Re: [Xen-devel] [PATCH v4 4/6] xen: add xen_pin_vcpu() to support calling functions on a dedicated pcpu

[Juergen Gross <jgross@xxxxxxxx>]

On 05/04/16 11:45, David Vrabel wrote:
> On 05/04/16 06:10, Juergen Gross wrote:
>> Some hardware models (e.g. Dell Studio 1555 laptops) require calls to
>> the firmware to be issued on cpu 0 only. As Dom0 might have to use
>> these calls, add xen_pin_vcpu() to achieve this functionality.
>>
>> In case either the domain doesn't have the privilege to make the
>> related hypercall or the hypervisor isn't supporting it, issue a
>> warning once and disable further pinning attempts.
> [...]
>> --- a/arch/x86/xen/enlighten.c
>> +++ b/arch/x86/xen/enlighten.c
>> @@ -1885,6 +1885,45 @@ static void xen_set_cpu_features(struct cpuinfo_x86 
>> *c)
>>      }
>>  }
>>  
>> +static void xen_pin_vcpu(int cpu)
>> +{
>> +    static bool disable_pinning;
>> +    struct sched_pin_override pin_override;
>> +    int ret;
>> +
>> +    if (disable_pinning)
>> +            return;
>> +
>> +    pin_override.pcpu = cpu;
>> +    ret = HYPERVISOR_sched_op(SCHEDOP_pin_override, &pin_override);
> 
>       /* Ignore errors when removing override. */

Okay.

>> +    if (cpu < 0)
>> +            return;
>> +
>> +    switch (ret) {
>> +    case -ENOSYS:
>> +            pr_warn("The kernel tried to call a function on physical cpu 
>> %d, but Xen isn't\n"
>> +                    "supporting this. In case of problems you might 
>> consider vcpu pinning.\n",
>> +                    cpu);
>> +            disable_pinning = true;
>> +            break;
>> +    case -EPERM:
>> +            WARN(1, "Trying to pin vcpu without having privilege to do 
>> so\n");
>> +            disable_pinning = true;
>> +            break;
>> +    case -EINVAL:
>> +    case -EBUSY:
>> +            pr_warn("The kernel tried to call a function on physical cpu 
>> %d, but this cpu\n"
>> +                    "seems not to be available. Please check your Xen cpu 
>> configuration.\n",
>> +                    cpu);
>> +            break;
>> +    case 0:
>> +            break;
>> +    default:
>> +            WARN(1, "rc %d while trying to pin vcpu\n", ret);
>> +            disable_pinning = true;
>> +    }
> 
> These messages are a bit wordy for my taste and since they don't say
> what function failed or what driver is affected they're not as useful as

Did you notice I used WARN() for the cases where a usage error is to
be suspected? This will print a stack backtrace helping to identify the
driver.

I can work on the message text, of course.

> they could be.  I'd probably turn these all into:
> 
>       if (cpu >= 0 && ret < 0) {
>               pr_warn("Failed to pin VCPU %d to physical CPU %d: %d",
>                       smp_processor_id(), cpu, ret);
>               disable_pinning = true;
>       }

No, I don't think this is a good idea. In the EINVAL or EBUSY case a
simple Xen admin command might be enough to make the next call succeed.
I don't want to disable pinning in this case.

> And look at getting the user of this API to print a more useful error.
> 
> "i8k: unable to call SMM BIOS on physical CPU %d: %d"

TBH: I think this should be done by another patch. This is something
the maintainers of the callers' code should decide.


Juergen

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