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charset="utf-8" Content-Transfer-Encoding: quoted-printable X-Rspam-User: X-Stat-Signature: hodd9yet7b16y5ka9hkp71guuop7hmwd X-Rspamd-Server: rspam12 X-Rspamd-Queue-Id: 7F2BD1C0003 X-HE-Tag: 1752000951-785180 X-HE-Meta: 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 h0V/HnRp 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 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Tue, Jul 08, 2025, Rick P Edgecombe wrote: > On Tue, 2025-07-08 at 11:03 -0700, Sean Christopherson wrote: > > > I think there is interest in de-coupling it? > >=20 > > No? >=20 > I'm talking about the intra-host migration/reboot optimization stuff. And= not > doing a good job, sorry. >=20 > > =C2=A0 Even if we get to a point where multiple distinct VMs can bind t= o a single > > guest_memfd, e.g. for inter-VM shared memory, there will still need to = be a > > sole > > owner of the memory.=C2=A0 AFAICT, fully decoupling guest_memfd from a = VM would add > > non-trivial complexity for zero practical benefit. >=20 > I'm talking about moving a gmem fd between different VMs or something usi= ng > KVM_LINK_GUEST_MEMFD [0]. Not advocating to try to support it. But trying= to > feel out where the concepts are headed. It kind of allows gmem fds (or ju= st > their source memory?) to live beyond a VM lifecycle. I think the answer is that we want to let guest_memfd live beyond the "stru= ct kvm" instance, but not beyond the Virtual Machine. From a past discussion on th= is topic[*]. : No go. Because again, the inode (physical memory) is coupled to the vir= tual machine : as a thing, not to a "struct kvm". Or more concretely, the inode is cou= pled to an : ASID or an HKID, and there can be multiple "struct kvm" objects associat= ed with a : single ASID. And at some point in the future, I suspect we'll have mult= iple KVM : objects per HKID too. :=20 : The current SEV use case is for the migration helper, where two KVM obje= cts share : a single ASID (the "real" VM and the helper). I suspect TDX will end up= with : similar behavior where helper "VMs" can use the HKID of the "real" VM. = For KVM, : that means multiple struct kvm objects being associated with a single HK= ID. :=20 : To prevent use-after-free, KVM "just" needs to ensure the helper instanc= es can't : outlive the real instance, i.e. can't use the HKID/ASID after the owning= virtual : machine has been destroyed. :=20 : To put it differently, "struct kvm" is a KVM software construct that _us= ually_, : but not always, is associated 1:1 with a virtual machine. :=20 : And FWIW, stashing the pointer without holding a reference would not be = a complete : solution, because it couldn't guard against KVM reusing a pointer. E.g.= if a : struct kvm was unbound and then freed, KVM could reuse the same memory f= or a new : struct kvm, with a different ASID/HKID, and get a false negative on the = rebinding : check. Exactly what that will look like in code is TBD, but the concept/logic hold= s up. [*] https://lore.kernel.org/all/ZOO782YGRY0YMuPu@google.com > [0] https://lore.kernel.org/all/cover.1747368092.git.afranji@google.com/ > https://lore.kernel.org/kvm/cover.1749672978.git.afranji@google.com/