Re: [PATCH 1/5] mm: Add support for unaccepted memory

From: Andi Kleen <ak@linux.intel.com>
To: Dave Hansen <dave.hansen@intel.com>,
	"Kirill A. Shutemov" <kirill@shutemov.name>,
	Borislav Petkov <bp@alien8.de>, Andy Lutomirski <luto@kernel.org>,
	Sean Christopherson <seanjc@google.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	Joerg Roedel <jroedel@suse.de>
Cc: 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>,
	x86@kernel.org, linux-mm@kvack.org, linux-coco@lists.linux.dev,
	linux-kernel@vger.kernel.org,
	"Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
Subject: Re: [PATCH 1/5] mm: Add support for unaccepted memory
Date: Tue, 10 Aug 2021 14:20:08 -0700	[thread overview]
Message-ID: <3caf5e73-c104-0057-680c-7851476e67ac@linux.intel.com> (raw)
In-Reply-To: <f445da8b-c044-3765-65f2-f911dbf6a507@intel.com>

> These boot blips are not the biggest issue in the world.  But, it is
> fully under the guest's control and I think the guest has some
> responsibility to provide *some* mitigation for it.

It sounds more like an exercise in preliminary optimization at this 
point. If it's a real problem we can still worry about it later.

> 1. Do background acceptance, as opposed to relying 100% on demand-driven
>     acceptance.  Guarantees a limited window in which blips can occur.

Like Kirill wrote this was abandoned because it always allocates all 
memory on the host even if the guest doesn't need it.

> 2. Do acceptance based on user input, like from sysfs.

You can easily do that by running "memhog" at boot. No need for anything 
in the kernel.

BTW I believe this is also configurable at the guest BIOS level.

> 3. Add a command-line argument to accept everything up front, or at
>     least before userspace runs.

Same.

> 4. Add some statistic for how much unaccepted memory remains.

Yes that makes sense. We should have statistic counters for all of this.

Also I agree with your suggestion that we should get the slow path out 
of the zone locks/interrupt disable region. That should be easy enough 
and is an obvious improvement.

-Andi