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[2003:cb:c706:2000:e44c:bc46:d8d3:be5]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-38f258b4123sm21580707f8f.17.2025.02.20.07.58.21 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 20 Feb 2025 07:58:22 -0800 (PST) Message-ID: <69467908-17a5-4700-b5da-efc0446b8663@redhat.com> Date: Thu, 20 Feb 2025 16:58:20 +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 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> 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 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? -- Cheers, David / dhildenb