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

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Date: Sat, 6 Jan 2024 03:59:01 +0300
From: "kirill.shutemov@...ux.intel.com" <kirill.shutemov@...ux.intel.com>
To: "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
Cc: "tglx@...utronix.de" <tglx@...utronix.de>,
	"mingo@...hat.com" <mingo@...hat.com>,
	"x86@...nel.org" <x86@...nel.org>, "bp@...en8.de" <bp@...en8.de>,
	"dave.hansen@...ux.intel.com" <dave.hansen@...ux.intel.com>,
	"kexec@...ts.infradead.org" <kexec@...ts.infradead.org>,
	"Reshetova, Elena" <elena.reshetova@...el.com>,
	"Nakajima, Jun" <jun.nakajima@...el.com>,
	"rafael@...nel.org" <rafael@...nel.org>,
	"peterz@...radead.org" <peterz@...radead.org>,
	"Huang, Kai" <kai.huang@...el.com>,
	"sathyanarayanan.kuppuswamy@...ux.intel.com" <sathyanarayanan.kuppuswamy@...ux.intel.com>,
	"Hunter, Adrian" <adrian.hunter@...el.com>,
	"thomas.lendacky@....com" <thomas.lendacky@....com>,
	"ashish.kalra@....com" <ashish.kalra@....com>,
	"linux-coco@...ts.linux.dev" <linux-coco@...ts.linux.dev>,
	"seanjc@...gle.com" <seanjc@...gle.com>,
	"bhe@...hat.com" <bhe@...hat.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCHv5 10/16] x86/tdx: Convert shared memory back to private
 on kexec

On Fri, Jan 05, 2024 at 07:38:48PM +0000, Edgecombe, Rick P wrote:
> The break apart looks good.
> 
> On Sat, 2023-12-23 at 02:52 +0300, Kirill A. Shutemov wrote:
> > +       while (addr < end) {
> > +               unsigned long size;
> > +               unsigned int level;
> > +               pte_t *pte;
> > +
> > +               pte = lookup_address(addr, &level);
> > +               size = page_level_size(level);
> > +
> > +               if (pte && pte_decrypted(*pte)) {
> > +                       int pages = size / PAGE_SIZE;
> > +
> > +                       /*
> > +                        * Touching memory with shared bit set
> > triggers implicit
> > +                        * conversion to shared.
> > +                        *
> > +                        * Make sure nobody touches the shared range
> > from
> > +                        * now on.
> > +                        */
> > +                       set_pte(pte, __pte(0));
> 
> Does this need a full shootdown at this point, at least in the crash
> case? A local flush for the non-crash case might be nice too.

See __flush_tlb_all() after the loop.

> Also, probably a barrier() between set_pte() and
> tdx_enc_status_changed().

But why? There's no concurrency at this point. Interrupts are disabled and
only one CPU is active. Nobody can touch the memory relevant for the PTE.

-- 
  Kiryl Shutsemau / Kirill A. Shutemov