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

[Jan Beulich <jbeulich@xxxxxxxx>]

On 06.03.2020 09:47, Jürgen Groß wrote:
> On 06.03.20 09:20, Jan Beulich wrote:
>> On 06.03.2020 07:42, Jürgen Groß wrote:
>>> On 05.03.20 09:26, Jan Beulich wrote:
>>>> 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.
>>>
>>> I was thinking of something like this:
>>>
>>> --- a/xen/arch/x86/hvm/vmx/vmcs.c
>>> +++ b/xen/arch/x86/hvm/vmx/vmcs.c
>>> @@ -313,12 +313,12 @@ static int vmx_init_vmcs_config(void)
>>>        {
>>>            rdmsrl(MSR_IA32_VMX_EPT_VPID_CAP, _vmx_ept_vpid_cap);
>>>
>>> +        if ( /* Work around Erratum AVR41 on Avoton processors. */
>>> +             boot_cpu_data.x86 == 6 && boot_cpu_data.x86_model == 0x4d &&
>>> +             opt_ept_ad < 0 )
>>> +            opt_ept_ad = 0;
>>>            if ( !opt_ept_ad )
>>>                _vmx_ept_vpid_cap &= ~VMX_EPT_AD_BIT;
>>> -        else if ( /* Work around Erratum AVR41 on Avoton processors. */
>>> -                  boot_cpu_data.x86 == 6 && boot_cpu_data.x86_model == 
>>> 0x4d &&
>>> -                  opt_ept_ad < 0 )
>>> -            _vmx_ept_vpid_cap &= ~VMX_EPT_AD_BIT;
>>>
>>>            /*
>>>             * Additional sanity checking before using EPT:
>>
>> And I was specifically hoping to avoid doing this in a non-__init
>> function.
> 
> Should be fairly easy (see attached patch).

Why not put the opt_ept_ad adjustment right into start_vmx(),
just like e.g. the opt_ept_exec_sp gets also done there? Pulling
the setting up of the 'v' key handler risks installing it ahead
of a potential future later error exit from start_vmx(). But I'm
not entirely opposed to the chosen approach either - it'll be up
to Kevin to judge, I guess.

Jan

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