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

[Jürgen Groß <jgross@xxxxxxxx>]

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:

Juergen

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