Re: [Qemu-devel] [PATCH] target/aarch64: exit to main loop after handling MSR

[Paolo Bonzini <pbonzini@redhat.com>]

On 14/06/2017 13:45, Alex Bennée wrote:
> 
> Paolo Bonzini <pbonzini@redhat.com> writes:
> 
>> On 14/06/2017 06:48, Richard Henderson wrote:
>>>>
>>>> Commit e75449a3 ("target/aarch64: optimize indirect branches") causes
>>>> a regression by which aarch64 guests freeze under TCG with -smp > 1,
>>>> even with `-accel accel=tcg,thread=single' (i.e. MTTCG disabled).
>>>>
>>>> I isolated the problem to the MSR handler. This patch forces an exit
>>>> after the handler is executed, which fixes the regression.
>>>
>>> Why would that be?  The cpu_get_tb_cpu_state within helper_lookup_tb_ptr
>>> is supposed to read the new state that the msr handler would have
>>> installed.
>>
>> Could some of these cause an interrupt, or some other change in the
>> cpu_exec flow?
> 
> Well what I was observing was the secondary_start_kernel stalling and
> leaving the main cpu spinning. The msr is actually:
> 
> 	local_irq_enable();
> 	local_fiq_enable();
> 
> Which I assume would re-enable IRQs if they are ready to go. However I
> guess if we sink into our cpu_idle without exiting the main loop we
> never set any pending IRQs?

Then Emilio's patch, if a bit of a heavy hammer, is correct. After
aa64_daif_write needs you need an exit_tb so that arm_cpu_exec_interrupt
is executed again.

Compare with this from the x86 front-end:

        /* if irq were inhibited with HF_INHIBIT_IRQ_MASK, we clear
           the flag and abort the translation to give the irqs a
           change to be happen */
        if (dc->tf || dc->singlestep_enabled ||
            (flags & HF_INHIBIT_IRQ_MASK)) {
            gen_jmp_im(pc_ptr - dc->cs_base);
            gen_eob(dc);
            break;
        }

(This triggers one instruction after a STI instruction, due to how x86's
"interrupt shadow" work, so it doesn't happen immediately after
helper_sti; but the idea is the same).

Paolo