Re: [PATCH 8/8] accel: make all calls to qemu_wait_io_event look the same

["Philippe Mathieu-Daudé" <philmd@linaro.org>]

On 8/8/25 20:59, Paolo Bonzini wrote:
> There is no reason for some accelerators to use qemu_wait_io_event_common
> (which is specifically separated for round robin).  They can also check
> on the first pass through the loop as well directly, without setting
> cpu->exit_request for no particular reason.
> 
> There is also no need to use qatomic_set_mb() because the ordering of
> the communication between I/O and vCPU threads is always the same.
> In the I/O thread:
> 
> (a) store other memory locations that will be checked if cpu->exit_request
>      or cpu->interrupt_request is 1 (for example cpu->stop or cpu->work_list
>      for cpu->exit_request)
> 
> (b) cpu_exit(): store-release cpu->exit_request, or
> (b) cpu_interrupt(): store-release cpu->interrupt_request
> 
>>>> at this point, cpu->halt_cond is broadcast and the BQL released
> 
> (c) do the accelerator-specific kick (e.g. write icount_decr for TCG,
>      pthread_kill for KVM, etc.)
> 
> In the vCPU thread instead the opposite order is respected:
> 
> (c) the accelerator's execution loop exits thanks to the kick
> 
> (b) then the inner execution loop checks cpu->interrupt_request
>      and cpu->exit_request.  If needed cpu->interrupt_request is
>      converted into cpu->exit_request when work is needed outside
>      the execution loop.
> 
> (a) then the other memory locations are checked.  Some may need
>      to be read under the BQL, and the vCPU thread may also take
>      for the vCPU thread can sleep on cpu->halt_cond; but in
>      principle this step is correct even without the BQL.
> 
> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
> ---
>   accel/dummy-cpus.c                |  2 +-
>   accel/hvf/hvf-accel-ops.c         |  2 +-
>   accel/kvm/kvm-accel-ops.c         |  3 ++-
>   accel/kvm/kvm-all.c               |  2 --
>   accel/tcg/cpu-exec.c              |  1 -
>   accel/tcg/tcg-accel-ops-mttcg.c   |  7 ++----
>   accel/tcg/tcg-accel-ops-rr.c      | 38 ++++++++++++++++---------------
>   accel/tcg/tcg-accel-ops.c         |  2 --
>   system/cpus.c                     |  1 +
>   target/i386/nvmm/nvmm-accel-ops.c |  6 ++---
>   target/i386/nvmm/nvmm-all.c       |  2 --
>   target/i386/whpx/whpx-accel-ops.c |  6 ++---
>   target/i386/whpx/whpx-all.c       |  2 --
>   13 files changed, 31 insertions(+), 43 deletions(-)

Great!

Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>