linux-kernel - Re: [RFC PATCH 0/4] KVM: ioctl for populating guest

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Message-ID: <efe6acf5-8e08-46cd-88e4-ad85d3af2688@redhat.com>
Date: Wed, 20 Nov 2024 16:13:07 +0100
From: David Hildenbrand <david@...hat.com>
To: kalyazin@...zon.com, pbonzini@...hat.com, corbet@....net,
 kvm@...r.kernel.org, linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org
Cc: jthoughton@...gle.com, brijesh.singh@....com, michael.roth@....com,
 graf@...zon.de, jgowans@...zon.com, roypat@...zon.co.uk, derekmn@...zon.com,
 nsaenz@...zon.es, xmarcalx@...zon.com,
 Sean Christopherson <seanjc@...gle.com>, linux-mm@...ck.org
Subject: Re: [RFC PATCH 0/4] KVM: ioctl for populating guest_memfd

>>
>> The questions are:
>>     - Is this a well-known behaviour?
>>     - Is there a way to mitigate that, ie make shared memory (including
>> guest_memfd) population faster/comparable to private memory?
> 
> Likely. But your experiment measures above something different than what
> guest_memfd vs. anon does: guest_memfd doesn't update page tables, so I
> would assume guest_memfd will be faster than MAP_POPULATE.
> 
> How do you end up allocating memory for guest_memfd? Using simple
> fallocate()?

Heh, now I spot that your comment was as reply to a series.

If your ioctl is supposed to to more than "allocating memory" like 
MAP_POPULATE/MADV_POPULATE+* ... then POPULATE is a suboptimal choice. 
Because for allocating memory, we would want to use fallocate() instead. 
I assume you want to "allocate+copy"?

I'll note that, as we're moving into the direction of moving 
guest_memfd.c into mm/guestmem.c, we'll likely want to avoid "KVM_*" 
ioctls, and think about something generic.

Any clue how your new ioctl will interact with the WIP to have shared 
memory as part of guest_memfd? For example, could it be reasonable to 
"populate" the shared memory first (via VMA) and then convert that 
"allocated+filled" memory to private?

-- 
Cheers,

David / dhildenb