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

[Jan Beulich <jbeulich@xxxxxxxx>]

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.

Jan

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