linux-kernel - Re: [PATCHv5 10/19] x86/mm: Implement page_keyid() using page

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Date:   Mon, 23 Jul 2018 12:45:17 +0300
From:   "Kirill A. Shutemov" <kirill@...temov.name>
To:     Dave Hansen <dave.hansen@...el.com>
Cc:     "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Ingo Molnar <mingo@...hat.com>, x86@...nel.org,
        Thomas Gleixner <tglx@...utronix.de>,
        "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,
        "Schofield, Alison" <alison.schofield@...el.com>
Subject: Re: [PATCHv5 10/19] x86/mm: Implement page_keyid() using page_ext

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.

> > 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