From: Sean Christopherson <seanjc@google.com>
To: Chao Peng <chao.p.peng@linux.intel.com>
Cc: Andy Lutomirski <luto@kernel.org>,
Vishal Annapurve <vannapurve@google.com>,
Marc Orr <marcorr@google.com>, kvm list <kvm@vger.kernel.org>,
LKML <linux-kernel@vger.kernel.org>,
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linux-api@vger.kernel.org, linux-doc@vger.kernel.org,
qemu-devel@nongnu.org, Paolo Bonzini <pbonzini@redhat.com>,
Jonathan Corbet <corbet@lwn.net>,
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Jim Mattson <jmattson@google.com>, Joerg Roedel <joro@8bytes.org>,
Thomas Gleixner <tglx@linutronix.de>,
Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
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"Maciej S . Szmigiero" <mail@maciej.szmigiero.name>,
Vlastimil Babka <vbabka@suse.cz>,
Yu Zhang <yu.c.zhang@linux.intel.com>,
"Kirill A . Shutemov" <kirill.shutemov@linux.intel.com>,
Jun Nakajima <jun.nakajima@intel.com>,
Dave Hansen <dave.hansen@intel.com>,
Andi Kleen <ak@linux.intel.com>,
David Hildenbrand <david@redhat.com>,
aarcange@redhat.com, ddutile@redhat.com, dhildenb@redhat.com,
Quentin Perret <qperret@google.com>,
Michael Roth <michael.roth@amd.com>,
mhocko@suse.com
Subject: Re: [PATCH v6 0/8] KVM: mm: fd-based approach for supporting KVM guest private memory
Date: Wed, 15 Jun 2022 14:29:45 +0000 [thread overview]
Message-ID: <Yqns2ar0TND4RP9P@google.com> (raw)
In-Reply-To: <20220615091759.GB1823790@chaop.bj.intel.com>
On Wed, Jun 15, 2022, Chao Peng wrote:
> On Tue, Jun 14, 2022 at 01:59:41PM -0700, Andy Lutomirski wrote:
> > On Tue, Jun 14, 2022 at 12:09 PM Sean Christopherson <seanjc@google.com> wrote:
> > >
> > > On Tue, Jun 14, 2022, Andy Lutomirski wrote:
> > > > This patch series is fairly close to implementing a rather more
> > > > efficient solution. I'm not familiar enough with hypervisor userspace
> > > > to really know if this would work, but:
> > > >
> > > > What if shared guest memory could also be file-backed, either in the
> > > > same fd or with a second fd covering the shared portion of a memslot?
> > > > This would allow changes to the backing store (punching holes, etc) to
> > > > be some without mmap_lock or host-userspace TLB flushes? Depending on
> > > > what the guest is doing with its shared memory, userspace might need
> > > > the memory mapped or it might not.
> > >
> > > That's what I'm angling for with the F_SEAL_FAULT_ALLOCATIONS idea. The issue,
> > > unless I'm misreading code, is that punching a hole in the shared memory backing
> > > store doesn't prevent reallocating that hole on fault, i.e. a helper process that
> > > keeps a valid mapping of guest shared memory can silently fill the hole.
> > >
> > > What we're hoping to achieve is a way to prevent allocating memory without a very
> > > explicit action from userspace, e.g. fallocate().
> >
> > Ah, I misunderstood. I thought your goal was to mmap it and prevent
> > page faults from allocating.
I don't think you misunderstood, that's also one of the goals. The use case is
that multiple processes in the host mmap() guest memory, and we'd like to be able
to punch a hole without having to rendezvous with all processes and also to prevent
an unintentional re-allocation.
> I think we still need the mmap, but want to prevent allocating when
> userspace touches previously mmaped area that has never filled the page.
Yes, or if a chunk was filled at some point but then was removed via PUNCH_HOLE.
> I don't have clear answer if other operations like read/write should be
> also prevented (probably yes). And only after an explicit fallocate() to
> allocate the page these operations would act normally.
I always forget about read/write. I believe reads should be ok, the semantics of
holes are that they return zeros, i.e. can use ZERO_PAGE() and not allocate a new
backing page. Not sure what to do about writes though. Allocating on direct writes
might be ok for our use case, but that could also result in a rather wierd API.
> > It is indeed the case (and has been since before quite a few of us
> > were born) that a hole in a sparse file is logically just a bunch of
> > zeros. A way to make a file for which a hole is an actual hole seems
> > like it would solve this problem nicely. It could also be solved more
> > specifically for KVM by making sure that the private/shared mode that
> > userspace programs is strict enough to prevent accidental allocations
> > -- if a GPA is definitively private, shared, neither, or (potentially,
> > on TDX only) both, then a page that *isn't* shared will never be
> > accidentally allocated by KVM.
>
> KVM is clever enough to not allocate since it knows a GPA is shared or
> not. This case it's the host userspace that can cause the allocating and
> is too complex to check on every access from guest.
Yes, KVM is not in the picture at all. KVM won't trigger allocation, but KVM also
is not in a position to prevent userspace from touching memory.
> > If the shared backing is not mmapped,
> > it also won't be accidentally allocated by host userspace on a stray
> > or careless write.
>
> As said above, mmap is still prefered, otherwise too many changes are
> needed for usespace VMM.
Forcing userspace to change doesn't bother me too much, the biggest concern is
having to take mmap_lock for write in a per-host process.
next prev parent reply other threads:[~2022-06-15 14:29 UTC|newest]
Thread overview: 58+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-05-19 15:37 [PATCH v6 0/8] KVM: mm: fd-based approach for supporting KVM guest private memory Chao Peng
2022-05-19 15:37 ` [PATCH v6 1/8] mm: Introduce memfile_notifier Chao Peng
2022-05-19 15:37 ` [PATCH v6 2/8] mm/shmem: Support memfile_notifier Chao Peng
2022-05-19 15:37 ` [PATCH v6 3/8] mm/memfd: Introduce MFD_INACCESSIBLE flag Chao Peng
2022-05-31 19:15 ` Vishal Annapurve
2022-06-01 10:17 ` Chao Peng
2022-06-01 12:11 ` Gupta, Pankaj
2022-06-02 10:07 ` Chao Peng
2022-06-14 20:23 ` Sean Christopherson
2022-06-15 8:53 ` Chao Peng
2022-05-19 15:37 ` [PATCH v6 4/8] KVM: Extend the memslot to support fd-based private memory Chao Peng
2022-05-20 17:57 ` Andy Lutomirski
2022-05-20 18:31 ` Sean Christopherson
2022-05-22 4:03 ` Andy Lutomirski
2022-05-23 13:21 ` Chao Peng
2022-05-23 15:22 ` Sean Christopherson
2022-05-30 13:26 ` Chao Peng
2022-06-10 16:14 ` Sean Christopherson
2022-06-14 6:45 ` Chao Peng
2022-06-23 22:59 ` Michael Roth
2022-06-24 8:54 ` Chao Peng
2022-06-24 13:01 ` Michael Roth
2022-06-17 20:52 ` Sean Christopherson
2022-06-17 21:27 ` Sean Christopherson
2022-06-20 14:09 ` Chao Peng
2022-06-20 14:08 ` Chao Peng
2022-05-19 15:37 ` [PATCH v6 5/8] KVM: Add KVM_EXIT_MEMORY_FAULT exit Chao Peng
2022-05-19 15:37 ` [PATCH v6 6/8] KVM: Handle page fault for private memory Chao Peng
2022-06-17 21:30 ` Sean Christopherson
2022-06-20 14:16 ` Chao Peng
2022-08-19 0:40 ` Kirill A. Shutemov
2022-08-25 23:43 ` Sean Christopherson
2022-06-24 3:58 ` Nikunj A. Dadhania
2022-06-24 9:02 ` Chao Peng
2022-06-30 19:14 ` Vishal Annapurve
2022-06-30 22:21 ` Michael Roth
2022-07-01 1:21 ` Xiaoyao Li
2022-07-07 20:08 ` Sean Christopherson
2022-07-08 3:29 ` Xiaoyao Li
2022-07-20 23:08 ` Vishal Annapurve
2022-07-21 9:45 ` Chao Peng
2022-05-19 15:37 ` [PATCH v6 7/8] KVM: Enable and expose KVM_MEM_PRIVATE Chao Peng
2022-06-23 22:07 ` Michael Roth
2022-06-24 8:43 ` Chao Peng
2022-05-19 15:37 ` [PATCH v6 8/8] memfd_create.2: Describe MFD_INACCESSIBLE flag Chao Peng
2022-06-06 20:09 ` [PATCH v6 0/8] KVM: mm: fd-based approach for supporting KVM guest private memory Vishal Annapurve
2022-06-07 6:57 ` Chao Peng
2022-06-08 0:55 ` Marc Orr
2022-06-08 2:18 ` Chao Peng
2022-06-08 19:37 ` Vishal Annapurve
2022-06-09 20:29 ` Sean Christopherson
2022-06-14 7:28 ` Chao Peng
2022-06-14 17:37 ` Andy Lutomirski
2022-06-14 19:08 ` Sean Christopherson
2022-06-14 20:59 ` Andy Lutomirski
2022-06-15 9:17 ` Chao Peng
2022-06-15 14:29 ` Sean Christopherson [this message]
2022-06-10 0:11 ` Marc Orr
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