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

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Date: Tue, 16 Jan 2024 13:45:48 +0300
From: "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
To: Nikolay Borisov <nik.borisov@...e.com>
Cc: Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>,
	Borislav Petkov <bp@...en8.de>,
	Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
	"Rafael J. Wysocki" <rafael@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Adrian Hunter <adrian.hunter@...el.com>,
	Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@...ux.intel.com>,
	Elena Reshetova <elena.reshetova@...el.com>,
	Jun Nakajima <jun.nakajima@...el.com>,
	Rick Edgecombe <rick.p.edgecombe@...el.com>,
	Tom Lendacky <thomas.lendacky@....com>,
	"Kalra, Ashish" <ashish.kalra@....com>,
	Sean Christopherson <seanjc@...gle.com>,
	"Huang, Kai" <kai.huang@...el.com>, Baoquan He <bhe@...hat.com>,
	kexec@...ts.infradead.org, linux-coco@...ts.linux.dev,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCHv5 10/16] x86/tdx: Convert shared memory back to private
 on kexec

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