On Tue, Apr 19, 2022, Maxim Levitsky wrote: > On Fri, 2022-04-15 at 00:43 +0000, Sean Christopherson wrote: > > Add wrappers to acquire/release KVM's SRCU lock when stashing the index > > in vcpu->src_idx, along with rudimentary detection of illegal usage, > > e.g. re-acquiring SRCU and thus overwriting vcpu->src_idx. Because the > > SRCU index is (currently) either 0 or 1, illegal nesting bugs can go > > unnoticed for quite some time and only cause problems when the nested > > lock happens to get a different index. > > > > Wrap the WARNs in PROVE_RCU=y, and make them ONCE, otherwise KVM will > > likely yell so loudly that it will bring the kernel to its knees. > > > > Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx> > > --- ... > Looks good to me overall. > > Note that there are still places that acquire the lock and store the idx into > a local variable, for example kvm_xen_vcpu_set_attr and such. > I didn't check yet if these should be converted as well. Using a local variable is ok, even desirable. Nested/multiple readers is not an issue, the bug fixed by patch 1 is purely that kvm_vcpu.srcu_idx gets corrupted. In an ideal world, KVM would _only_ track the SRCU index in local variables, but that would require plumbing the local variable down into vcpu_enter_guest() and kvm_vcpu_block() so that SRCU can be unlocked prior to entering the guest or scheduling out the vCPU.
