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[2003:d8:2f36:d800:164f:7689:99fe:7f58]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-390e4848252sm5848249f8f.69.2025.02.28.09.33.31 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Fri, 28 Feb 2025 09:33:34 -0800 (PST) Message-ID: <5c394c80-bb2b-4f9c-9b76-78b0696fa316@redhat.com> Date: Fri, 28 Feb 2025 18:33:30 +0100 Precedence: bulk X-Mailing-List: linux-arm-msm@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v4 04/10] KVM: guest_memfd: Add KVM capability to check if guest_memfd is shared To: Fuad Tabba , Peter Xu Cc: kvm@vger.kernel.org, linux-arm-msm@vger.kernel.org, linux-mm@kvack.org, pbonzini@redhat.com, chenhuacai@kernel.org, mpe@ellerman.id.au, anup@brainfault.org, paul.walmsley@sifive.com, palmer@dabbelt.com, aou@eecs.berkeley.edu, seanjc@google.com, 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 References: <20250218172500.807733-1-tabba@google.com> <20250218172500.807733-5-tabba@google.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 28.02.25 18:22, Fuad Tabba wrote: > Hi Peter, > > On Fri, 28 Feb 2025 at 08:24, Peter Xu wrote: >> >> On Tue, Feb 18, 2025 at 05:24:54PM +0000, Fuad Tabba wrote: >>> Add the KVM capability KVM_CAP_GMEM_SHARED_MEM, which indicates >>> that the VM supports shared memory in guest_memfd, or that the >>> host can create VMs that support shared memory. Supporting shared >>> memory implies that memory can be mapped when shared with the >>> host. >>> >>> Signed-off-by: Fuad Tabba >>> --- >>> include/uapi/linux/kvm.h | 1 + >>> virt/kvm/kvm_main.c | 4 ++++ >>> 2 files changed, 5 insertions(+) >>> >>> diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h >>> index 45e6d8fca9b9..117937a895da 100644 >>> --- a/include/uapi/linux/kvm.h >>> +++ b/include/uapi/linux/kvm.h >>> @@ -929,6 +929,7 @@ struct kvm_enable_cap { >>> #define KVM_CAP_PRE_FAULT_MEMORY 236 >>> #define KVM_CAP_X86_APIC_BUS_CYCLES_NS 237 >>> #define KVM_CAP_X86_GUEST_MODE 238 >>> +#define KVM_CAP_GMEM_SHARED_MEM 239 >> >> I think SHARED_MEM is ok. Said that, to me the use case in this series is >> more about "in-place" rather than "shared". >> >> In comparison, what I'm recently looking at is a "more" shared mode of >> guest-memfd where it works almost like memfd. So all pages will be shared >> there. >> >> That helps me e.g. for the N:1 kvm binding issue I mentioned in another >> email (in one of my relies in previous version), in which case I want to >> enable gmemfd folios to be mapped more than once in a process. >> >> That'll work there as long as it's fully shared, because all things can be >> registered in the old VA way, then there's no need to have N:1 restriction. >> IOW, gmemfd will still rely on mmu notifier for tearing downs, and the >> gmem->bindings will always be empty. >> >> So if this one would be called "in-place", then I'll have my use case as >> "shared". > > I understand what you mean. The naming here is to be consistent with > the rest of the series. I don't really have a strong opinion. It means > SHARED_IN_PLACE, but then that would be a mouthful. :) I'll note that Patrick is also driving it in "all shared" mode for his direct-map removal series IIRC. So we would have a) All private b) Mixing of private and shared (incl conversion) c) All shared "IN_PLACE" might be the wrong angle to look at it. -- Cheers, David / dhildenb