linux-kernel - Re: [PATCH 3/3] KVM: guest_memfd: GUP source pages prior to populating guest memory

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Message-ID: <5649e224-bb6a-4b63-bb27-5541216df0b6@suse.cz>
Date: Mon, 8 Dec 2025 12:07:00 +0100
From: Vlastimil Babka <vbabka@...e.cz>
To: Michael Roth <michael.roth@....com>,
 FirstName LastName <vannapurve@...gle.com>
Cc: ackerleytng@...gle.com, aik@....com, ashish.kalra@....com,
 david@...hat.com, ira.weiny@...el.com, kvm@...r.kernel.org,
 liam.merwick@...cle.com, linux-coco@...ts.linux.dev,
 linux-kernel@...r.kernel.org, linux-mm@...ck.org, pbonzini@...hat.com,
 seanjc@...gle.com, thomas.lendacky@....com, yan.y.zhao@...el.com
Subject: Re: [PATCH 3/3] KVM: guest_memfd: GUP source pages prior to
 populating guest memory

On 12/4/25 00:12, Michael Roth wrote:
> On Wed, Dec 03, 2025 at 08:59:10PM +0000, FirstName LastName wrote:
>> 
>> e.g. 4K page based population logic will keep things simple and can be
>> further simplified if we can add PAGE_ALIGNED(params.uaddr) restriction.
> 
> I'm still hesitant to pull the trigger on retroactively enforcing
> page-aligned uaddr for SNP, but if the maintainers are good with it then
> no objection from me.

IMHO it would be for the best. If there are no known users that would break,
it's worth trying. The "do not break userspace" rule isn't about eliminating
any theoretical possibility, but indeed about known breakages (and reacting
appropriately to reports about previously unknown breakages). Perhaps any
such users would be also willing to adjust and not demand a revert.