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[2003:cb:c70a:1c00:b8d4:bc62:e7ed:ec0e]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-438e23dea58sm50657605e9.15.2025.01.31.02.52.44 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Fri, 31 Jan 2025 02:52:47 -0800 (PST) Message-ID: <1ce8fd8e-893b-4867-b690-298bb3048d75@redhat.com> Date: Fri, 31 Jan 2025 11:52:43 +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: [RFC PATCH v2 04/11] KVM: guest_memfd: Add KVM capability to check if guest_memfd is shared To: Fuad Tabba 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, yu.c.zhang@linux.intel.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: <20250129172320.950523-1-tabba@google.com> <20250129172320.950523-5-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 31.01.25 10:52, Fuad Tabba wrote: > Hi David, > > On Fri, 31 Jan 2025 at 09:11, David Hildenbrand wrote: >> >> On 29.01.25 18:23, 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. >>> >>> For now, this checks only whether the VM type supports sharing >>> guest_memfd backed memory. In the future, it will be expanded to >>> check whether the specific memory address is shared with the >>> host. >>> >>> Signed-off-by: Fuad Tabba >>> --- >>> include/uapi/linux/kvm.h | 1 + >>> virt/kvm/guest_memfd.c | 13 +++++++++++++ >>> virt/kvm/kvm_main.c | 4 ++++ >>> 3 files changed, 18 insertions(+) >>> >>> diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h >>> index 502ea63b5d2e..3ac805c5abf1 100644 >>> --- a/include/uapi/linux/kvm.h >>> +++ b/include/uapi/linux/kvm.h >>> @@ -933,6 +933,7 @@ 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 >>> >>> struct kvm_irq_routing_irqchip { >>> __u32 irqchip; >>> diff --git a/virt/kvm/guest_memfd.c b/virt/kvm/guest_memfd.c >>> index 86441581c9ae..4e1144ed3446 100644 >>> --- a/virt/kvm/guest_memfd.c >>> +++ b/virt/kvm/guest_memfd.c >>> @@ -308,6 +308,13 @@ static pgoff_t kvm_gmem_get_index(struct kvm_memory_slot *slot, gfn_t gfn) >>> } >>> >>> #ifdef CONFIG_KVM_GMEM_SHARED_MEM >>> +static bool kvm_gmem_is_shared(struct file *file, pgoff_t pgoff) >> >> I assume you want to call this something like: >> >> kvm_gmem_folio_is_shared >> kvm_gmem_offset_is_shared >> kvm_gmem_range_is_shared >> ... >> >> To make it clearer that you are only checking one piece and not the >> whole thing. >> >> But then, I wonder if that could be handled in kvm_gmem_get_folio(), >> and e.g., specified via a flag? >> >> kvm_gmem_get_folio(inode, vmf->pgoff, KVM_GMEM_GF_SHARED); >> >> Maybe existing callers would want to pass KVM_GMEM_GF_PRIVATE, and the >> ones that "don't care" don't pas anything? > > I agree that naming this function to make it clearer would be a good > idea. That said, I think with the patches I removed, it doesn't even > need to be exposed at all, even looking at future patches --- it has > no callers other than kvm_gmem_fault(). Therefore, I don't think we > need to add a flag to kvm_gmem_get_folio(). > > I'll have a closer look while preparing the respin and fix it either way. Okay, having some indication in the name that we only want a shared folio etc. might make sense. -- Cheers, David / dhildenb