Re: [PATCH] i386/apic: Add hint on boot failure because of disabling x2APIC

[Xiaoyao Li <xiaoyao.li@intel.com>]

On 6/7/2024 3:46 PM, Zhao Liu wrote:
> Hi Philippe,
> 
> On Fri, Jun 07, 2024 at 08:17:36AM +0200, Philippe Mathieu-Daudé wrote:
>> Date: Fri, 7 Jun 2024 08:17:36 +0200
>> From: Philippe Mathieu-Daudé <philmd@linaro.org>
>> Subject: Re: [PATCH] i386/apic: Add hint on boot failure because of
>>   disabling x2APIC
>>
>> On 6/6/24 16:08, Zhao Liu wrote:
>>> Currently, the Q35 supports up to 4096 vCPUs (since v9.0), but for TCG
>>> cases, if x2APIC is not actively enabled to boot more than 255 vCPUs (

why bother mentioning TCG case?

you below example is not for TCG. And the issue is not caused by TCG but 
default cpu model doesn't have X2APIC enabled.

>>> e.g., qemu-system-i386 -M pc-q35-9.0 -smp 666), the following error is
>>> reported:
>>>
>>> Unexpected error in apic_common_set_id() at ../hw/intc/apic_common.c:449:
>>> qemu-system-i386: APIC ID 255 requires x2APIC feature in CPU
>>> Aborted (core dumped)
>>>
>>> This error can be resolved by setting x2apic=on in -cpu. In order to
>>> better help users deal with this scenario, add the error hint to
>>> instruct users on how to enable the x2apic feature.
>>
>> Why not automatically set x2apic=on in this case instead?
> 
> The default CPU model is qemu64 without x2APIC. I think it might be
> necessary to update the version to add x2APIC in qemu64, and I'd like to
> look into it again for any other potential issues.
> 
> In addition to adding x2APIC directly to the qemu64, this hint can also
> help some users who want a large number of vCPUs based on other older
> CPU models. Though, it's not very common but this hint would be helpful.
> 
>>> Then, the error
>>> report becomes the following:
>>>
>>> Unexpected error in apic_common_set_id() at ../hw/intc/apic_common.c:448:
>>> qemu-system-i386: APIC ID 255 requires x2APIC feature in CPU
>>> Try x2apic=on in -cpu.
>>> Aborted (core dumped)
>>>
>>> Note since @errp is &error_abort, error_append_hint() can't be applied
>>> on @errp. And in order to separate the exact error message from the
>>> (perhaps effectively) hint, adding a hint via error_append_hint() is
>>> also necessary. Therefore, introduce @local_error in
>>> apic_common_set_id() to handle both the error message and the error
>>> hint.
>>>
>>> Suggested-by: Philippe Mathieu-Daudé <philmd@linaro.org>
>>> Signed-off-by: Zhao Liu <zhao1.liu@intel.com>
>>> ---
>>>    hw/intc/apic_common.c | 7 ++++++-
>>>    1 file changed, 6 insertions(+), 1 deletion(-)
>>
>> Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
> 
> Thanks!
> 
>