On Tue, 2022-04-19 at 15:45 +0000, Sean Christopherson wrote:
> 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 <sea...@google.com>
> > > ---
>
> ...
>
> > 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.
Makes sense. I still recal *that* bug in AVIC inhibition where that srcu lock
was
a major PITA, but now I remember that it was not due to nesting of the lock,
but rather fact that we attempted to call syncronize_srcu or something like that
with it held.
>
> 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.
>
It all makes sense now - thanks.
Best regards,
Maxim Levistky
