Re: How to synchronize CPUs on MMIO read?

[Richard Henderson]

On 8/16/23 09:31, Igor Lesik wrote:
> Hi.
>
> I need to model some custom HW that synchronizes CPUs when they read MMIO register N: MMIO 
> read does not return until another CPU writes to MMIO register M. I modeled this behavior 
> with a) on MMIO read of N, save CPU into a list of waiting CPUs and put it asleep with 
> cpu_interrupt(current_cpu, CPU_INTERRUPT_HALT) and b) on MMIO write to M, wake all waiting 
> CPUs with cpu->halted = 0; qemu_cpu_kick(cpu). It seems to work fine. However, this HW has 
> a twist: MMIO read of N returns a value that was written by MMIO write to M. Can anyone 
> please advise how this could be done?

You'll want to add something to allow each cpu to latch the value written.

Something like

    CPU_FOREACH(cpu) {
        if (cpu != write_cpu) {
            *cpu->mmio_latch = value;
            qemu_cpu_kick(cpu);
        }
    }

where cpu->sync_latch = &cpu->env.reg[N] for the register destination of the 
MMIO read.

This is easy if you can identify the hw sync mmio during translation.  If this sync is 
mapped somewhere arbitrary within the address space, you may have to work harder.



r~