From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from bombadil.infradead.org (bombadil.infradead.org [198.137.202.133]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature ECDSA (prime256v1) server-digest SHA256) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id C1B63606A88D for ; Thu, 12 Nov 2026 10:43:28 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=lists.infradead.org header.i=@lists.infradead.org header.a=rsa-sha256 header.s=bombadil.20210309 header.b=VHLFsMAx; dkim=pass (1024-bit key) header.d=arm.com header.i=@arm.com header.a=rsa-sha256 header.s=foss header.b=sz/bRcPX DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=lists.infradead.org; s=bombadil.20210309; h=Sender:List-Subscribe:List-Help :List-Post:List-Archive:List-Unsubscribe:List-Id:Content-Transfer-Encoding: Content-Type:In-Reply-To:From:References:Cc:To:Subject:MIME-Version:Date: Message-ID:Reply-To:Content-ID:Content-Description:Resent-Date:Resent-From: Resent-Sender:Resent-To:Resent-Cc:Resent-Message-ID:List-Unsubscribe-Post: List-Owner; bh=X2j4x6ETnjsd3Kk7UiXLGfpl+QRCIcV4hL4S2HQyUOs=; b=VHLFsMAxsoL07S PqiomIVRmjpzAjGi7BHJT/IlXhpjlWhw0hAraEQM6+z/l49HfICTSxVc/i1TDlIrOEo1ZqW5lL/Yh UU4M+H92tEBRpKcDAAXW8tcKXZTGPOK8FrPZ4AFjSow0r78MDN2FkKu14y1//MEnQlUWzpGDxOXsB ROF0pDKwBhhqfYF77G66PKZCfSneDecCyCQjiSEAPaZLtXVSXkrXhdLIraGmJr3khW25k1BXOdtPE L01AuFmP7b2vdHXnpGyrIqWz3bGHOWdHMlvvrKgBVL33lbHznfiDkGvfaDJi8yR5vXWqdgpBWr6k5 I9ZcKW2w4WJdRSKajAvA==; Received: from localhost ([::1] helo=bombadil.infradead.org) by bombadil.infradead.org with esmtp (Exim 4.100.1 #2 (Red Hat Linux)) id 1xRSH4-00000009Z3z-21w3; Thu, 12 Nov 2026 10:43:22 +0000 Received: from foss.arm.com ([217.140.110.172]) by bombadil.infradead.org with esmtp (Exim 4.100.1 #2 (Red Hat Linux)) id 1xRSH2-00000009Z2C-2Zsn for linux-arm-kernel@lists.infradead.org; Thu, 12 Nov 2026 10:43:21 +0000 Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id 9CC8C153B; Thu, 12 Nov 2026 02:43:14 -0800 (PST) Received: from [10.0.129.47] (e122027.cambridge.arm.com [10.0.129.47]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 76C8C3F673; Thu, 12 Nov 2026 02:43:14 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=simple/simple; d=arm.com; s=foss; t=1794480197; bh=xDR73f4nxpZk5DSaOewf66eooDmTQ5dy0pwmOSiLIuw=; h=Date:Subject:To:Cc:References:From:In-Reply-To:From; b=sz/bRcPXFEyNa7belfSW5kTpVnMxp65VhQF1lieCCf0rIAi0oiwk3xn+JZFcig9CD 1gcmn08lsRczfyJ6K8ZbBvOpIDzJ9EnXmgUZqfKoqEarzelQvs12nZW7bi6qrs3iCC KiUIJYqG1GFIcWLl7RHP5oXDUOBXgOfs70RBQC7E= Message-ID: <1e02dfff-c1ad-46bb-9834-fd4bbaa46179@arm.com> Date: Thu, 12 Nov 2026 10:43:13 +0000 X-Archived-By: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v2 02/12] KVM: arm64: guest_memfd: Check mmap support when preparing memslot To: Suzuki K Poulose , kvm@vger.kernel.org, kvmarm@lists.linux.dev Cc: Catalin Marinas , Marc Zyngier , Will Deacon , James Morse , Oliver Upton , Zenghui Yu , linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, Joey Gouly , Alexandru Elisei , Christoffer Dall , Fuad Tabba , linux-coco@lists.linux.dev, Ganapatrao Kulkarni , Gavin Shan , Shanker Donthineni , Alper Gun , "Aneesh Kumar K . V" , Emi Kisanuki , Vishal Annapurve , WeiLin.Chang@arm.com, Lorenzo Pieralisi , Kohei Enju References: <20261111145758.217946-1-steven.price@arm.com> <20261111145758.217946-3-steven.price@arm.com> From: Steven Price Content-Language: en-GB In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.9.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20261112_024320_734390_83E45EF1 X-CRM114-Status: GOOD ( 23.47 ) X-Spam-Score: -4.4 (----) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: List-Archive: List-Post: List-Help: List-Subscribe: , Sender: "linux-arm-kernel" X-Spam-Status: No, score=-17.4 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,DKIM_VALID_EF, HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_CR_TRAILER,INCLUDES_PATCH, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 autolearn=ham autolearn_force=no version=4.0.1 X-Spam-Checker-Version: SpamAssassin 4.0.1 (2024-03-26) on aws-us-west-2-korg-listserv-alma10-1 On 11/11/2026 15:21, Suzuki K Poulose wrote: > On 11/11/2026 14:57, Steven Price wrote: >> Check the guest_memfd backing file when deciding whether an arm64 >> memslot has mappable memory rather than checking whether the slot is >> gmem only. A public helper function is added to be able to reuse the >> static kvm_gmem_supports_mmap() function. >> > > Please could you explain why we need this ? Why not rely on > kvm_memslot_is_gmem_only(), which already tells > kvm_gmem_supports_mmap() ? With inplace conversion support gmem supports > mmap. Unless I'm missing something it's still possible to create a guest_memfd that doesn't support mmap() by not passing the GUEST_MEMFD_FLAG_MMAP flag. So kvm_memslot_is_gmem_only() doesn't actually guarantee that it's a guest_memfd that supports mmap(). > >> Signed-off-by: Steven Price >> >> --- >> Changes since v1: >> - New patch in v2. Check the guest_memfd backing file rather than the >>    memslot flag when deciding whether arm64 can map the backing memory. >> --- >>   arch/arm64/kvm/mmu.c     | 7 ++++--- >>   include/linux/kvm_host.h | 1 + >>   virt/kvm/guest_memfd.c   | 6 ++++++ >>   3 files changed, 11 insertions(+), 3 deletions(-) >> >> diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c >> index 1db10d8d6db7..a146f6cde3d0 100644 >> --- a/arch/arm64/kvm/mmu.c >> +++ b/arch/arm64/kvm/mmu.c >> @@ -2839,10 +2839,11 @@ int kvm_arch_prepare_memory_region(struct kvm >> *kvm, >>           return -EFAULT; >>         /* >> -     * Only support guest_memfd backed memslots with mappable memory, >> since >> -     * there aren't any CoCo VMs that support only private memory on >> arm64. >> +     * Protected VMs currently require guest_memfd backed memslots with >> +     * mappable memory to support shared mappings. >>        */ >> -    if (kvm_slot_has_gmem(new) && !kvm_memslot_is_gmem_only(new)) >> +    if (kvm_arch_has_private_mem(kvm) && kvm_slot_has_gmem(new) && >> +        !kvm_gmem_slot_supports_mmap(new)) Sashiko has identified I've screwed up this condition though. kvm_arch_has_private_mem(kvm) shouldn't be in there as that allows a non-private VM to bind a non-mmappable gmem. >>           return -EINVAL; >>         /* guest_memfd is incompatible with MTE. */ >> diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h >> index e7fc94e2910a..f65728a713ec 100644 >> --- a/include/linux/kvm_host.h >> +++ b/include/linux/kvm_host.h >> @@ -2614,6 +2614,7 @@ static inline bool kvm_is_private_gfn(struct kvm >> *kvm, gfn_t gfn) >>     #ifdef CONFIG_KVM_GUEST_MEMFD >>   bool kvm_gmem_is_private_gfn(struct kvm *kvm, gfn_t gfn); >> +bool kvm_gmem_slot_supports_mmap(const struct kvm_memory_slot *slot); >>   bool kvm_arch_supports_gmem_init_shared(struct kvm *kvm); >>     static inline u64 kvm_gmem_get_supported_flags(struct kvm *kvm) >> diff --git a/virt/kvm/guest_memfd.c b/virt/kvm/guest_memfd.c >> index 8c97a69b16dc..3dd8d227bebb 100644 >> --- a/virt/kvm/guest_memfd.c >> +++ b/virt/kvm/guest_memfd.c >> @@ -442,6 +442,12 @@ static bool kvm_gmem_supports_mmap(struct inode >> *inode) >>       return GMEM_I(inode)->flags & GUEST_MEMFD_FLAG_MMAP; >>   } >>   +bool kvm_gmem_slot_supports_mmap(const struct kvm_memory_slot *slot) >> +{ >> +    /* The guest_memfd file is pinned while preparing a new memslot. */ > > We should gate this on kvm_slot_has_gmem() and how do we ensure that the > call comes after it is prepared ? I'm not sure what you mean by "gate this on kvm_slot_has_gmem()" - yes this function only makes sense on a gmem slot - the user above checks that first. I'm not sure what value the function should call if you call it on a non-gmem slot? Thanks, Steve >> +    return kvm_gmem_supports_mmap(file_inode(slot->gmem.file)); >> +} >> + >>   static vm_fault_t kvm_gmem_fault_user_mapping(struct vm_fault *vmf) >>   { >>       struct inode *inode = file_inode(vmf->vma->vm_file); > > > Cheers > Suzuki