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[2003:cb:c706:2000:e44c:bc46:d8d3:be5]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-38f64b0f565sm2341513f8f.51.2025.02.20.09.12.04 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 20 Feb 2025 09:12:06 -0800 (PST) Message-ID: <404807ee-a5d0-4409-a0d4-f475a6eb39ac@redhat.com> Date: Thu, 20 Feb 2025 18:12:03 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v4 03/10] KVM: guest_memfd: Allow host to map guest_memfd() pages 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, 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> <20250218172500.807733-4-tabba@google.com> <2cdc5414-a280-4c47-86d5-4261a12deab6@redhat.com> <69467908-17a5-4700-b5da-efc0446b8663@redhat.com> From: David Hildenbrand Autocrypt: addr=david@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Rspam-User: X-Rspamd-Queue-Id: 20B8D18001F X-Rspamd-Server: rspam07 X-Stat-Signature: nwnx97hjgxw9kkhfh87151zaifunp4r5 X-HE-Tag: 1740071531-367119 X-HE-Meta: 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 Mr3r4PFL 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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 20.02.25 18:10, Fuad Tabba wrote: > On Thu, 20 Feb 2025 at 15:58, David Hildenbrand wrote: >> >> On 20.02.25 16:45, Fuad Tabba wrote: >>> Hi David, >>> >>> On Thu, 20 Feb 2025 at 12:04, Fuad Tabba wrote: >>>> >>>> On Thu, 20 Feb 2025 at 11:58, David Hildenbrand wrote: >>>>> >>>>> On 18.02.25 18:24, Fuad Tabba wrote: >>>>>> Add support for mmap() and fault() for guest_memfd backed memory >>>>>> in the host for VMs that support in-place conversion between >>>>>> shared and private. To that end, this patch adds the ability to >>>>>> check whether the VM type supports in-place conversion, and only >>>>>> allows mapping its memory if that's the case. >>>>>> >>>>>> This behavior is also gated by the configuration option >>>>>> KVM_GMEM_SHARED_MEM. >>>>>> >>>>>> Signed-off-by: Fuad Tabba >>>>>> --- >>>>>> include/linux/kvm_host.h | 11 +++++ >>>>>> virt/kvm/guest_memfd.c | 103 +++++++++++++++++++++++++++++++++++++++ >>>>>> 2 files changed, 114 insertions(+) >>>>>> >>>>>> diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h >>>>>> index 3ad0719bfc4f..f9e8b10a4b09 100644 >>>>>> --- a/include/linux/kvm_host.h >>>>>> +++ b/include/linux/kvm_host.h >>>>>> @@ -728,6 +728,17 @@ static inline bool kvm_arch_has_private_mem(struct kvm *kvm) >>>>>> } >>>>>> #endif >>>>>> >>>>>> +/* >>>>>> + * Arch code must define kvm_arch_gmem_supports_shared_mem if support for >>>>>> + * private memory is enabled and it supports in-place shared/private conversion. >>>>>> + */ >>>>>> +#if !defined(kvm_arch_gmem_supports_shared_mem) && !IS_ENABLED(CONFIG_KVM_PRIVATE_MEM) >>>>>> +static inline bool kvm_arch_gmem_supports_shared_mem(struct kvm *kvm) >>>>>> +{ >>>>>> + return false; >>>>>> +} >>>>>> +#endif >>>>>> + >>>>>> #ifndef kvm_arch_has_readonly_mem >>>>>> static inline bool kvm_arch_has_readonly_mem(struct kvm *kvm) >>>>>> { >>>>>> diff --git a/virt/kvm/guest_memfd.c b/virt/kvm/guest_memfd.c >>>>>> index c6f6792bec2a..30b47ff0e6d2 100644 >>>>>> --- a/virt/kvm/guest_memfd.c >>>>>> +++ b/virt/kvm/guest_memfd.c >>>>>> @@ -317,9 +317,112 @@ void kvm_gmem_handle_folio_put(struct folio *folio) >>>>>> { >>>>>> WARN_ONCE(1, "A placeholder that shouldn't trigger. Work in progress."); >>>>>> } >>>>>> + >>>>>> +static bool kvm_gmem_offset_is_shared(struct file *file, pgoff_t index) >>>>>> +{ >>>>>> + struct kvm_gmem *gmem = file->private_data; >>>>>> + >>>>>> + /* For now, VMs that support shared memory share all their memory. */ >>>>>> + return kvm_arch_gmem_supports_shared_mem(gmem->kvm); >>>>>> +} >>>>>> + >>>>>> +static vm_fault_t kvm_gmem_fault(struct vm_fault *vmf) >>>>>> +{ >>>>>> + struct inode *inode = file_inode(vmf->vma->vm_file); >>>>>> + struct folio *folio; >>>>>> + vm_fault_t ret = VM_FAULT_LOCKED; >>>>>> + >>>>>> + filemap_invalidate_lock_shared(inode->i_mapping); >>>>>> + >>>>>> + folio = kvm_gmem_get_folio(inode, vmf->pgoff); >>>>>> + if (IS_ERR(folio)) { >>>>>> + switch (PTR_ERR(folio)) { >>>>>> + case -EAGAIN: >>>>>> + ret = VM_FAULT_RETRY; >>>>>> + break; >>>>>> + case -ENOMEM: >>>>>> + ret = VM_FAULT_OOM; >>>>>> + break; >>>>>> + default: >>>>>> + ret = VM_FAULT_SIGBUS; >>>>>> + break; >>>>>> + } >>>>>> + goto out_filemap; >>>>>> + } >>>>>> + >>>>>> + if (folio_test_hwpoison(folio)) { >>>>>> + ret = VM_FAULT_HWPOISON; >>>>>> + goto out_folio; >>>>>> + } >>>>>> + >>>>>> + /* Must be called with folio lock held, i.e., after kvm_gmem_get_folio() */ >>>>>> + if (!kvm_gmem_offset_is_shared(vmf->vma->vm_file, vmf->pgoff)) { >>>>>> + ret = VM_FAULT_SIGBUS; >>>>>> + goto out_folio; >>>>>> + } >>>>>> + >>>>>> + /* >>>>>> + * Only private folios are marked as "guestmem" so far, and we never >>>>>> + * expect private folios at this point. >>>>>> + */ >>>>>> + if (WARN_ON_ONCE(folio_test_guestmem(folio))) { >>>>>> + ret = VM_FAULT_SIGBUS; >>>>>> + goto out_folio; >>>>>> + } >>>>>> + >>>>>> + /* No support for huge pages. */ >>>>>> + if (WARN_ON_ONCE(folio_test_large(folio))) { >>>>>> + ret = VM_FAULT_SIGBUS; >>>>>> + goto out_folio; >>>>>> + } >>>>>> + >>>>>> + if (!folio_test_uptodate(folio)) { >>>>>> + clear_highpage(folio_page(folio, 0)); >>>>>> + kvm_gmem_mark_prepared(folio); >>>>>> + } >>>>> >>>>> kvm_gmem_get_pfn()->__kvm_gmem_get_pfn() seems to call >>>>> kvm_gmem_prepare_folio() instead. >>>>> >>>>> Could we do the same here? >>>> >>>> Will do. >>> >>> I realized it's not that straightforward. __kvm_gmem_prepare_folio() >>> requires the kvm_memory_slot, which is used to calculate the gfn. At >>> that point we have neither, and it's not just an issue of access, but >>> there might not be a slot associated with that yet. >> >> Hmm, right ... I wonder if that might be problematic. I assume no >> memslot == no memory attribute telling us if it is private or shared at >> least for now? >> >> Once guest_memfd maintains that state, it might be "cleaner" ? What's >> your thought? > > The idea is that this doesn't determine whether it's shared or private > by the guest_memfd's attributes, but by the new state added in the > other patch series. That's independent of memslots and guest addresses > altogether. > > One scenario you can imagine is the host wanting to fault in memory to > initialize it before associating it with a memslot. I guess we could > make it a requirement that you cannot fault-in pages unless they are > associated with a memslot, but that might be too restrictive. Okay, just what I thought, thanks! -- Cheers, David / dhildenb