linux-kernel - Re: [PATCHv5 08/19] x86/mm: Introduce variables to store number, shift and mask of KeyIDs

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Date:   Fri, 20 Jul 2018 15:34:16 +0300
From:   "Kirill A. Shutemov" <kirill@...temov.name>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     Dave Hansen <dave.hansen@...el.com>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Ingo Molnar <mingo@...hat.com>, x86@...nel.org,
        "H. Peter Anvin" <hpa@...or.com>,
        Tom Lendacky <thomas.lendacky@....com>,
        Kai Huang <kai.huang@...ux.intel.com>,
        Jacob Pan <jacob.jun.pan@...ux.intel.com>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCHv5 08/19] x86/mm: Introduce variables to store number,
 shift and mask of KeyIDs

On Thu, Jul 19, 2018 at 03:40:41PM +0200, Thomas Gleixner wrote:
> On Thu, 19 Jul 2018, Kirill A. Shutemov wrote:
> > On Thu, Jul 19, 2018 at 03:18:03PM +0200, Thomas Gleixner wrote:
> > > On Thu, 19 Jul 2018, Kirill A. Shutemov wrote:
> > > > On Thu, Jul 19, 2018 at 02:37:35PM +0200, Thomas Gleixner wrote:
> > > > > On Thu, 19 Jul 2018, Kirill A. Shutemov wrote:
> > > > > > On Wed, Jul 18, 2018 at 04:19:10PM -0700, Dave Hansen wrote:
> > > > > > > >  	} else {
> > > > > > > >  		/*
> > > > > > > >  		 * Reset __PHYSICAL_MASK.
> > > > > > > > @@ -591,6 +592,9 @@ static void detect_tme(struct cpuinfo_x86 *c)
> > > > > > > >  		 * between CPUs.
> > > > > > > >  		 */
> > > > > > > >  		physical_mask = (1ULL << __PHYSICAL_MASK_SHIFT) - 1;
> > > > > > > > +		mktme_keyid_mask = 0;
> > > > > > > > +		mktme_keyid_shift = 0;
> > > > > > > > +		mktme_nr_keyids = 0;
> > > > > > > >  	}
> > > > > > > 
> > > > > > > Should be unnecessary.  These are zeroed by the compiler.
> > > > > > 
> > > > > > No. detect_tme() called for each CPU in the system.
> > > > > 
> > > > > And then the variables are cleared out while other CPUs can access them?
> > > > > How is that supposed to work?
> > > > 
> > > > This code path only matter in patalogical case: when MKTME configuation is
> > > > inconsitent between CPUs. Basically if BIOS screwed things up we disable
> > > > MKTME.
> > > 
> > > I still don't see how that's supposed to work.
> > > 
> > > When the inconsistent CPU is brought up _AFTER_ MKTME is enabled, then how
> > > does clearing the variables help? It does not magically make all the other
> > > stuff go away.
> > 
> > We don't actually enable MKTME in kernel. BIOS does. Kernel makes choose
> > to use it or not. Current design targeted to be used by userspace.
> > So until init we don't have any other stuff to go away. We can just
> > pretend that MKTME was never there.
> 
> Hotplug is not guaranteed to happen _BEFORE_ init. Think about physical
> hotplug.

Ouch. I didn't think about this. :/

In this case I don't see how to handle the situation properly.
Is it okay to WARN() && pray()?

-- 
 Kirill A. Shutemov