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

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Date:	Tue, 5 Apr 2016 12:01:50 +0200
From:	Juergen Gross <jgross@...e.com>
To:	David Vrabel <david.vrabel@...rix.com>,
	linux-kernel@...r.kernel.org, xen-devel@...ts.xenproject.org
Cc:	jeremy@...p.org, jdelvare@...e.com, peterz@...radead.org,
	hpa@...or.com, akataria@...are.com, x86@...nel.org,
	rusty@...tcorp.com.au, virtualization@...ts.linux-foundation.org,
	chrisw@...s-sol.org, mingo@...hat.com, Douglas_Warzecha@...l.com,
	pali.rohar@...il.com, boris.ostrovsky@...cle.com,
	tglx@...utronix.de, linux@...ck-us.net
Subject: Re: [Xen-devel] [PATCH v4 4/6] xen: add xen_pin_vcpu() to support
 calling functions on a dedicated pcpu

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