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Date: Fri, 26 Jun 2015 17:54:56 +0200
From: Frederic Konrad <fred.konrad@greensocs.com>
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Subject: Re: [Qemu-devel] [RFC PATCH V6 15/18] cpu: introduce tlb_flush*_all.
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To: Paolo Bonzini <pbonzini@redhat.com>, qemu-devel@nongnu.org, mttcg@greensocs.com
Cc: peter.maydell@linaro.org, a.spyridakis@virtualopensystems.com, mark.burton@greensocs.com, agraf@suse.de, alistair.francis@xilinx.com, guillaume.delbergue@greensocs.com, alex.bennee@linaro.org

On 26/06/2015 17:15, Paolo Bonzini wrote:
>
> On 26/06/2015 16:47, fred.konrad@greensocs.com wrote:
>> +    CPU_FOREACH(cpu) {
>> +        if (qemu_cpu_is_self(cpu)) {
>> +            /* async_run_on_cpu handle this case but this just avoid a malloc
>> +             * here.
>> +             */
>> +            tlb_flush(cpu, flush_global);
>> +        } else {
>> +            params = g_malloc(sizeof(struct TLBFlushParams));
>> +            params->cpu = cpu;
>> +            params->flush_global = flush_global;
>> +            async_run_on_cpu(cpu, tlb_flush_async_work, params);
> Shouldn't this be synchronous (which you cannot do straightforwardly
> because of deadlocks---hence the need to hook cpu_has_work as discussed
> earlier)?
>
> Paolo
>
I think it doesn't requires to be synchronous as each VCPUs only clear 
it's own
tlb here:

void tlb_flush(CPUState *cpu, int flush_global)
{
     CPUArchState *env = cpu->env_ptr;

#if defined(DEBUG_TLB)
     printf("tlb_flush:\n");
#endif
     /* must reset current TB so that interrupts cannot modify the
        links while we are modifying them */
     cpu->current_tb = NULL;

     memset(env->tlb_table, -1, sizeof(env->tlb_table));
     memset(env->tlb_v_table, -1, sizeof(env->tlb_v_table));
     memset(cpu->tb_jmp_cache, 0, sizeof(cpu->tb_jmp_cache));

     env->vtlb_index = 0;
     env->tlb_flush_addr = -1;
     env->tlb_flush_mask = 0;
     tlb_flush_count++;
}

So what happen is:
An arm instruction want to clear tlb of all VCPUs eg: IS version of TLBIALL.
The VCPU which execute the TLBIALL_IS can't flush tlb of other VCPU.
It will just ask all VCPU thread to exit and to do tlb_flush hence the 
async_work.

Maybe the big issue might be memory barrier instruction here which I didn't
checked.

Fred
>> +        }
>> +    }