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

[Jan Beulich <jbeulich@xxxxxxxx>]

On 05.03.2020 07:01, Jürgen Groß wrote:
> On 04.03.20 17:56, Jan Beulich wrote:
>> 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 disagree here.
> 
> This is important only for cases where the hypervisor treats the
> parameter as a tristate: true/false/unspecified. In all cases where
> the bool value is really true or false it can be reported as such.

The problem I'm having with this is the resulting inconsistency:
When we write the variable with 0 or 1 in case we find it to be
-1 after command line parsing, the externally visible effect will
be different from the case where we leave it to be -1 yet still
treat it as (pseudo-)boolean. This, however, is an implementation
detail, while imo the hypfs presentation should not depend on
such implementation details.

> Reporting 0/1 for e.g. "ad" if opt_ept_ad==-1 would add a latent problem
> if any other action would be derived from the parameter variable being
> -1.
> 
> So either opt_ept_ad should be modified to change it to 0/1 instead of
> only setting the VCMS flag,

That's what I did suggest.

> or the logic should be kept as is in this
> patch. IMO changing the setting of opt_ept_ad should be done in another
> patch if this is really wanted.

And of course I don't mind at all doing so in a prereq patch.
It's just that the patch here provides a good place _where_ to
actually do such an adjustment.

Jan

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