Re: [PATCHv5 10/19] x86/mm: Implement page_keyid() using page_ext

From: Alison Schofield <alison.schofield@intel.com>
To: "Kirill A. Shutemov" <kirill@shutemov.name>
Cc: Dave Hansen <dave.hansen@intel.com>,
	"Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
	Ingo Molnar <mingo@redhat.com>,
	x86@kernel.org, Thomas Gleixner <tglx@linutronix.de>,
	"H. Peter Anvin" <hpa@zytor.com>,
	Tom Lendacky <thomas.lendacky@amd.com>,
	Kai Huang <kai.huang@linux.intel.com>,
	Jacob Pan <jacob.jun.pan@linux.intel.com>,
	linux-kernel@vger.kernel.org, linux-mm@kvack.org
Subject: Re: [PATCHv5 10/19] x86/mm: Implement page_keyid() using page_ext
Date: Mon, 23 Jul 2018 10:22:49 -0700	[thread overview]
Message-ID: <20180723172249.GA13530@alison-desk.jf.intel.com> (raw)
In-Reply-To: <20180723094517.7sxt62p3h75htppw@kshutemo-mobl1>

On Mon, Jul 23, 2018 at 12:45:17PM +0300, Kirill A. Shutemov wrote:
> On Wed, Jul 18, 2018 at 04:38:02PM -0700, Dave Hansen wrote:
> > On 07/17/2018 04:20 AM, Kirill A. Shutemov wrote:
> > > Store KeyID in bits 31:16 of extended page flags. These bits are unused.
> > 
> > I'd love a two sentence remind of what page_ext is and why you chose to
> > use it.  Yes, you need this.  No, not everybody that you want to review
> > this patch set knows what it is or why you chose it.
> 
> Okay.
> 
> > > page_keyid() returns zero until page_ext is ready.
> > 
> > Is there any implication of this?  Or does it not matter because we
> > don't run userspace until after page_ext initialization is done?
> 
> It matters in sense that we shouldn't reference page_ext before it's
> initialized otherwise we will get garbage and crash.
> 
> > > page_ext initializer enables static branch to indicate that
> > 
> > 			"enables a static branch"
> > 
> > > page_keyid() can use page_ext. The same static branch will gate MKTME
> > > readiness in general.
> > 
> > Can you elaborate on this a bit?  It would also be a nice place to hint
> > to the folks working hard on the APIs to ensure she checks this.
> 
> Okay.

At API init time we can check if (MKTME_ENABLED &&  mktme_nr_keyids > 0)
Sounds like this is another dependency we need to check and 'wait' on?
It happens after MKTME_ENABLED is set?  Let me know.

> 
> > > We don't yet set KeyID for the page. It will come in the following
> > > patch that implements prep_encrypted_page(). All pages have KeyID-0 for
> > > now.
> > 
> > It also wouldn't hurt to mention why you don't use an X86_FEATURE_* for
> > this rather than an explicit static branch.  I'm sure the x86
> > maintainers will be curious.
> 
> Sure.
> 
> -- 
>  Kirill A. Shutemov