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

[Sean Christopherson]

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.