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Re: [Qemu-devel] [PATCH v1 RFC 09/10] QEMU: s390: cpu model QMP query-cp

From: Eric Blake
Subject: Re: [Qemu-devel] [PATCH v1 RFC 09/10] QEMU: s390: cpu model QMP query-cpu-model
Date: Tue, 13 May 2014 09:29:37 -0600
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:24.0) Gecko/20100101 Thunderbird/24.5.0

On 05/13/2014 09:00 AM, Michael Mueller wrote:
> This patch implements a new QMP request named "query-cpu-model". It returns
> the cpu model of cpu 0. eg:
> request: '{"execute" : "query-cpu-model" }
> answer:  '{"return" : {"name": "2827-ga2"}}
> Alias names are resolved to their respective machine type and GA names
> already during cpu instantiation. Thus, also a cpu name like "host",
> which is implemented as alias, will return its normalized cpu model name.

> +    }
> +    cpu_model = g_try_malloc0(sizeof(*cpu_model));

It's simpler to just use g_malloc0 and rely on glibc's exit-on-OOM
behavior than to try and deal with NULL - this isn't user input (so
unlikely to be so huge as to cause OOM), and would be more in line with
what most other QMP code does.  But that said...

> +    if (!cpu_model) {
> +        goto out_no_mem;
> +    }
> +    cpu_model->name = g_try_malloc0(CPU_MODEL_NAME_LEN);
> +    if (!cpu_model->name) {
> +        goto out_no_mem;
> +    }
> +    snprintf(cpu_model->name, CPU_MODEL_NAME_LEN - 1, "%04x-ga%u",
> +             cc->proc->type, cc->mach->ga);

...why not just use g_strdup_printf() instead of trying to size a buffer
yourself?  In other words, skip the g_try_malloc0 to begin with.

The fact that you are packing two pieces of information into one string
is a bit worrisome - that means that the client of the QMP command has
to parse the string back into two pieces of information if they ever
need either item in isolation.  If the user never has a need to split
the name down into parts, you are okay; I don't know S390 well enough to
know whether anyone will care about type separate from ga.  But if
someone DOES care, then the QMP command should return the parts already
split, as in { "type": 2827, "ga": 2 }, or even as convenience provide
both split and combined forms: { "name": "2827-ga2", "type": 2827, "ga": 2 }

Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org

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