Re: [PATCH] q35: catch invalid cpu hotplug configuration

["Daniel P. Berrangé" <berrange@redhat.com>]

On Thu, Aug 12, 2021 at 12:23:41PM +0200, Gerd Hoffmann wrote:
> Related: https://bugzilla.redhat.com//show_bug.cgi?id=1985924
> Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
> ---
>  hw/isa/lpc_ich9.c | 13 +++++++++++++
>  1 file changed, 13 insertions(+)
> 
> diff --git a/hw/isa/lpc_ich9.c b/hw/isa/lpc_ich9.c
> index 5f9de0239cf9..5f143dca17aa 100644
> --- a/hw/isa/lpc_ich9.c
> +++ b/hw/isa/lpc_ich9.c
> @@ -31,6 +31,7 @@
>  #include "qemu/osdep.h"
>  #include "qemu/log.h"
>  #include "cpu.h"
> +#include "qapi/error.h"
>  #include "qapi/visitor.h"
>  #include "qemu/range.h"
>  #include "hw/isa/isa.h"
> @@ -676,6 +677,18 @@ static void ich9_lpc_realize(PCIDevice *d, Error **errp)
>      DeviceState *dev = DEVICE(d);
>      ISABus *isa_bus;
>  
> +    if ((lpc->smi_host_features & BIT_ULL(ICH9_LPC_SMI_F_CPU_HOT_UNPLUG_BIT)) &&
> +        !(lpc->smi_host_features & BIT_ULL(ICH9_LPC_SMI_F_CPU_HOTPLUG_BIT))) {
> +        /*
> +         * smi_features_ok_callback() throws an error on this.
> +         *
> +         * So bail out here instead of advertizing the invalid
> +         * configuration and get obscure firmware failures from that.
> +         */
> +        error_setg(errp, "cpu hot-unplug requires cpu hot-plug");
> +        return;
> +    }

What does this actually mean in practice ? AFAIK from the libvirt side
we don't do anything special to enable/disable CPU hotplug. What QEMU
config setting is wrong to lead to this scenario, thta the user/mgmt
app has to fix ?


Regards,
Daniel
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