linux-kernel - Re: [PATCH RFC 0/5] mm/gup: Introduce exclusive GUP pinning

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Date: Wed, 19 Jun 2024 14:17:23 +0200
From: David Hildenbrand <david@...hat.com>
To: Jason Gunthorpe <jgg@...dia.com>, Fuad Tabba <tabba@...gle.com>
Cc: John Hubbard <jhubbard@...dia.com>,
 Elliot Berman <quic_eberman@...cinc.com>,
 Andrew Morton <akpm@...ux-foundation.org>, Shuah Khan <shuah@...nel.org>,
 Matthew Wilcox <willy@...radead.org>, maz@...nel.org, kvm@...r.kernel.org,
 linux-arm-msm@...r.kernel.org, linux-mm@...ck.org,
 linux-kernel@...r.kernel.org, linux-kselftest@...r.kernel.org,
 pbonzini@...hat.com
Subject: Re: [PATCH RFC 0/5] mm/gup: Introduce exclusive GUP pinning

> If the memory can't be accessed by the CPU then it shouldn't be mapped
> into a PTE in the first place. The fact you made userspace faults
> (only) work is nifty but still an ugly hack to get around the fact you
> shouldn't be mapping in the first place.
> 
> We already have ZONE_DEVICE/DEVICE_PRIVATE to handle exactly this
> scenario. "memory" that cannot be touched by the CPU but can still be
> specially accessed by enlightened components.
> 
> guest_memfd, and more broadly memfd based instead of VMA based, memory
> mapping in KVM is a similar outcome to DEVICE_PRIVATE.
> 
> I think you need to stay in the world of not mapping the memory, one
> way or another.

Fully agreed. Private memory shall not be mapped.

-- 
Cheers,

David / dhildenb