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[35.230.65.123]) by smtp.gmail.com with ESMTPSA id z15-20020a170903018f00b0015eb200cc00sm9366660plg.138.2022.06.15.07.29.49 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 15 Jun 2022 07:29:49 -0700 (PDT) Date: Wed, 15 Jun 2022 14:29:45 +0000 From: Sean Christopherson To: Chao Peng Cc: Andy Lutomirski , Vishal Annapurve , Marc Orr , kvm list , LKML , linux-mm@kvack.org, linux-fsdevel@vger.kernel.org, linux-api@vger.kernel.org, linux-doc@vger.kernel.org, qemu-devel@nongnu.org, Paolo Bonzini , Jonathan Corbet , Vitaly Kuznetsov , Wanpeng Li , Jim Mattson , Joerg Roedel , Thomas Gleixner , Ingo Molnar , Borislav Petkov , x86 , "H . Peter Anvin" , Hugh Dickins , Jeff Layton , "J . Bruce Fields" , Andrew Morton , Mike Rapoport , Steven Price , "Maciej S . Szmigiero" , Vlastimil Babka , Yu Zhang , "Kirill A . Shutemov" , Jun Nakajima , Dave Hansen , Andi Kleen , David Hildenbrand , aarcange@redhat.com, ddutile@redhat.com, dhildenb@redhat.com, Quentin Perret , Michael Roth , mhocko@suse.com Subject: Re: [PATCH v6 0/8] KVM: mm: fd-based approach for supporting KVM guest private memory Message-ID: References: <20220607065749.GA1513445@chaop.bj.intel.com> <20220608021820.GA1548172@chaop.bj.intel.com> <20220614072800.GB1783435@chaop.bj.intel.com> <20220615091759.GB1823790@chaop.bj.intel.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20220615091759.GB1823790@chaop.bj.intel.com> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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 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.