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Re: [Qemu-devel] [PATCH] Add another sanity check to smp_parse() functio


From: Eduardo Habkost
Subject: Re: [Qemu-devel] [PATCH] Add another sanity check to smp_parse() function
Date: Wed, 19 Aug 2015 08:58:13 -0700
User-agent: Mutt/1.5.23 (2014-03-12)

On Wed, Jul 22, 2015 at 03:59:50PM +0200, Thomas Huth wrote:
> The code in smp_parse already checks the topology information for
> sockets * cores * threads < cpus and bails out with an error in
> that case. However, it is still possible to supply a bad configuration
> the other way round, e.g. with:
> 
>  qemu-system-xxx -smp 4,sockets=1,cores=4,threads=2
> 
> QEMU then still starts the guest, with topology configuration that
> is rather incomprehensible and likely not what the user wanted.
> So let's add another check to refuse such wrong configurations.
> 
> Signed-off-by: Thomas Huth <address@hidden>
> ---
>  vl.c | 8 +++++++-
>  1 file changed, 7 insertions(+), 1 deletion(-)
> 
> diff --git a/vl.c b/vl.c
> index 5856396..c8d24b1 100644
> --- a/vl.c
> +++ b/vl.c
> @@ -1224,7 +1224,13 @@ static void smp_parse(QemuOpts *opts)
>              exit(1);
>          }
>  
> -        max_cpus = qemu_opt_get_number(opts, "maxcpus", 0);
> +        max_cpus = qemu_opt_get_number(opts, "maxcpus", cpus);
> +        if (sockets * cores * threads > max_cpus) {
> +            fprintf(stderr, "cpu topology: error: "
> +                    "sockets (%u) * cores (%u) * threads (%u) > maxcpus 
> (%u)\n",
> +                    sockets, cores, threads, max_cpus);
> +            exit(1);
> +        }

I am always afraid of breaking existing setups when we do that, because
there may be existing VMs running with these weird configurations, and
people won't be able to live-migrate them to a newer QEMU.

But I think we really have to start refusing to run obviously broken
configurations one day, or we will never fix this mess, so:

Reviewed-by: Eduardo Habkost <address@hidden>

I want to apply this through the x86 tree, but I would like to get some
Acked-by from other maintainers first.

-- 
Eduardo



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