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X-CSE-ConnectionGUID: YrcLLfDbS+CenVt5wbGrzA== X-CSE-MsgGUID: MIx7IPmdQvea93mfGH04gA== X-IronPort-AV: E=McAfee;i="6800,10657,11499"; a="58985278" X-IronPort-AV: E=Sophos;i="6.16,329,1744095600"; d="scan'208";a="58985278" Received: from fmviesa010.fm.intel.com ([10.60.135.150]) by orvoesa107.jf.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 21 Jul 2025 07:42:16 -0700 X-CSE-ConnectionGUID: qL0fP/21RrO2BIIqh5xu1g== X-CSE-MsgGUID: xd+ebKGrR4KwX8aDa1+/mA== X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="6.16,329,1744095600"; d="scan'208";a="159582796" Received: from xiaoyaol-hp-g830.ccr.corp.intel.com (HELO [10.124.247.1]) ([10.124.247.1]) by fmviesa010-auth.fm.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 21 Jul 2025 07:42:02 -0700 Message-ID: <131a3b2f-616f-428d-a41c-d490b9ca1b88@intel.com> Date: Mon, 21 Jul 2025 22:42:00 +0800 Precedence: bulk X-Mailing-List: kvm@vger.kernel.org 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: 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, seanjc@google.com, 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 9:45 PM, 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? > > If kvm allows mappable guest_memfd files for TDX VMs without > conversion support, userspace will be able to use those for backing > 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. yes, this is the additional change required. > 2) KVM fetches shared faults through userspace page tables and not > guest_memfd directly. current KVM supports it already. And as I described above, current userspace can just mmap the gmem and pass the gotten addr to userspace_addr without passing guest_memfd, to force KVM to fetch through userspace page tables. So if we want KVM to fetch page from guest memfd directly, should we add something in KVM to enforce it? > I don't see value in trying to go out of way to support such a usecase. From my perspective, it's intuitive to think about this usecase when I tried to enable gmem mmap in QEMU. It seems a little strange that mmap'ed gmem can only serve as normal memory for non-CoCo VMs. If KVM mandates any gmem must be passed via kvm_userspace_memory_region2.guest_memfd, no matter it's mmap'ed or not, it would make more sense (to me) to not support such usecase, since there is only one guest_memfd field in struct kvm_userspace_memory_region2. At last, I just put it on the table, so people can be aware of it and make the call to support it or not. Either is OK to me.