[Qemu-devel] Round robin of multi-vcpu in qemu

[解壁伟 <xiebiwei@ict.ac.cn>]

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Hi there:

I'm studying the execution details of multi-vcpu in TCG mode.

The vcpus in TCG mode are executed one by one in a sequencial way.

I read the function [tcg_exec_all] in Qemu 1.3.0 as bellow, but the
implementation is not as I expected.

[tcg_exec_all] will finally call [cpu_exec] to excute a loop to excute
the TBs in code cache

So,how does these functions control the running time of each VCPU?

That is,when will the execution of one VCPU return, in order to execute
the next_cpu in the loop of [tcg_exec_all].

I've examined the alarm timmer, this timer is used to interrupt the
execution of VCPUs to handle interruptions.

Does Qemu use a similar way (timer) to control the round robin of these
VCPUs?

Or, does Qemu use the iCount to prevent a VCPU from running too long?

        staticvoidtcg_exec_all(void)
        {
        intr;

        /* Account partial waits to the vm_clock. */
        qemu_clock_warp(vm_clock);

        if(next_cpu==NULL){
        next_cpu=first_cpu;
        }
        for(;next_cpu!=NULL&&!exit_request;next_cpu=next_cpu->next_cpu){
        CPUArchState*env=next_cpu;
        CPUState*cpu=ENV_GET_CPU(env);

        qemu_clock_enable(vm_clock,
        (env->singlestep_enabled&SSTEP_NOTIMER)==0);

        if(cpu_can_run(cpu)){
        r=tcg_cpu_exec(env);
        if(r==EXCP_DEBUG){
        cpu_handle_guest_debug(env);
        break;
        }
        }elseif(cpu->stop||cpu->stopped){
        break;
        }
        }
        exit_request=0;
        }

Yours
Biwei

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