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Tue, 06 May 2025 17:03:37 -0700 (PDT) Date: Tue, 6 May 2025 17:03:35 -0700 In-Reply-To: <20250109204929.1106563-4-jthoughton@google.com> Mime-Version: 1.0 References: <20250109204929.1106563-1-jthoughton@google.com> <20250109204929.1106563-4-jthoughton@google.com> Message-ID: Subject: Re: [PATCH v2 03/13] KVM: Allow late setting of KVM_MEM_USERFAULT on guest_memfd memslot From: Sean Christopherson To: James Houghton Cc: Paolo Bonzini , Jonathan Corbet , Marc Zyngier , Oliver Upton , Yan Zhao , Nikita Kalyazin , Anish Moorthy , Peter Gonda , Peter Xu , David Matlack , wei.w.wang@intel.com, kvm@vger.kernel.org, linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org, kvmarm@lists.linux.dev Content-Type: text/plain; charset="us-ascii" X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20250506_170350_197898_0FDE8DC2 X-CRM114-Status: GOOD ( 24.69 ) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org On Thu, Jan 09, 2025, James Houghton wrote: > Currently guest_memfd memslots can only be deleted. Slightly change the > logic to allow KVM_MR_FLAGS_ONLY changes when the only flag being > changed is KVM_MEM_USERFAULT. > > Signed-off-by: James Houghton > --- > virt/kvm/kvm_main.c | 15 +++++++++++---- > 1 file changed, 11 insertions(+), 4 deletions(-) > > diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c > index 4bceae6a6401..882c1f7b4aa8 100644 > --- a/virt/kvm/kvm_main.c > +++ b/virt/kvm/kvm_main.c > @@ -2015,9 +2015,6 @@ int __kvm_set_memory_region(struct kvm *kvm, > if ((kvm->nr_memslot_pages + npages) < kvm->nr_memslot_pages) > return -EINVAL; > } else { /* Modify an existing slot. */ > - /* Private memslots are immutable, they can only be deleted. */ > - if (mem->flags & KVM_MEM_GUEST_MEMFD) > - return -EINVAL; > if ((mem->userspace_addr != old->userspace_addr) || > (npages != old->npages) || > ((mem->flags ^ old->flags) & KVM_MEM_READONLY)) > @@ -2031,6 +2028,16 @@ int __kvm_set_memory_region(struct kvm *kvm, > return 0; > } > > + /* > + * Except for being able to set KVM_MEM_USERFAULT, private memslots are > + * immutable, they can only be deleted. > + */ > + if (mem->flags & KVM_MEM_GUEST_MEMFD && > + !(change == KVM_MR_CREATE || > + (change == KVM_MR_FLAGS_ONLY && > + (mem->flags ^ old->flags) == KVM_MEM_USERFAULT))) > + return -EINVAL; Oof. I don't even want to decipher this. Let's just drop the blankent immutable restriction, and simply say guest_memfd slots can't be MOVED. guest_memfd doesn't support RO memslots (and never will), and doesn't support dirty logging, so this is effectively dead code anyways. Then the diff is much nicer: @@ -2012,14 +2027,15 @@ static int kvm_set_memory_region(struct kvm *kvm, if ((kvm->nr_memslot_pages + npages) < kvm->nr_memslot_pages) return -EINVAL; } else { /* Modify an existing slot. */ - /* Private memslots are immutable, they can only be deleted. */ - if (mem->flags & KVM_MEM_GUEST_MEMFD) - return -EINVAL; if ((mem->userspace_addr != old->userspace_addr) || (npages != old->npages) || ((mem->flags ^ old->flags) & KVM_MEM_READONLY)) return -EINVAL; + /* Moving a guest_memfd memslot isn't supported. */ + if (base_gfn != old->base_gfn && mem->flags & KVM_MEM_GUEST_MEMFD) + return -EINVAL; + if (base_gfn != old->base_gfn) change = KVM_MR_MOVE; else if (mem->flags != old->flags) > + > if ((change == KVM_MR_CREATE || change == KVM_MR_MOVE) && > kvm_check_memslot_overlap(slots, id, base_gfn, base_gfn + npages)) > return -EEXIST; > @@ -2046,7 +2053,7 @@ int __kvm_set_memory_region(struct kvm *kvm, > new->npages = npages; > new->flags = mem->flags; > new->userspace_addr = mem->userspace_addr; > - if (mem->flags & KVM_MEM_GUEST_MEMFD) { > + if (mem->flags & KVM_MEM_GUEST_MEMFD && change == KVM_MR_CREATE) { > r = kvm_gmem_bind(kvm, new, mem->guest_memfd, mem->guest_memfd_offset); > if (r) > goto out; > -- > 2.47.1.613.gc27f4b7a9f-goog >