Re: [Xen-devel] [PATCH v6 09/12] xen: add runtime parameter access support to hypfs

[Jan Beulich <jbeulich@suse.com>]

On 04.03.2020 17:31, Jürgen Groß wrote:
> On 04.03.20 16:19, Jan Beulich wrote:
>> On 04.03.2020 16:07, Jürgen Groß wrote:
>>> On 04.03.20 12:32, Jan Beulich wrote:
>>>> On 26.02.2020 13:47, Juergen Gross wrote:
>>>>> +static void update_ept_param_append(const char *str, int val)
>>>>> +{
>>>>> +    char *pos = opt_ept_setting + strlen(opt_ept_setting);
>>>>> +
>>>>> +    snprintf(pos, sizeof(opt_ept_setting) - (pos - opt_ept_setting),
>>>>> +             ",%s=%d", str, val);
>>>>> +}
>>>>> +
>>>>> +static void update_ept_param(void)
>>>>> +{
>>>>> +    snprintf(opt_ept_setting, sizeof(opt_ept_setting), "pml=%d", opt_ept_pml);
>>>>> +    if ( opt_ept_ad >= 0 )
>>>>> +        update_ept_param_append("ad", opt_ept_ad);
>>>>
>>>> This won't correctly reflect reality: If you look at
>>>> vmx_init_vmcs_config(), even a negative value means "true" here,
>>>> unless on a specific Atom model. I think init_ept_param() wants
>>>> to have that erratum workaround logic moved there, such that
>>>> you can then assme the value to be non-negative here.
>>>
>>> But isn't not mentioning it in the -1 case correct? -1 means: do the
>>> correct thing on the current hardware.
>>
>> Well, I think the output here should represent effective settings,
> 
> The minimum requirement is to reflect the effective parameters, like
> cmdline is doing for boot-time only parameters. With runtime parameters
> we had no way of telling what was set, and this is now possible.
> 
>> and a sub-item should be suppressed only if a setting has no effect
>> at all in the current setup, like ...
>>
>>>>> +    if ( opt_ept_exec_sp >= 0 )
>>>>> +        update_ept_param_append("exec-sp", opt_ept_exec_sp);
>>>>
>>>> I agree for this one - if the value is still -1, it has neither
>>>> been set nor is its value of any interest.
>>
>> ... here.
> 
> I think we should not mix up specified parameters and effective
> settings. In case an effective setting is of common interest it should
> be reported via a specific node (like e.g. specific mitigation settings
> where the cmdline is not providing enough details).

But then a boolean option that wasn't specified on the command line
should produce no output at all. And hence we'd need a way to tell
whether an option was set from command line for _all_ of them. I
don't think this would be very helpful.

I'm curious if anyone else has any opinion either way (or yet
another one) here:

Jan

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