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[2003:d8:2f2d:2400:4052:3b5:fff9:4ed0]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-3a568a547d2sm16556533f8f.19.2025.06.18.01.15.56 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 18 Jun 2025 01:15:58 -0700 (PDT) Message-ID: <3fb0e82b-f4ef-402d-a33c-0b12e8aa990c@redhat.com> Date: Wed, 18 Jun 2025 10:15:55 +0200 Precedence: bulk X-Mailing-List: kvm@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v12 08/18] KVM: guest_memfd: Allow host to map guest_memfd pages To: Sean Christopherson Cc: Fuad Tabba , Ira Weiny , kvm@vger.kernel.org, linux-arm-msm@vger.kernel.org, linux-mm@kvack.org, kvmarm@lists.linux.dev, pbonzini@redhat.com, chenhuacai@kernel.org, mpe@ellerman.id.au, anup@brainfault.org, paul.walmsley@sifive.com, palmer@dabbelt.com, aou@eecs.berkeley.edu, viro@zeniv.linux.org.uk, brauner@kernel.org, willy@infradead.org, akpm@linux-foundation.org, xiaoyao.li@intel.com, yilun.xu@intel.com, chao.p.peng@linux.intel.com, jarkko@kernel.org, amoorthy@google.com, dmatlack@google.com, isaku.yamahata@intel.com, mic@digikod.net, vbabka@suse.cz, vannapurve@google.com, ackerleytng@google.com, mail@maciej.szmigiero.name, michael.roth@amd.com, wei.w.wang@intel.com, liam.merwick@oracle.com, isaku.yamahata@gmail.com, kirill.shutemov@linux.intel.com, suzuki.poulose@arm.com, steven.price@arm.com, quic_eberman@quicinc.com, quic_mnalajal@quicinc.com, quic_tsoni@quicinc.com, quic_svaddagi@quicinc.com, quic_cvanscha@quicinc.com, quic_pderrin@quicinc.com, quic_pheragu@quicinc.com, catalin.marinas@arm.com, james.morse@arm.com, yuzenghui@huawei.com, oliver.upton@linux.dev, maz@kernel.org, will@kernel.org, qperret@google.com, keirf@google.com, roypat@amazon.co.uk, shuah@kernel.org, hch@infradead.org, jgg@nvidia.com, rientjes@google.com, jhubbard@nvidia.com, fvdl@google.com, hughd@google.com, jthoughton@google.com, peterx@redhat.com, pankaj.gupta@amd.com References: <20250611133330.1514028-1-tabba@google.com> <20250611133330.1514028-9-tabba@google.com> <68501fa5dce32_2376af294d1@iweiny-mobl.notmuch> <701c8716-dd69-4bf6-9d36-4f8847f96e18@redhat.com> From: David Hildenbrand Content-Language: en-US Autocrypt: addr=david@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 18.06.25 02:40, Sean Christopherson wrote: > On Mon, Jun 16, 2025, David Hildenbrand wrote: >> On 16.06.25 16:16, Fuad Tabba wrote: >>> On Mon, 16 Jun 2025 at 15:03, David Hildenbrand wrote: >>>>>> IMO, GUEST_MEMFD_FLAG_SHAREABLE would be more appropriate. But even that is >>>>>> weird to me. For non-CoCo VMs, there is no concept of shared vs. private. What's >>>>>> novel and notable is that the memory is _mappable_. Yeah, yeah, pKVM's use case >>>>>> is to share memory, but that's a _use case_, not the property of guest_memfd that >>>>>> is being controlled by userspace. >>>>>> >>>>>> And kvm_gmem_memslot_supports_shared() is even worse. It's simply that the >>>>>> memslot is bound to a mappable guest_memfd instance, it's that the guest_memfd >>>>>> instance is the _only_ entry point to the memslot. >>>>>> >>>>>> So my vote would be "GUEST_MEMFD_FLAG_MAPPABLE", and then something like >>>>> >>>>> If we are going to change this; FLAG_MAPPABLE is not clear to me either. >>>>> The guest can map private memory, right? I see your point about shared >>>>> being overloaded with file shared but it would not be the first time a >>>>> term is overloaded. kvm_slot_has_gmem() does makes a lot of sense. >>>>> >>>>> If it is going to change; how about GUEST_MEMFD_FLAG_USER_MAPPABLE? >>>> >>>> If "shared" is not good enough terminology ... >>>> >>>> ... can we please just find a way to name what this "non-private" memory >>>> is called? > > guest_memfd? Not trying to be cheeky, I genuinely don't understand the need > to come up with a different name. Before CoCo came along, I can't think of a > single time where we felt the need to describe guest memory. There have been > *many* instances of referring to the underlying backing store (e.g. HugeTLB vs. > THP), and many instances where we've needed to talk about the types of mappings > for guest memory, but I can't think of any cases where describing the state of > guest memory itself was ever necessary or even useful. > >>>> That something is mappable into $whatever is not the right >>>> way to look at this IMHO. > > Why not? Honest question. USER_MAPPABLE is very literal, but I think it's the > right granularity. E.g. we _could_ support read()/write()/etc, but it's not > clear to me that we need/want to. And so why bundle those under SHARED, or any > other one-size-fits-all flag? Let's take a step back. There are various ways to look at this: 1) Indicate support for guest_memfd operations: "GUEST_MEMFD_FLAG_MMAP": we support the mmap() operation "GUEST_MEMFD_FLAG_WRITE": we support the write() operation "GUEST_MEMFD_FLAG_READ": we support the read() operation ... "GUEST_MEMFD_FLAG_UFFD": we support userfaultfd operations Absolutely fine with me. In this series, we'd be advertising GUEST_MEMFD_FLAG_MMAP. Because we support the mmap operation. If the others are ever required remains to be seen [1]. 2) Indicating the mmap mapping type (support for MMAP flags) As you write below, one could indicate that we support "mmap(MAP_SHARED)" vs "mmap(MAP_PRIVATE)". I don't think that's required for now, as MAP_SHARED is really the default that anything that supports mmap() supports. If someone ever needs MAP_PRIVATE (CoW) support they can add such a flag (GUEST_MEMFD_FLAG_MMAP_MAP_PRIVATE). I doubt we want that, but who knows. As expressed elsewhere, the mmap mapping type was never what the "SHARED" in KVM_GMEM_SHARED_MEM implied. 3) *guest-memfd specific* memory access characteristics "private (non-accessible, private, secure, protected, ...) vs. "non-private". Traditionally, all was memory in guest-memfd was private, now we will make guest_memfd also support non-private memory. As this memory is "inaccessible" from a host point of view, any access to read/write it (fault it into user page tables, read(), write(), etc) will fail. Mempolicy support wanted to support mmap() without that, though [2], which was one of the reasons I agreed that exposing the access characteristics (that affect what you can actually mmap() ) made sense. In the last upstream meeting we agreed that we will not do that, but rather built up on MMAP+support for non-private memory support. [1] https://lore.kernel.org/kvm/20250303130838.28812-1-kalyazin@amazon.com/T/ [2] https://lore.kernel.org/linux-mm/20250408112402.181574-1-shivankg@amd.com/ [...] >>>> I'll further note that in the doc of KVM_SET_USER_MEMORY_REGION2 we talk >>>> about "private" vs "shared" memory ... so that would have to be improved >>>> as well. >>> >>> To add to what David just wrote, V1 of this series used the term >>> "mappable" [1]. After a few discussions, I thought the consensus was >>> that "shared" was a more accurate description --- i.e., mappability >>> was a side effect of it being shared with the host. > > As I mentioned in the other thread with respect to sharing between other > entities, simply SHARED doesn't provide sufficient granularity. HOST_SHAREABLE > gets us closer, but I still don't like that because it implies the memory is > 100% shareable, e.g. can be accessed just like normal memory. > > And for non-CoCo x86 VMs, sharing with host userspace isn't even necessarily the > goal, i.e. "sharing" is a side effect of needing to allow mmap() so that KVM can > continue to function. Does mmap() support imply "support for non-private" memory or does "support for non-private" imply mmap() support? :) In this series we went for the latter. If I got you correctly, you argue for the former. Maybe both things should simply be separated. > >>> One could argue that non-CoCo VMs have no concept of "shared" vs >>> "private". > > I am that one :-) Well, if the concept of "private" does not exist, I'd argue everything is "non-private" :) > >> A different way of looking at it is, non-CoCo VMs have >>> their state as shared by default. > > Eh, there has to be another state for there to be a default. > >> All memory of these VMs behaves similar to other memory-based shared memory >> backends (memfd, shmem) in the system, yes. You can map it into multiple >> processes and use it like shmem/memfd. > > Ya, but that's more because guest_memfd only supports MAP_SHARED, versus KVM > really wanting to truly share the memory with the entire system. > > Of course, that's also an argument to some extent against USER_MAPPABLE, because > that name assumes we'll never want to support MAP_PRIVATE. But letting userspace > MAP_PRIVATE guest_memfd would completely defeat the purpose of guest_memfd, so > unless I'm forgetting a wrinkle with MAP_PRIVATE vs. MAP_SHARED, that's an > assumption I'm a-ok making. So, first important question, are we okay with adding: "GUEST_MEMFD_FLAG_MMAP": we support the mmap() operation > > If we are really dead set on having SHARED in the name, it could be > GUEST_MEMFD_FLAG_USER_MAPPABLE_SHARED or GUEST_MEMFD_FLAG_USER_MAP_SHARED? But > to me that's _too_ specific and again somewhat confusing given the unfortunate > private vs. shared usage in CoCo-land. And just playing the odds, I'm fine taking > a risk of ending up with GUEST_MEMFD_FLAG_USER_MAPPABLE_PRIVATE or whatever, > because I think that is comically unlikely to happen. I think in addition to GUEST_MEMFD_FLAG_MMAP we want something to express "this is not your old guest_memfd that only supports private memory". And that's what I am struggling with. Now, if you argue "support for mmap() implies support for non-private memory", I'm probably okay for that. I could envision support for non-private memory even without mmap() support, how useful that might be, I don't know. But that's why I was arguing that we mmap() is just one way to consume non-private memory. -- Cheers, David / dhildenb