
> 
> I'm not disputing whether those are needed for AMD or not, that way I see it
> you make those callbacks mandatory in the case of CC_ATTR_GUEST_MEM_ENCRYPT
> being present, yet only implement them for TDX. So in the case of AMD they
> will be NULL and so AMD with kexec enabled (albeit erroneously) will crash,
> no ?

As I said, I will fix it.

-- 
  Kiryl Shutsemau / Kirill A. Shutemov