From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 74A543C5DD0; Mon, 23 Mar 2026 18:05:44 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1774289144; cv=none; b=DqxRTxMNHrXcg/RP3qR/72CLm1HehCrqNuGbIp4nNAgFP7KOQqogDg7cz1v46eRh0ormcX4DGfP8eb+SuMo0Jf3AiSMxPgAboXrM29XNqvCrcLJYiEiFueyxGuoAJRQJubymoJOFLx7We1vF0hXb+GosY3vnlJ3yMmFV+YN4vNs= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1774289144; c=relaxed/simple; bh=4YF94x65bzJ3rKIUDmxtedMV/tAeS/S9+gKVJXnd5qE=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=I52MORLj3EwzHSMcrnuB5449kHisZ+EuGP/AklZ2D6hFOj1r7YYURehZx90IPWt39QsTMrF4eRaR+mksfbb7oVMNxiRQdJVx7xQa7wJDS3DXA7Y6Ng3eawG3O4XXpS+L4XCxFGKLrc2DgwlzU4Wey0qPXhqF8O0X+EPChkBcFas= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=AHKvItoJ; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="AHKvItoJ" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 004F1C2BC9E; Mon, 23 Mar 2026 18:05:12 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1774289144; bh=4YF94x65bzJ3rKIUDmxtedMV/tAeS/S9+gKVJXnd5qE=; h=Date:Subject:To:Cc:References:From:In-Reply-To:From; b=AHKvItoJGXAujQc/G8P6Gc8rqqw2ndF2rJFpzDL1wCRIM4zePKLLUYmkzEcl4rV5x C9dQIrOKubc/M7z8daRTOn08rZp1JjjdrEF5HWZM/Eq5XE3bvu/GyUJruGyRc1Wh41 xeXWPV/3ivjsJOz9LrxSGSDT3g9tz/IE3u7wW5ThW3KCHSrdwbWdUqenmm4LlO7Odb vswG3r2NfoxosJhfVYTmOKg3LA6fkMLbIqPCelZHfI4QqFYbcVlvKcbd7kQQ6YMVUo ytodebTDvx9Zcc/e3Zptz0rtqxGcsPJnMCmMFBTcBJaaF8I52M8NLfU80excBckG1C I1GpEc5mzI6rw== Message-ID: <50bfaeb5-551e-403f-bd00-a7d8b6bbf6e2@kernel.org> Date: Mon, 23 Mar 2026 19:05:11 +0100 Precedence: bulk X-Mailing-List: linux-doc@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v11 10/16] KVM: guest_memfd: Add flag to remove from direct map To: "Kalyazin, Nikita" , "kvm@vger.kernel.org" , "linux-doc@vger.kernel.org" , "linux-kernel@vger.kernel.org" , "linux-arm-kernel@lists.infradead.org" , "kvmarm@lists.linux.dev" , "linux-fsdevel@vger.kernel.org" , "linux-mm@kvack.org" , "bpf@vger.kernel.org" , "linux-kselftest@vger.kernel.org" , "kernel@xen0n.name" , "linux-riscv@lists.infradead.org" , "linux-s390@vger.kernel.org" , "loongarch@lists.linux.dev" , "linux-pm@vger.kernel.org" Cc: "pbonzini@redhat.com" , "corbet@lwn.net" , "maz@kernel.org" , "oupton@kernel.org" , "joey.gouly@arm.com" , "suzuki.poulose@arm.com" , "yuzenghui@huawei.com" , "catalin.marinas@arm.com" , "will@kernel.org" , "seanjc@google.com" , "tglx@kernel.org" , "mingo@redhat.com" , "bp@alien8.de" , "dave.hansen@linux.intel.com" , "x86@kernel.org" , "hpa@zytor.com" , "luto@kernel.org" , "peterz@infradead.org" , "willy@infradead.org" , "akpm@linux-foundation.org" , "lorenzo.stoakes@oracle.com" , "vbabka@kernel.org" , "rppt@kernel.org" , "surenb@google.com" , "mhocko@suse.com" , "ast@kernel.org" , "daniel@iogearbox.net" , "andrii@kernel.org" , "martin.lau@linux.dev" , "eddyz87@gmail.com" , "song@kernel.org" , "yonghong.song@linux.dev" , "john.fastabend@gmail.com" , "kpsingh@kernel.org" , "sdf@fomichev.me" , "haoluo@google.com" , "jolsa@kernel.org" , "jgg@ziepe.ca" , "jhubbard@nvidia.com" , "peterx@redhat.com" , "jannh@google.com" , "pfalcato@suse.de" , "skhan@linuxfoundation.org" , "riel@surriel.com" , "ryan.roberts@arm.com" , "jgross@suse.com" , "yu-cheng.yu@intel.com" , "kas@kernel.org" , "coxu@redhat.com" , "kevin.brodsky@arm.com" , "ackerleytng@google.com" , "yosry@kernel.org" , "ajones@ventanamicro.com" , "maobibo@loongson.cn" , "tabba@google.com" , "prsampat@amd.com" , "wu.fei9@sanechips.com.cn" , "mlevitsk@redhat.com" , "jmattson@google.com" , "jthoughton@google.com" , "agordeev@linux.ibm.com" , "alex@ghiti.fr" , "aou@eecs.berkeley.edu" , "borntraeger@linux.ibm.com" , "chenhuacai@kernel.org" , "dev.jain@arm.com" , "gor@linux.ibm.com" , "hca@linux.ibm.com" , "palmer@dabbelt.com" , "pjw@kernel.org" , "shijie@os.amperecomputing.com" , "svens@linux.ibm.com" , "thuth@redhat.com" , "wyihan@google.com" , "yang@os.amperecomputing.com" , "Jonathan.Cameron@huawei.com" , "Liam.Howlett@oracle.com" , "urezki@gmail.com" , "zhengqi.arch@bytedance.com" , "gerald.schaefer@linux.ibm.com" , "jiayuan.chen@shopee.com" , "lenb@kernel.org" , "osalvador@suse.de" , "pavel@kernel.org" , "rafael@kernel.org" , "vannapurve@google.com" , "jackmanb@google.com" , "aneesh.kumar@kernel.org" , "patrick.roy@linux.dev" , "Thomson, Jack" , "Itazuri, Takahiro" , "Manwaring, Derek" References: <20260317141031.514-1-kalyazin@amazon.com> <20260317141031.514-11-kalyazin@amazon.com> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 7bit On 3/17/26 15:12, Kalyazin, Nikita wrote: > From: Patrick Roy > > Add GUEST_MEMFD_FLAG_NO_DIRECT_MAP flag for KVM_CREATE_GUEST_MEMFD() > ioctl. When set, guest_memfd folios will be removed from the direct map > after preparation, with direct map entries only restored when the folios > are freed. > > To ensure these folios do not end up in places where the kernel cannot > deal with them, set AS_NO_DIRECT_MAP on the guest_memfd's struct > address_space if GUEST_MEMFD_FLAG_NO_DIRECT_MAP is requested. > > Note that this flag causes removal of direct map entries for all > guest_memfd folios independent of whether they are "shared" or "private" > (although current guest_memfd only supports either all folios in the > "shared" state, or all folios in the "private" state if > GUEST_MEMFD_FLAG_MMAP is not set). The usecase for removing direct map > entries of also the shared parts of guest_memfd are a special type of > non-CoCo VM where, host userspace is trusted to have access to all of > guest memory, but where Spectre-style transient execution attacks > through the host kernel's direct map should still be mitigated. In this > setup, KVM retains access to guest memory via userspace mappings of > guest_memfd, which are reflected back into KVM's memslots via > userspace_addr. This is needed for things like MMIO emulation on x86_64 > to work. > > Direct map entries are zapped right before guest or userspace mappings > of gmem folios are set up, e.g. in kvm_gmem_fault_user_mapping() or > kvm_gmem_get_pfn() [called from the KVM MMU code]. The only place where > a gmem folio can be allocated without being mapped anywhere is > kvm_gmem_populate(), where handling potential failures of direct map > removal is not possible (by the time direct map removal is attempted, > the folio is already marked as prepared, meaning attempting to re-try > kvm_gmem_populate() would just result in -EEXIST without fixing up the > direct map state). These folios are then removed form the direct map > upon kvm_gmem_get_pfn(), e.g. when they are mapped into the guest later. > > Signed-off-by: Patrick Roy I you changed this patch significantly, you should likely add a Co-developed-by: Nikita Kalyazin above your sob. (applies to other patches as well, please double check) > Signed-off-by: Nikita Kalyazin > --- > Documentation/virt/kvm/api.rst | 21 ++++++----- > include/linux/kvm_host.h | 3 ++ > include/uapi/linux/kvm.h | 1 + > virt/kvm/guest_memfd.c | 67 ++++++++++++++++++++++++++++++++-- > 4 files changed, 79 insertions(+), 13 deletions(-) > > diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst > index 032516783e96..8feec77b03fe 100644 > --- a/Documentation/virt/kvm/api.rst > +++ b/Documentation/virt/kvm/api.rst > @@ -6439,15 +6439,18 @@ a single guest_memfd file, but the bound ranges must not overlap). > The capability KVM_CAP_GUEST_MEMFD_FLAGS enumerates the `flags` that can be > specified via KVM_CREATE_GUEST_MEMFD. Currently defined flags: > > - ============================ ================================================ > - GUEST_MEMFD_FLAG_MMAP Enable using mmap() on the guest_memfd file > - descriptor. > - GUEST_MEMFD_FLAG_INIT_SHARED Make all memory in the file shared during > - KVM_CREATE_GUEST_MEMFD (memory files created > - without INIT_SHARED will be marked private). > - Shared memory can be faulted into host userspace > - page tables. Private memory cannot. > - ============================ ================================================ > + ============================== ================================================ > + GUEST_MEMFD_FLAG_MMAP Enable using mmap() on the guest_memfd file > + descriptor. > + GUEST_MEMFD_FLAG_INIT_SHARED Make all memory in the file shared during > + KVM_CREATE_GUEST_MEMFD (memory files created > + without INIT_SHARED will be marked private). > + Shared memory can be faulted into host userspace > + page tables. Private memory cannot. > + GUEST_MEMFD_FLAG_NO_DIRECT_MAP The guest_memfd instance will unmap the memory > + backing it from the kernel's address space > + before passing it off to userspace or the guest. > + ============================== ================================================ > > When the KVM MMU performs a PFN lookup to service a guest fault and the backing > guest_memfd has the GUEST_MEMFD_FLAG_MMAP set, then the fault will always be > diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h > index ce8c5fdf2752..c95747e2278c 100644 > --- a/include/linux/kvm_host.h > +++ b/include/linux/kvm_host.h > @@ -738,6 +738,9 @@ static inline u64 kvm_gmem_get_supported_flags(struct kvm *kvm) > if (!kvm || kvm_arch_supports_gmem_init_shared(kvm)) > flags |= GUEST_MEMFD_FLAG_INIT_SHARED; > > + if (!kvm || kvm_arch_gmem_supports_no_direct_map(kvm)) > + flags |= GUEST_MEMFD_FLAG_NO_DIRECT_MAP; > + > return flags; > } > #endif > diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h > index 80364d4dbebb..d864f67efdb7 100644 > --- a/include/uapi/linux/kvm.h > +++ b/include/uapi/linux/kvm.h > @@ -1642,6 +1642,7 @@ struct kvm_memory_attributes { > #define KVM_CREATE_GUEST_MEMFD _IOWR(KVMIO, 0xd4, struct kvm_create_guest_memfd) > #define GUEST_MEMFD_FLAG_MMAP (1ULL << 0) > #define GUEST_MEMFD_FLAG_INIT_SHARED (1ULL << 1) > +#define GUEST_MEMFD_FLAG_NO_DIRECT_MAP (1ULL << 2) > > struct kvm_create_guest_memfd { > __u64 size; > diff --git a/virt/kvm/guest_memfd.c b/virt/kvm/guest_memfd.c > index 651649623448..c9344647579c 100644 > --- a/virt/kvm/guest_memfd.c > +++ b/virt/kvm/guest_memfd.c > @@ -7,6 +7,7 @@ > #include > #include > #include > +#include > > #include "kvm_mm.h" > > @@ -76,6 +77,35 @@ static int __kvm_gmem_prepare_folio(struct kvm *kvm, struct kvm_memory_slot *slo > return 0; > } > > +#define KVM_GMEM_FOLIO_NO_DIRECT_MAP BIT(0) > + > +static bool kvm_gmem_folio_no_direct_map(struct folio *folio) > +{ > + return ((u64)folio->private) & KVM_GMEM_FOLIO_NO_DIRECT_MAP; > +} > + > +static int kvm_gmem_folio_zap_direct_map(struct folio *folio) > +{ > + u64 gmem_flags = GMEM_I(folio_inode(folio))->flags; > + int r = 0; > + > + if (kvm_gmem_folio_no_direct_map(folio) || !(gmem_flags & GUEST_MEMFD_FLAG_NO_DIRECT_MAP)) The function is only called when kvm_gmem_no_direct_map(folio_inode(folio)) Does it really make sense to check for GUEST_MEMFD_FLAG_NO_DIRECT_MAP again? If, at all, it should be a warning if GUEST_MEMFD_FLAG_NO_DIRECT_MAP is not set? Further, kvm_gmem_folio_zap_direct_map() uses the folio lock to synchronize, right? Might be worth pointing that out somehow (e.g., lockdep check if possible). > + goto out; > + > + r = folio_zap_direct_map(folio); > + if (!r) > + folio->private = (void *)((u64)folio->private | KVM_GMEM_FOLIO_NO_DIRECT_MAP); > + > +out: > + return r; > +} > + > +static void kvm_gmem_folio_restore_direct_map(struct folio *folio) > +{ kvm_gmem_folio_zap_direct_map() is allowed to be called on folios that already have the directmap remove, kvm_gmem_folio_restore_direct_map() cannot be called if the directmap was already restored. Should we make that more consistent? Hoping Sean can find some time to review -- Cheers, David