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

[Jan Beulich <jbeulich@suse.com>]

On 06.03.2020 10:20, Jürgen Groß wrote:
> On 06.03.20 10:04, Jan Beulich wrote:
>> 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
> 
> Is this really problematic?

Not _really_, but still. In particular I'd prefer the 'v' key to
not even be listed among 'h' key output in such a case.

Jan

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