linux-kernel - Re: [PATCH] x86/mm: Fix boot with some memory above MAXMEM

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Date:   Wed, 3 Jun 2020 12:18:57 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     "Kirill A. Shutemov" <kirill@...temov.name>
Cc:     Mike Rapoport <rppt@...nel.org>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Andy Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Tony Luck <tony.luck@...el.com>, x86@...nel.org,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        stable@...r.kernel.org
Subject: Re: [PATCH] x86/mm: Fix boot with some memory above MAXMEM

On 6/2/20 4:18 PM, Kirill A. Shutemov wrote:
> On Tue, May 26, 2020 at 07:27:15AM -0700, Dave Hansen wrote:
>> On 5/25/20 8:08 AM, Kirill A. Shutemov wrote:
>>>>>> +	if (not_addressable) {
>>>>>> +		pr_err("%lldGB of physical memory is not addressable in the paging mode\n",
>>>>>> +		       not_addressable >> 30);
>>>>>> +		if (!pgtable_l5_enabled())
>>>>>> +			pr_err("Consider enabling 5-level paging\n");
>>>> Could this happen at all when l5 is enabled?
>>>> Does it mean we need kmap() for 64-bit?
>>> It's future-profing. Who knows what paging modes we would have in the
>>> future.
>>
>> Future-proofing and firmware-proofing. :)
>>
>> In any case, are we *really* limited to 52 bits of physical memory with
>> 5-level paging?
> 
> Yes. It's architectural. SDM says "MAXPHYADDR is at most 52" (Vol 3A,
> 4.1.4).

Right you are.

I'm glad it's in the architecture.  Makes all of this a lot easier!

>> So shouldn't it technically be this:
>>
>> #define MAX_PHYSMEM_BITS (pgtable_l5_enabled() ? 55 : 46)
>>
>> ?
> 
> Bits above 52 are ignored in the page table entries and accessible to
> software. Some of them got claimed by HW features (XD-bit, protection
> keys), but such features require explicit opt-in on software side.
> 
> Kernel could claim bits 53-55 for the physical address, but it doesn't get
> us anything: if future HW would provide such feature it would require
> opt-in. On other hand claiming them now means we cannot use them for other
> purposes as SW bit. I don't see a point.

Yep, agreed.