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charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 8/5/24 18:26, Sean Christopherson wrote: > On Sun, Aug 04, 2024, Michal Luczaj wrote: >> On 8/2/24 22:29, Sean Christopherson wrote: >>> [...] >>> Making the x2APIC ID fully readonly fixes a WARN in KVM's optimized map >>> calculation, which expects the LDR to align with the x2APIC ID. >>> >>> WARNING: CPU: 2 PID: 958 at arch/x86/kvm/lapic.c:331 kvm_recalculate_apic_map+0x609/0xa00 [kvm] >>> CPU: 2 PID: 958 Comm: recalc_apic_map Not tainted 6.4.0-rc3-vanilla+ #35 >>> Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS Arch Linux 1.16.2-1-1 04/01/2014 >>> RIP: 0010:kvm_recalculate_apic_map+0x609/0xa00 [kvm] >>> Call Trace: >>> >>> kvm_apic_set_state+0x1cf/0x5b0 [kvm] >>> kvm_arch_vcpu_ioctl+0x1806/0x2100 [kvm] >>> kvm_vcpu_ioctl+0x663/0x8a0 [kvm] >>> __x64_sys_ioctl+0xb8/0xf0 >>> do_syscall_64+0x56/0x80 >>> entry_SYSCALL_64_after_hwframe+0x46/0xb0 >>> RIP: 0033:0x7fade8b9dd6f >> >> Isn't this WARN_ON_ONCE() inherently racy, though? With your patch applied, >> it can still be hit by juggling the APIC modes. > > Doh, right, the logic is unfortunately cross-vCPU. The sanity check could be > conditioned on the APIC belonging to the running/loaded vCPU, but I'm leaning > towards deleting it entirely. Though it did detect the KVM_SET_LAPIC backdoor... Yeah, let's drop it since we do have a test (in userspace). Paolo