Re: [PATCH 4/8] accel/tcg: introduce tcg_kick_vcpu_thread

[Paolo Bonzini <pbonzini@redhat.com>]

On 8/10/25 01:26, Richard Henderson wrote:
> On 8/9/25 04:59, Paolo Bonzini wrote:
>> +void tcg_kick_vcpu_thread(CPUState *cpu)
>> +{
>> +    /*
>> +     * Ensure cpu_exec will see the reason why the exit request was set.
>> +     * FIXME: this is not always needed.  Other accelerators instead
>> +     * read interrupt_request and set exit_request on demand from the
>> +     * CPU thread; see kvm_arch_pre_run() for example.
>> +     */
>> +    qatomic_store_release(&cpu->exit_request, 1);
>> +
>> +    /* Ensure cpu_exec will see the exit request after TCG has 
>> exited.  */
>> +    qatomic_store_release(&cpu->neg.icount_decr.u16.high, -1);
>> +}
> 
> So, now both cpu_exit and cpu_kick set exit_request.
> 
> You ifdef this out again for user-only in patch 7, but this does suggest 
> that kick and exit are essentially interchangeable.  You rearrange 
> things a bit in patch 6, but it's still not clear to me what the 
> difference between the two should be.  There's certainly nothing at all 
> in include/hw/core/cpu.h to differentiate them.
> 
> Should we instead eliminate one of kick or exit, unifying the paths?
In cpu-exec.c terms, qemu_cpu_kick() *should* go out to 
cpu_handle_interrupt() whereas cpu_exit() *should* go out to 
cpu_handle_exception().  The difference matters for some accelerators 
where qemu_cpu_kick() tries not to take the BQL in the vCPU thread.

Until now TCG's implementation of kick_vcpu_thread set both exit_request 
and interrupt_request, and I'm not changing that yet for system 
emulation.  Patch 7 does that for user-mode emulation, because it's 
trivial: neither linux-user not bsd-user use qemu_cpu_kick() directly.

Paolo