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X-CSE-ConnectionGUID: NOm1iGgLS0acM/1W3CHWuA== X-CSE-MsgGUID: MneSdCz6SXiykg0sDvXPyw== X-IronPort-AV: E=McAfee;i="6800,10657,11499"; a="80772511" X-IronPort-AV: E=Sophos;i="6.16,329,1744095600"; d="scan'208";a="80772511" Received: from fmviesa005.fm.intel.com ([10.60.135.145]) by fmvoesa101.fm.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 21 Jul 2025 08:07:42 -0700 X-CSE-ConnectionGUID: Y2/p+6F3S7qzVEpfid72dw== X-CSE-MsgGUID: Fsb9V4BJTea2+1zJTwr/dA== X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="6.16,329,1744095600"; d="scan'208";a="163160761" Received: from xiaoyaol-hp-g830.ccr.corp.intel.com (HELO [10.124.247.1]) ([10.124.247.1]) by fmviesa005-auth.fm.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 21 Jul 2025 08:07:28 -0700 Message-ID: <1fe0f46a-152a-4b5b-99e2-2a74873dafdc@intel.com> Date: Mon, 21 Jul 2025 23:07:25 +0800 Precedence: bulk X-Mailing-List: kvmarm@lists.linux.dev List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v15 14/21] KVM: x86: Enable guest_memfd mmap for default VM type To: Sean Christopherson , Vishal Annapurve Cc: Fuad Tabba , kvm@vger.kernel.org, linux-arm-msm@vger.kernel.org, linux-mm@kvack.org, kvmarm@lists.linux.dev, pbonzini@redhat.com, chenhuacai@kernel.org, mpe@ellerman.id.au, anup@brainfault.org, paul.walmsley@sifive.com, palmer@dabbelt.com, aou@eecs.berkeley.edu, viro@zeniv.linux.org.uk, brauner@kernel.org, willy@infradead.org, akpm@linux-foundation.org, 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, ackerleytng@google.com, mail@maciej.szmigiero.name, david@redhat.com, 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: <20250717162731.446579-1-tabba@google.com> <20250717162731.446579-15-tabba@google.com> <505a30a3-4c55-434c-86a5-f86d2e9dc78a@intel.com> Content-Language: en-US From: Xiaoyao Li In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit On 7/21/2025 10:42 PM, Sean Christopherson wrote: > On Mon, Jul 21, 2025, Vishal Annapurve wrote: >> On Mon, Jul 21, 2025 at 5:22 AM Xiaoyao Li wrote: >>> >>> On 7/18/2025 12:27 AM, Fuad Tabba wrote: >>>> +/* >>>> + * CoCo VMs with hardware support that use guest_memfd only for backing private >>>> + * memory, e.g., TDX, cannot use guest_memfd with userspace mapping enabled. >>>> + */ >>>> +#define kvm_arch_supports_gmem_mmap(kvm) \ >>>> + (IS_ENABLED(CONFIG_KVM_GMEM_SUPPORTS_MMAP) && \ >>>> + (kvm)->arch.vm_type == KVM_X86_DEFAULT_VM) >>> >>> I want to share the findings when I do the POC to enable gmem mmap in QEMU. >>> >>> Actually, QEMU can use gmem with mmap support as the normal memory even >>> without passing the gmem fd to kvm_userspace_memory_region2.guest_memfd >>> on KVM_SET_USER_MEMORY_REGION2. >>> >>> Since the gmem is mmapable, QEMU can pass the userspace addr got from >>> mmap() on gmem fd to kvm_userspace_memory_region(2).userspace_addr. It >>> works well for non-coco VMs on x86. >>> >>> Then it seems feasible to use gmem with mmap for the shared memory of >>> TDX, and an additional gmem without mmap for the private memory. i.e., >>> For struct kvm_userspace_memory_region, the @userspace_addr is passed >>> with the uaddr returned from gmem0 with mmap, while @guest_memfd is >>> passed with another gmem1 fd without mmap. >>> >>> However, it fails actually, because the kvm_arch_suports_gmem_mmap() >>> returns false for TDX VMs, which means userspace cannot allocate gmem >>> with mmap just for shared memory for TDX. >> >> Why do you want such a usecase to work? > > I'm guessing Xiaoyao was asking an honest question in response to finding a > perceived flaw when trying to get this all working in QEMU. I'm not sure if it is an flaw. Such usecase is not supported is just anti-intuition to me. >> If kvm allows mappable guest_memfd files for TDX VMs without >> conversion support, userspace will be able to use those for backing > > s/able/unable? I think vishal meant "able", because ... >> private memory unless: >> 1) KVM checks at binding time if the guest_memfd passed during memslot >> creation is not a mappable one and doesn't enforce "not mappable" >> requirement for TDX VMs at creation time. > > Xiaoyao's question is about "just for shared memory", so this is irrelevant for > the question at hand. ... if we allow gmem mmap for TDX, KVM needs to ensure the mmapable gmem should only be passed via userspace_addr. IOW, KVM needs to forbid userspace from passing the mmap'able guest_memfd to kvm_userspace_memory_region2.guest_memfd. Because it allows userspace to access the private mmeory. >> 2) KVM fetches shared faults through userspace page tables and not >> guest_memfd directly. > > This is also irrelevant. KVM _already_ supports resolving shared faults through > userspace page tables. That support won't go away as KVM will always need/want > to support mapping VM_IO and/or VM_PFNMAP memory into the guest (even for TDX). > >> I don't see value in trying to go out of way to support such a usecase. > > But if/when KVM gains support for tracking shared vs. private in guest_memfd > itself, i.e. when TDX _does_ support mmap() on guest_memfd, KVM won't have to go > out of its to support using guest_memfd for the @userspace_addr backing store. > Unless I'm missing something, the only thing needed to "support" this scenario is: As above, we need 1) mentioned by Vishal as well, to prevent userspace from passing mmapable guest_memfd to serve as private memory. > diff --git a/virt/kvm/guest_memfd.c b/virt/kvm/guest_memfd.c > index d01bd7a2c2bd..34403d2f1eeb 100644 > --- a/virt/kvm/guest_memfd.c > +++ b/virt/kvm/guest_memfd.c > @@ -533,7 +533,7 @@ int kvm_gmem_create(struct kvm *kvm, struct kvm_create_guest_memfd *args) > u64 flags = args->flags; > u64 valid_flags = 0; > > - if (kvm_arch_supports_gmem_mmap(kvm)) > + // if (kvm_arch_supports_gmem_mmap(kvm)) > valid_flags |= GUEST_MEMFD_FLAG_MMAP; > > if (flags & ~valid_flags) > > I think the question we actually want to answer is: do we want to go out of our > way to *prevent* such a usecase. E.g. is there any risk/danger that we need to > mitigate, and would the cost of the mitigation be acceptable? > > I think the answer is "no", because preventing userspace from using guest_memfd > as shared-only memory would require resolving the VMA during hva_to_pfn() in order > to fully prevent such behavior, and I defintely don't want to take mmap_lock > around hva_to_pfn_fast(). > > I don't see any obvious danger lurking. KVM's pre-guest_memfd memory management > scheme is all about effectively making KVM behave like "just another" userspace > agent. E.g. if/when TDX/SNP support comes along, guest_memfd must not allow mapping > private memory into userspace regardless of what KVM supports for page faults. > > So unless I'm missing something, for now we do nothing, and let this support come > along naturally once TDX support mmap() on guest_memfd.