Re: [PATCH 2/2] KVM: arm64: selftests: Disable single-step without relying on ucall()

[Sean Christopherson]

On Thu, Nov 17, 2022, Sean Christopherson wrote:
> On Thu, Nov 17, 2022, Oliver Upton wrote:
> > On Thu, Nov 17, 2022 at 12:23:50AM +0000, Sean Christopherson wrote:
> > > Automatically disable single-step when the guest reaches the end of the
> > > verified section instead of using an explicit ucall() to ask userspace to
> > > disable single-step.  An upcoming change to implement a pool-based scheme
> > > for ucall() will add an atomic operation (bit test and set) in the guest
> > > ucall code, and if the compiler generate "old school" atomics, e.g.
> > 
> > Off topic, but I didn't ask when we were discussing this issue. What is
> > the atomic used for in the pool-based ucall implementation?
> 
> To avoid having to plumb an "id" into the guest, vCPUs grab a ucall entry from
> the pool on a first-come first-serve basis, and then release the entry when the
> ucall is complete.  The current implementation is a bitmap, e.g. every possible
> entry has a bit in the map, and vCPUs do an atomic bit-test-and-set to claim an
> entry.
> 
> Ugh.  And there's a bug.  Of course I notice it after sending the pull request.
> Depsite being defined in atomic.h, and despite clear_bit() being atomic in the
> kernel, tools' clear_bit() isn't actually atomic.  Grr.
> 
> Doesn't cause problems because there are so few multi-vCPU selftests, but that
> needs to be fixed.  Best thing would be to fix clear_bit() itself.

Ha!  And I bet when clear_bit() is fixed, this test will start failing again
because the ucall() to activate single-step needs to release the entry _after_
exiting to the host, i.e. single-step will be enabled across the atomic region
again.