Re: [PATCH 04/13] cpufreq: Add Hardware P-State (HWP) driver

From: Jan Beulich <jbeulich@suse.com>
To: Jason Andryuk <jandryuk@gmail.com>
Cc: "Andrew Cooper" <andrew.cooper3@citrix.com>,
	"George Dunlap" <george.dunlap@citrix.com>,
	"Ian Jackson" <iwj@xenproject.org>,
	"Julien Grall" <julien@xen.org>,
	"Stefano Stabellini" <sstabellini@kernel.org>,
	"Wei Liu" <wl@xen.org>, "Roger Pau Monné" <roger.pau@citrix.com>,
	xen-devel <xen-devel@lists.xenproject.org>
Subject: Re: [PATCH 04/13] cpufreq: Add Hardware P-State (HWP) driver
Date: Fri, 4 Jun 2021 08:39:12 +0200	[thread overview]
Message-ID: <2d88b1db-3064-63e1-e197-9318624e6cc6@suse.com> (raw)
In-Reply-To: <CAKf6xpvCkzHOZsBY2yMQSVxq844_muaAaKd-JZUQfd7UCrXLVg@mail.gmail.com>

On 03.06.2021 13:55, Jason Andryuk wrote:
> On Fri, May 28, 2021 at 2:35 AM Jan Beulich <jbeulich@suse.com> wrote:
>> On 27.05.2021 20:50, Jason Andryuk wrote:
>>> On Wed, May 26, 2021 at 11:00 AM Jan Beulich <jbeulich@suse.com> wrote:
>>>> On 03.05.2021 21:28, Jason Andryuk wrote:
>>>>> +    hwp_verbose("HWP: FAST_IA32_HWP_REQUEST %ssupported\n",
>>>>> +                eax & CPUID6_EAX_FAST_HWP_MSR ? "" : "not ");
>>>>> +    if ( eax & CPUID6_EAX_FAST_HWP_MSR )
>>>>> +    {
>>>>> +        if ( rdmsr_safe(MSR_FAST_UNCORE_MSRS_CAPABILITY, val) )
>>>>> +            hwp_err("error rdmsr_safe(MSR_FAST_UNCORE_MSRS_CAPABILITY)\n");
>>>>> +
>>>>> +        hwp_verbose("HWP: MSR_FAST_UNCORE_MSRS_CAPABILITY: %016lx\n", val);
>>>>
>>>> Missing "else" above here?
>>>
>>> Are unbalanced braces acceptable or must they be balanced?  Is this acceptable:
>>> if ()
>>>   one;
>>> else {
>>>   two;
>>>   three;
>>> }
>>
>> Yes, it is. But I don't see how the question relates to my comment.
>> All that needs to go in the else's body is the hwp_verbose().
> 
> 'val' shouldn't be used to set features when the rdmsr fails, so the
> following code needs to be within the else.  Unless you want to rely
> on a failed rdmsr returning 0.

It is intentional for rdmsr_safe() to return a zero value when the
access faulted, so I certainly think you may rely on this.

Jan