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([2a01:599:916:16c6:54bd:1780:84bf:8f90]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-447f1ef0b20sm99223265e9.14.2025.05.22.01.14.56 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 22 May 2025 01:14:59 -0700 (PDT) Message-ID: Date: Thu, 22 May 2025 10:14:55 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v9 09/17] KVM: x86/mmu: Handle guest page faults for guest_memfd with shared memory To: Fuad Tabba Cc: Ackerley Tng , 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, 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, ira.weiny@intel.com References: <56e17793-efa2-44ea-9491-3217a059d3f3@redhat.com> From: David Hildenbrand Autocrypt: addr=david@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Rspamd-Queue-Id: 837634000A X-Stat-Signature: sdp7jpob6aimut3uz5g7er3n1ppnzcxq X-Rspam-User: X-Rspamd-Server: rspam07 X-HE-Tag: 1747901704-20669 X-HE-Meta: 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 JOa3KDxw 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 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 22.05.25 09:46, Fuad Tabba wrote: > Hi David, > > On Thu, 22 May 2025 at 08:16, David Hildenbrand wrote: >> >> >>>>> + * shared (i.e., non-CoCo VMs). >>>>> + */ >>>>> static inline bool kvm_mem_is_private(struct kvm *kvm, gfn_t gfn) >>>>> { >>>>> - return IS_ENABLED(CONFIG_KVM_GMEM) && >>>>> - kvm_get_memory_attributes(kvm, gfn) & KVM_MEMORY_ATTRIBUTE_PRIVATE; >>>>> + struct kvm_memory_slot *slot; >>>>> + >>>>> + if (!IS_ENABLED(CONFIG_KVM_GMEM)) >>>>> + return false; >>>>> + >>>>> + slot = gfn_to_memslot(kvm, gfn); >>>>> + if (kvm_slot_has_gmem(slot) && kvm_gmem_memslot_supports_shared(slot)) { >>>>> + /* >>>>> + * For now, memslots only support in-place shared memory if the >>>>> + * host is allowed to mmap memory (i.e., non-Coco VMs). >>>>> + */ >>>> >>>> Not accurate: there is no in-place conversion support in this series, >>>> because there is no such itnerface. So the reason is that all memory is >>>> shared for there VM types? >>>> >>> >>> True that there's no in-place conversion yet. >>> >>> In this patch series, guest_memfd memslots support shared memory only >>> for specific VM types (on x86, that would be KVM_X86_DEFAULT_VM and >>> KVM_X86_SW_PROTECTED_VMs). >>> >>> How about this wording: >>> >>> Without conversion support, if the guest_memfd memslot supports shared >>> memory, all memory must be used as not private (implicitly shared). >>> >> >> LGTM >> >>>>> + return false; >>>>> + } >>>>> + >>>>> + return kvm_get_memory_attributes(kvm, gfn) & KVM_MEMORY_ATTRIBUTE_PRIVATE; >>>>> } >>>>> #else >>>>> static inline bool kvm_mem_is_private(struct kvm *kvm, gfn_t gfn) >>>>> diff --git a/virt/kvm/guest_memfd.c b/virt/kvm/guest_memfd.c >>>>> index 2f499021df66..fe0245335c96 100644 >>>>> --- a/virt/kvm/guest_memfd.c >>>>> +++ b/virt/kvm/guest_memfd.c >>>>> @@ -388,6 +388,23 @@ static int kvm_gmem_mmap(struct file *file, struct vm_area_struct *vma) >>>>> >>>>> return 0; >>>>> } >>>>> + >>>>> +bool kvm_gmem_memslot_supports_shared(const struct kvm_memory_slot *slot) >>>>> +{ >>>>> + struct file *file; >>>>> + bool ret; >>>>> + >>>>> + file = kvm_gmem_get_file((struct kvm_memory_slot *)slot); >>>>> + if (!file) >>>>> + return false; >>>>> + >>>>> + ret = kvm_gmem_supports_shared(file_inode(file)); >>>>> + >>>>> + fput(file); >>>>> + return ret; >>>> >>>> Would it make sense to cache that information in the memslot, to avoid >>>> the get/put? >>>> >>>> We could simply cache when creating the memslot I guess. >>>> >>> >>> When I wrote it I was assuming that to ensure correctness we should >>> check with guest memfd, like what if someone closed the gmem file in the >>> middle of the fault path? >>> >>> But I guess after the discussion at the last call, since the faulting >>> process is long and racy, if this check passed and we go to guest memfd >>> and the file was closed, it would just fail so I guess caching is fine. >> >> Yes, that would be my assumption. I mean, we also msut make sure that if >> the user does something stupid like that, that we won't trigger other >> undesired code paths (like, suddenly the guest_memfd being !shared). >> >>> >>>> As an alternative ... could we simple get/put when managing the memslot? >>> >>> What does a simple get/put mean here? >> >> s/simple/simply/ >> >> So when we create the memslot, we'd perform the get, and when we destroy >> the memslot, we'd do the put. >> >> Just an idea. > > I'm not sure we can do that. The comment in kvm_gmem_bind() on > dropping the reference to the file explains why: > https://elixir.bootlin.com/linux/v6.14.7/source/virt/kvm/guest_memfd.c#L526 Right, although it is rather suboptimal; we have to constantly get/put the file, even in kvm_gmem_get_pfn() right now. Repeatedly two atomics and a bunch of checks ... for something a sane use case should never trigger. Anyhow, that's probably something to optimize also for kvm_gmem_get_pfn() later on? Of course, the caching here is rather straight forward. -- Cheers, David / dhildenb