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:
<TASK>
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
