linux-kernel - Re: [PATCHv4 3/8] efi/x86: Implement support for unaccepted memory

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Date:   Thu, 14 Apr 2022 17:55:13 +0200
From:   Borislav Petkov <bp@...en8.de>
To:     Dave Hansen <dave.hansen@...el.com>
Cc:     "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Andy Lutomirski <luto@...nel.org>,
        Sean Christopherson <seanjc@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Joerg Roedel <jroedel@...e.de>,
        Ard Biesheuvel <ardb@...nel.org>,
        Andi Kleen <ak@...ux.intel.com>,
        Kuppuswamy Sathyanarayanan 
        <sathyanarayanan.kuppuswamy@...ux.intel.com>,
        David Rientjes <rientjes@...gle.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Tom Lendacky <thomas.lendacky@....com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Ingo Molnar <mingo@...hat.com>,
        Varad Gautam <varad.gautam@...e.com>,
        Dario Faggioli <dfaggioli@...e.com>,
        Brijesh Singh <brijesh.singh@....com>,
        Mike Rapoport <rppt@...nel.org>,
        David Hildenbrand <david@...hat.com>, x86@...nel.org,
        linux-mm@...ck.org, linux-coco@...ts.linux.dev,
        linux-efi@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCHv4 3/8] efi/x86: Implement support for unaccepted memory

On Fri, Apr 08, 2022 at 10:26:14AM -0700, Dave Hansen wrote:
> > +	/*
> > +	 * Accept small regions that might not be able to be represented
> > +	 * in the bitmap:
> > +	 */
> > +	if (end - start < 2 * PMD_SIZE) {
> > +		__accept_memory(start, end);
> > +		return;
> > +	}
> 
> This is not my first time looking at this code and I still had to think
> about this a bit.  That's not good.  That pathological case here is
> actually something like this:
> 
> | 4k | 2044k + 2044k | 4k |
> ^ 0x0 	     ^ 2MB	  ^ 4MB
> 
> Where we have a 2MB-aligned 4k accepted area, a 4088k unaccepted area,
> then another 4k accepted area.  That will not result in any bits being
> set in the accepted memory bitmap because no 2MB region is fully accepted.

I could use that ascii art very well in a comment above it instead of
having to paint it in my mind each time.

Thx.

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette