linux-kernel - Re: [PATCHv11 09/19] x86/tdx: Account shared memory

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Date: Tue, 4 Jun 2024 19:24:35 +0300
From: "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
To: Dave Hansen <dave.hansen@...el.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>, 
	Ard Biesheuvel <ardb@...nel.org>, Baoquan He <bhe@...hat.com>, "H. Peter Anvin" <hpa@...or.com>, 
	"K. Y. Srinivasan" <kys@...rosoft.com>, Haiyang Zhang <haiyangz@...rosoft.com>, 
	kexec@...ts.infradead.org, linux-hyperv@...r.kernel.org, linux-acpi@...r.kernel.org, 
	linux-coco@...ts.linux.dev, linux-kernel@...r.kernel.org, Tao Liu <ltao@...hat.com>
Subject: Re: [PATCHv11 09/19] x86/tdx: Account shared memory

On Tue, Jun 04, 2024 at 09:08:25AM -0700, Dave Hansen wrote:
> On 5/28/24 02:55, Kirill A. Shutemov wrote:
> > Keep track of the number of shared pages. This will allow for 
> > cross-checking against the shared information in the direct mapping
> > and reporting if the shared bit is lost.
> 
> It's probably also worth mentioning that conversions are slow and
> relatively rare and even though a global atomic isn't really scalable,
> it also isn't worth doing anything fancier.

Okay, will do.

> > diff --git a/arch/x86/coco/tdx/tdx.c b/arch/x86/coco/tdx/tdx.c
> > index 26fa47db5782..979891e97d83 100644
> > --- a/arch/x86/coco/tdx/tdx.c
> > +++ b/arch/x86/coco/tdx/tdx.c
> > @@ -38,6 +38,8 @@
> >  
> >  #define TDREPORT_SUBTYPE_0	0
> >  
> > +static atomic_long_t nr_shared;
> 
> Doesn't this technically need to be:
> 
> 	static atomic_long_t nr_shared = ATOMIC_LONG_INIT(0);
> 
> ?  I thought we had some architectures where the 0 logical value wasn't
> actually all 0's.

Hm. I am not aware of such requirement. I see plenty uninitilized
atomic_long_t in generic code. For instance, invalid_kread_bytes.

And I doubt TDX will ever be built for non-x86 :P

-- 
  Kiryl Shutsemau / Kirill A. Shutemov