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

From: Borislav Petkov <bp@alien8.de>
To: Dave Hansen <dave.hansen@intel.com>
Cc: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
	Andy Lutomirski <luto@kernel.org>,
	Sean Christopherson <seanjc@google.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	Joerg Roedel <jroedel@suse.de>, Ard Biesheuvel <ardb@kernel.org>,
	Andi Kleen <ak@linux.intel.com>,
	Kuppuswamy Sathyanarayanan
	<sathyanarayanan.kuppuswamy@linux.intel.com>,
	David Rientjes <rientjes@google.com>,
	Vlastimil Babka <vbabka@suse.cz>,
	Tom Lendacky <thomas.lendacky@amd.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	Peter Zijlstra <peterz@infradead.org>,
	Paolo Bonzini <pbonzini@redhat.com>,
	Ingo Molnar <mingo@redhat.com>,
	Varad Gautam <varad.gautam@suse.com>,
	Dario Faggioli <dfaggioli@suse.com>,
	Brijesh Singh <brijesh.singh@amd.com>,
	Mike Rapoport <rppt@kernel.org>,
	David Hildenbrand <david@redhat.com>,
	x86@kernel.org, linux-mm@kvack.org, linux-coco@lists.linux.dev,
	linux-efi@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCHv4 3/8] efi/x86: Implement support for unaccepted memory
Date: Thu, 14 Apr 2022 17:55:13 +0200	[thread overview]
Message-ID: <YlhD4fajtABnLNvw@zn.tnic> (raw)
In-Reply-To: <c4b987d5-00d3-40ea-4c20-bf82b7512dec@intel.com>

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