Re: [PATCH 0/2] KVM: s390: Fix latent guest entry/exit bugs

[Mark Rutland]

On Tue, Jul 08, 2025 at 07:27:40PM +1000, Andrew Donnellan wrote:
> In [0], the guest_{enter,exit}_irqoff() helpers were deprecated, in favour
> of guest_timing_{enter,exit}_irqoff() and
> guest_context_{enter,exit}_irqoff(). This was to fix a number of latent
> guest entry/exit bugs, relating to the enabling of interrupts during an
> RCU extended quiescent state, instrumentation code, and correct handling
> of lockdep and tracing.
> 
> However, while arm64, mips, riscv and x86 have been migrated to the new
> helpers, s390 hasn't been. There was an initial attempt at [1] to do this,
> but that didn't work for reasons discussed at [2].
> 
> Since then, Claudio Imbrenda has reworked much of the interrupt handling.
> Moving interrupt handling into vcpu_post_run() avoids the issues in [2],
> so we can now move to the new helpers.

Nice!

> I've rebased Mark's patches from [1]. kvm-unit-tests, the kvm selftests,
> and IBM's internal test suites pass under debug_defconfig.

I took a quick look at this and Claudio's preparatory work, and this all
looks like what I was hoping for back in one of the replies to [2]:

  https://lore.kernel.org/all/YerRbhqvJ5nEcQYT@FVFF77S0Q05N/

I am not aware of any additional problems, and this all looks good to
me. Thanks for picking this up!

Mark.

> These patches do introduce some overhead - in my testing, a few of the
> tests in the kvm-unit-tests exittime test suite appear 6-11% slower, but
> some noticeable overhead may be unavoidable (we introduce a new function
> call and the irq entry/exit paths change a bit).
> 
> [0] https://lore.kernel.org/lkml/20220201132926.3301912-1-mark.rutland@xxxxxxx/
> [1] https://lore.kernel.org/all/20220119105854.3160683-7-mark.rutland@xxxxxxx/
> [2] https://lore.kernel.org/all/a4a26805-3a56-d264-0a7e-60bed1ada9f3@xxxxxxxxxxxxx/
> [3] https://lore.kernel.org/all/20241022120601.167009-1-imbrenda@xxxxxxxxxxxxx/
> 
> Mark Rutland (2):
>   entry: Add arch_in_rcu_eqs()
>   KVM: s390: Rework guest entry logic
> 
>  arch/s390/include/asm/entry-common.h | 10 ++++++
>  arch/s390/include/asm/kvm_host.h     |  3 ++
>  arch/s390/kvm/kvm-s390.c             | 51 +++++++++++++++++++++-------
>  arch/s390/kvm/vsie.c                 | 17 ++++------
>  include/linux/entry-common.h         | 16 +++++++++
>  kernel/entry/common.c                |  3 +-
>  6 files changed, 77 insertions(+), 23 deletions(-)
> 
> -- 
> 2.50.0