linux-kernel - Re: [PATCHv4 18/18] x86: Introduce CONFIG_X86_INTEL

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Date:   Mon, 9 Jul 2018 11:59:33 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     Konrad Rzeszutek Wilk <konrad.wilk@...cle.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
Subject: Re: [PATCHv4 18/18] x86: Introduce CONFIG_X86_INTEL_MKTME

On 07/09/2018 11:52 AM, Konrad Rzeszutek Wilk wrote:
> On Mon, Jul 09, 2018 at 11:44:33AM -0700, Dave Hansen wrote:
>> On 07/09/2018 11:36 AM, Konrad Rzeszutek Wilk wrote:
>>> On Tue, Jun 26, 2018 at 05:22:45PM +0300, Kirill A. Shutemov wrote:
>>> Rip out the X86?
>>>> +	bool "Intel Multi-Key Total Memory Encryption"
>>>> +	select DYNAMIC_PHYSICAL_MASK
>>>> +	select PAGE_EXTENSION
>>>
>>> And maybe select 5-page?
>>
>> Why?  It's not a strict dependency.  You *can* build a 4-level kernel
>> and run it on smaller systems.
> 
> Sure, but in one of his commits he mentions that we may run in overlapping
> physical memory if we use 4-level paging. Hence why not just move to 5-level
> paging and simplify this.

I'm not sure it _actually_ simplifies anything.  We still need code to
handle the cases where we bump into the limits because even 5-level
paging systems can hit the *architectural* limits.  We just don't think
we'll bump into those limits any time soon in practice since they're
512x larger on 5-level systems.

But, a future system that needs physical address space or has a bunch
more KeyID bits might bump into the limits.

It's also _possible_ that a processor could come out that supports MKTME
but not 5-level paging, or a hypervisor would expose such a
configuration to a guest.  We've asked our colleagues very nicely that
Intel not make a processor that does this, but it's still possible one
shows up.