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X-CSE-ConnectionGUID: 3GuGIKWoRI26U5rN+Q8+Ew== X-CSE-MsgGUID: tuMliPqVSLmc4z3hG6PwXA== X-IronPort-AV: E=McAfee;i="6800,10657,11493"; a="80329688" X-IronPort-AV: E=Sophos;i="6.16,315,1744095600"; d="scan'208";a="80329688" Received: from orviesa002.jf.intel.com ([10.64.159.142]) by fmvoesa101.fm.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 15 Jul 2025 23:11:25 -0700 X-CSE-ConnectionGUID: jZPozQdBTsOjGre4uRsADw== X-CSE-MsgGUID: jcMR3HMmTvCVSp7FSE7gIQ== X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="6.16,315,1744095600"; d="scan'208";a="188386724" Received: from xiaoyaol-hp-g830.ccr.corp.intel.com (HELO [10.124.247.1]) ([10.124.247.1]) by orviesa002-auth.jf.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 15 Jul 2025 23:10:59 -0700 Message-ID: Date: Wed, 16 Jul 2025 14:10:56 +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 v14 09/21] KVM: guest_memfd: Track guest_memfd mmap support in memslot To: Fuad Tabba , kvm@vger.kernel.org, linux-arm-msm@vger.kernel.org, linux-mm@kvack.org, kvmarm@lists.linux.dev Cc: 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, vannapurve@google.com, 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: <20250715093350.2584932-1-tabba@google.com> <20250715093350.2584932-10-tabba@google.com> Content-Language: en-US From: Xiaoyao Li In-Reply-To: <20250715093350.2584932-10-tabba@google.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 7/15/2025 5:33 PM, Fuad Tabba wrote: > Add a new internal flag, KVM_MEMSLOT_GMEM_ONLY, to the top half of > memslot->flags. This flag tracks when a guest_memfd-backed memory slot > supports host userspace mmap operations. It's strictly for KVM's > internal use. I would expect some clarification of why naming it with KVM_MEMSLOT_GMEM_ONLY, not something like KVM_MEMSLOT_GMEM_MMAP_ENABLED There was a patch to check the userspace_addr of the memslot refers to the same memory as guest memfd[1], but that patch was dropped. Without the background that when guest memfd is mmapable, userspace doesn't need to provide separate memory via userspace_addr, it's hard to understand and accept the name of GMEM_ONLY. [1] https://lore.kernel.org/all/20250513163438.3942405-9-tabba@google.com/ > This optimization avoids repeatedly checking the underlying guest_memfd > file for mmap support, which would otherwise require taking and > releasing a reference on the file for each check. By caching this > information directly in the memslot, we reduce overhead and simplify the > logic involved in handling guest_memfd-backed pages for host mappings. > > Reviewed-by: Gavin Shan > Reviewed-by: Shivank Garg > Acked-by: David Hildenbrand > Suggested-by: David Hildenbrand > Signed-off-by: Fuad Tabba > --- > include/linux/kvm_host.h | 11 ++++++++++- > virt/kvm/guest_memfd.c | 2 ++ > 2 files changed, 12 insertions(+), 1 deletion(-) > > diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h > index 9ac21985f3b5..d2218ec57ceb 100644 > --- a/include/linux/kvm_host.h > +++ b/include/linux/kvm_host.h > @@ -54,7 +54,8 @@ > * used in kvm, other bits are visible for userspace which are defined in > * include/uapi/linux/kvm.h. > */ > -#define KVM_MEMSLOT_INVALID (1UL << 16) > +#define KVM_MEMSLOT_INVALID (1UL << 16) > +#define KVM_MEMSLOT_GMEM_ONLY (1UL << 17) > > /* > * Bit 63 of the memslot generation number is an "update in-progress flag", > @@ -2536,6 +2537,14 @@ static inline void kvm_prepare_memory_fault_exit(struct kvm_vcpu *vcpu, > vcpu->run->memory_fault.flags |= KVM_MEMORY_EXIT_FLAG_PRIVATE; > } > > +static inline bool kvm_memslot_is_gmem_only(const struct kvm_memory_slot *slot) > +{ > + if (!IS_ENABLED(CONFIG_KVM_GMEM_SUPPORTS_MMAP)) > + return false; > + > + return slot->flags & KVM_MEMSLOT_GMEM_ONLY; > +} > + > #ifdef CONFIG_KVM_GENERIC_MEMORY_ATTRIBUTES > static inline unsigned long kvm_get_memory_attributes(struct kvm *kvm, gfn_t gfn) > { > diff --git a/virt/kvm/guest_memfd.c b/virt/kvm/guest_memfd.c > index 07a4b165471d..2b00f8796a15 100644 > --- a/virt/kvm/guest_memfd.c > +++ b/virt/kvm/guest_memfd.c > @@ -592,6 +592,8 @@ int kvm_gmem_bind(struct kvm *kvm, struct kvm_memory_slot *slot, > */ > WRITE_ONCE(slot->gmem.file, file); > slot->gmem.pgoff = start; > + if (kvm_gmem_supports_mmap(inode)) > + slot->flags |= KVM_MEMSLOT_GMEM_ONLY; > > xa_store_range(&gmem->bindings, start, end - 1, slot, GFP_KERNEL); > filemap_invalidate_unlock(inode->i_mapping);