Re: [Qemu-devel] [PATCH v4 3/6] hypertrace: [*-user] Add QEMU-side proxy to "guest_hypertrace" event

[Stefan Hajnoczi]

On Mon, Dec 26, 2016 at 09:34:54PM +0100, Lluís Vilanova wrote:
> +static void segv_handler(int signum, siginfo_t *siginfo, void *sigctxt)
> +{
> +    CPUState *vcpu = current_cpu;
> +    void *control_0 = vcpu->hypertrace_control;
> +    void *control_1 = vcpu->hypertrace_control + config.control_size / 2;
> +    void *control_2 = control_1 + config.control_size / 2;
> +
> +    if (control_0 <= siginfo->si_addr && siginfo->si_addr < control_1) {
> +
> +        /* 1st fault (guest will write cmd) */
> +        assert(((unsigned long)siginfo->si_addr % sizeof(uint64_t)) == 0);
> +        swap_control(control_0, control_1);
> +
> +    } else if (control_1 <= siginfo->si_addr && siginfo->si_addr < 
> control_2) {
> +        size_t client = (siginfo->si_addr - control_1) / sizeof(uint64_t);
> +        uint64_t vcontrol = ((uint64_t *)control_0)[client];
> +        uint64_t *data_ptr = &qemu_data[client * config.client_data_size];
> +
> +        /* 2nd fault (invoke) */
> +        assert(((unsigned long)siginfo->si_addr % sizeof(uint64_t)) == 0);
> +        hypertrace_emit(current_cpu, vcontrol, data_ptr);
> +        swap_control(control_1, control_0);

A simpler and faster approach is to permanently mprotect just one region
and load all arguments from data[] (including the first argument).  Then
swapping isn't necessary.

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