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From: Oliver Upton To: Peter Xu Cc: Gavin Shan , kvmarm@lists.linux.dev, kvmarm@lists.cs.columbia.edu, kvm@vger.kernel.org, catalin.marinas@arm.com, bgardon@google.com, shuah@kernel.org, andrew.jones@linux.dev, will@kernel.org, dmatlack@google.com, pbonzini@redhat.com, zhenyzha@redhat.com, james.morse@arm.com, suzuki.poulose@arm.com, alexandru.elisei@arm.com, seanjc@google.com, shan.gavin@gmail.com Subject: Re: [PATCH v5 3/7] KVM: x86: Allow to use bitmap in ring-based dirty page tracking Message-ID: References: <20221005004154.83502-1-gshan@redhat.com> <20221005004154.83502-4-gshan@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-Migadu-Flow: FLOW_OUT Precedence: bulk List-ID: X-Mailing-List: kvm@vger.kernel.org On Mon, Oct 10, 2022 at 07:49:15PM -0400, Peter Xu wrote: > On Mon, Oct 10, 2022 at 11:18:55PM +0000, Oliver Upton wrote: > > On Fri, Oct 07, 2022 at 10:31:49AM -0400, Peter Xu wrote: > > > > [...] > > > > > > - In kvm_vm_ioctl_enable_dirty_log_ring(), set 'dirty_ring_allow_bitmap' to > > > > true when the capability is KVM_CAP_DIRTY_LONG_RING_ACQ_REL > > > > > > What I wanted to do is to decouple the ACQ_REL with ALLOW_BITMAP, so mostly > > > as what you suggested, except.. > > > > +1 > > > > > > > > > > static int kvm_vm_ioctl_enable_dirty_log_ring(struct kvm *kvm, u32 cap, u32 size) > > > > { > > > > : > > > > mutex_lock(&kvm->lock); > > > > > > > > if (kvm->created_vcpus) { > > > > /* We don't allow to change this value after vcpu created */ > > > > r = -EINVAL; > > > > } else { > > > > kvm->dirty_ring_size = size; > > > > > > .. here I'd not set dirty_ring_allow_bitmap at all so I'd drop below line, > > > instead.. > > > > > > > kvm->dirty_ring_allow_bitmap = (cap == KVM_CAP_DIRTY_LOG_RING_ACQ_REL); > > > > r = 0; > > > > } > > > > > > > > mutex_unlock(&kvm->lock); > > > > return r; > > > > } > > > > - In kvm_vm_ioctl_check_extension_generic(), KVM_CAP_DIRTY_LOG_RING_ALLOW_BITMAP > > > > is always flase until KVM_CAP_DIRTY_LOG_RING_ACQ_REL is enabled. > > > > > > > > static long kvm_vm_ioctl_check_extension_generic(...) > > > > { > > > > : > > > > case KVM_CAP_DIRTY_LOG_RING_ALLOW_BITMAP: > > > > return kvm->dirty_ring_allow_bitmap ? 1 : 0; > > > > > > ... here we always return 1, OTOH in kvm_vm_ioctl_enable_cap_generic(): > > > > > > case KVM_CAP_DIRTY_LOG_RING_ALLOW_BITMAP: > > > if (kvm->dirty_ring_size) > > > return -EINVAL; > > > kvm->dirty_ring_allow_bitmap = true; > > > return 0; > > > > > > A side effect of checking dirty_ring_size is then we'll be sure to have no > > > vcpu created too. Maybe we should also check no memslot created to make > > > sure the bitmaps are not created. > > > > I'm not sure I follow... What prevents userspace from creating a vCPU > > between enabling the two caps? > > Enabling of dirty ring requires no vcpu created, so as to make sure all the > vcpus will have the ring structures allocated as long as ring enabled for > the vm. Done in kvm_vm_ioctl_enable_dirty_log_ring(): > > if (kvm->created_vcpus) { > /* We don't allow to change this value after vcpu created */ > r = -EINVAL; > } else { > kvm->dirty_ring_size = size; > r = 0; > } > > Then if we have KVM_CAP_DIRTY_LOG_RING_ALLOW_BITMAP checking > dirty_ring_size first then we make sure we need to configure both > ALLOW_BITMAP and DIRTY_RING before any vcpu creation. Ah, right. Sorry, I had the 'if' condition inverted in my head. > > > > > Then if the userspace wants to use the bitmap altogether with the ring, it > > > needs to first detect KVM_CAP_DIRTY_LOG_RING_ALLOW_BITMAP and enable it > > > before it enables KVM_CAP_DIRTY_LOG_RING. > > > > > > One trick on ALLOW_BITMAP is in mark_page_dirty_in_slot() - after we allow > > > !vcpu case we'll need to make sure it won't accidentally try to set bitmap > > > for !ALLOW_BITMAP, because in that case the bitmap pointer is NULL so > > > set_bit_le() will directly crash the kernel. > > > > > > We could keep the old flavor of having a WARN_ON_ONCE(!vcpu && > > > !ALLOW_BITMAP) then return, but since now the userspace can easily trigger > > > this (e.g. on ARM, a malicious userapp can have DIRTY_RING && > > > !ALLOW_BITMAP, then it can simply trigger the gic ioctl to trigger host > > > warning), I think the better approach is we can kill the process in that > > > case. Not sure whether there's anything better we can do. > > > > I don't believe !ALLOW_BITMAP && DIRTY_RING is a valid configuration for > > arm64 given the fact that we'll dirty memory outside of a vCPU context. > > Yes it's not, but after Gavin's current series it'll be possible, IOW a > malicious app can leverage this to trigger host warning, which is IMHO not > wanted. > > > > > Could ALLOW_BITMAP be a requirement of DIRTY_RING, thereby making > > userspace fail fast? Otherwise (at least on arm64) your VM is DOA on the > > target. With that the old WARN() could be preserved, as you suggest. > > It's just that x86 doesn't need the bitmap, so it'll be a pure waste there > otherwise. It's not only about the memory that will be wasted (that's > guest mem size / 32k), but also the sync() process for x86 will be all > zeros and totally meaningless - note that the sync() of bitmap will be part > of VM downtime in this case (we need to sync() after turning VM off), so it > will make x86 downtime larger but without any benefit. Ah, my follow-up [1] missed by just a few minutes :) I think this further drives the point home -- there's zero need for the bitmap with dirty ring on x86, so why even support it? The proposal of ALLOW_BITMAP && DIRTY_RING should be arm64-specific. Any other arch that needs to dirty memory outside of a vCPU context can opt-in to the behavior. [1]: https://lore.kernel.org/kvm/Y0SuJee3oWL2QCqM@google.com/ -- Thanks, Oliver