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[2003:d8:2f25:bd00:949:b5a9:e02a:f265]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-3a568b18f96sm11366180f8f.66.2025.06.16.07.25.27 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 16 Jun 2025 07:25:29 -0700 (PDT) Message-ID: <701c8716-dd69-4bf6-9d36-4f8847f96e18@redhat.com> Date: Mon, 16 Jun 2025 16:25:26 +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: Fuad Tabba Cc: Ira Weiny , Sean Christopherson , 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> 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 16.06.25 16:16, Fuad Tabba wrote: > On Mon, 16 Jun 2025 at 15:03, David Hildenbrand wrote: >> >> On 16.06.25 15:44, Ira Weiny wrote: >>> Sean Christopherson wrote: >>>> On Wed, Jun 11, 2025, Fuad Tabba wrote: >>>>> This patch enables support for shared memory in guest_memfd, including >>>> >>>> Please don't lead with with "This patch", simply state what changes are being >>>> made as a command. >>>> >>>>> mapping that memory from host userspace. >>>> >>>>> This functionality is gated by the KVM_GMEM_SHARED_MEM Kconfig option, >>>>> and enabled for a given instance by the GUEST_MEMFD_FLAG_SUPPORT_SHARED >>>>> flag at creation time. >>>> >>>> Why? I can see that from the patch. >>>> >>>> This changelog is way, way, waaay too light on details. Sorry for jumping in at >>>> the 11th hour, but we've spent what, 2 years working on this? >>>> >>>>> Reviewed-by: Gavin Shan >>>>> Acked-by: David Hildenbrand >>>>> Co-developed-by: Ackerley Tng >>>>> Signed-off-by: Ackerley Tng >>>>> Signed-off-by: Fuad Tabba >>>>> --- >>>>> diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h >>>>> index d00b85cb168c..cb19150fd595 100644 >>>>> --- a/include/uapi/linux/kvm.h >>>>> +++ b/include/uapi/linux/kvm.h >>>>> @@ -1570,6 +1570,7 @@ struct kvm_memory_attributes { >>>>> #define KVM_MEMORY_ATTRIBUTE_PRIVATE (1ULL << 3) >>>>> >>>>> #define KVM_CREATE_GUEST_MEMFD _IOWR(KVMIO, 0xd4, struct kvm_create_guest_memfd) >>>>> +#define GUEST_MEMFD_FLAG_SUPPORT_SHARED (1ULL << 0) >>>> >>>> I find the SUPPORT_SHARED terminology to be super confusing. I had to dig quite >>>> deep to undesrtand that "support shared" actually mean "userspace explicitly >>>> enable sharing on _this_ guest_memfd instance". E.g. I was surprised to see >>>> >>>> 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? That something is mappable into $whatever is not the right >> way to look at this IMHO. As raised in the past, we can easily support >> read()/write()/etc to this non-private memory. >> >> >> I'll note, the "non-private" memory in guest-memfd behaves just like ... >> the "shared" memory in shmem ... well, or like other memory in memfd. >> (which is based on mm/shmem.c). >> >> "Private" is also not the best way to describe the "protected\encrypted" >> memory, but that ship has sailed with KVM_MEMORY_ATTRIBUTE_PRIVATE. >> >> 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. > > One could argue that non-CoCo VMs have no concept of "shared" vs > "private". A different way of looking at it is, non-CoCo VMs have > their state as shared by 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. I'm still thinking about another way to call non-private memory ... no success so far. "ordinary" or "generic" is .... not better. -- Cheers, David / dhildenb