Re: [PATCH v7 05/15] machine: Improve the error reporting of smp parsing

["wangyanan (Y)" <wangyanan55@huawei.com>]

On 2021/8/24 15:29, Philippe Mathieu-Daudé wrote:
> On 8/24/21 6:51 AM, wangyanan (Y) wrote:
>> On 2021/8/23 21:17, Philippe Mathieu-Daudé wrote:
>>> On 8/23/21 2:27 PM, Yanan Wang wrote:
>>>> We have two requirements for a valid SMP configuration:
>>>> the product of "sockets * cores * threads" must represent all the
>>>> possible cpus, i.e., max_cpus, and then must include the initially
>>>> present cpus, i.e., smp_cpus.
>>>>
>>>> So we only need to ensure 1) "sockets * cores * threads == maxcpus"
>>>> at first and then ensure 2) "maxcpus >= cpus". With a reasonable
>>>> order of the sanity check, we can simplify the error reporting code.
>>>> When reporting an error message we also report the exact value of
>>>> each topology member to make users easily see what's going on.
>>>>
>>>> Signed-off-by: Yanan Wang <wangyanan55@huawei.com>
>>>> Reviewed-by: Andrew Jones <drjones@redhat.com>
>>>> Reviewed-by: Pankaj Gupta <pankaj.gupta@ionos.com>
>>>> ---
>>>>    hw/core/machine.c | 22 +++++++++-------------
>>>>    hw/i386/pc.c      | 24 ++++++++++--------------
>>>>    2 files changed, 19 insertions(+), 27 deletions(-)
>>>>
>>>> diff --git a/hw/core/machine.c b/hw/core/machine.c
>>>> index 85908abc77..093c0d382d 100644
>>>> --- a/hw/core/machine.c
>>>> +++ b/hw/core/machine.c
>>>> @@ -779,25 +779,21 @@ static void smp_parse(MachineState *ms,
>>>> SMPConfiguration *config, Error **errp)
>>>>        maxcpus = maxcpus > 0 ? maxcpus : sockets * cores * threads;
>>>>        cpus = cpus > 0 ? cpus : maxcpus;
>>>>    -    if (sockets * cores * threads < cpus) {
>>>> -        error_setg(errp, "cpu topology: "
>>>> -                   "sockets (%u) * cores (%u) * threads (%u) < "
>>>> -                   "smp_cpus (%u)",
>>>> -                   sockets, cores, threads, cpus);
>>>> +    if (sockets * cores * threads != maxcpus) {
>>>> +        error_setg(errp, "Invalid CPU topology: "
>>>> +                   "product of the hierarchy must match maxcpus: "
>>>> +                   "sockets (%u) * cores (%u) * threads (%u) "
>>>> +                   "!= maxcpus (%u)",
>>>> +                   sockets, cores, threads, maxcpus);
>>>>            return;
>>>>        }
>>> Thinking about scalability, MachineClass could have a
>>> parse_cpu_topology() handler, and this would be the
>>> generic one. Principally because architectures don't
>>> use the same terms, and die/socket/core/thread arrangement
>>> is machine specific (besides being arch-spec).
>>> Not a problem as of today, but the way we try to handle
>>> this generically seems over-engineered to me.
>> Hi Philippe,
>>
>> The reason for introducing a generic implementation and avoiding
>> specific ones is that we thought there is little difference in parsing
>> logic between the specific parsers. Most part of the parsing is the
>> automatic calculation of missing values and the related error reporting,
>> in which the only difference between parsers is the handling of specific
>> (no matter of arch-specific or machine-specifc) parameters.
>>
>> So it may be better to keep the parsing logic unified if we can easily
>> realize that. And actually we can use compat stuff to handle specific
>> topology parameters well. See implementation in patch #10.
>>
>> There have been patches on list introducing new specific members
>> (s390 related in [1] and ARM related in [2]), and in each of them there
>> is a specific parser needed. However, based on generic one we can
>> extend without the increasing code duplication.
>>
>> There is also some discussion about generic/specific parser in [1],
>> which can be a reference.
>>
>> [1]
>> https://lore.kernel.org/qemu-devel/1626281596-31061-2-git-send-email-pmorel@linux.ibm.com/
>>
>> [2]
>> https://lore.kernel.org/qemu-devel/20210516103228.37792-1-wangyanan55@huawei.com/
> OK I read Daniel's rationale here:
> https://lore.kernel.org/qemu-devel/YPFN83pKBt7F97kW@redhat.com/
exactly. :)

Thanks,
Yanan
.
> Thanks,
>
> Phil.
>
> .