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

[Jürgen Groß <jgross@xxxxxxxx>]

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.

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, 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.

> I'm curious if anyone else has any opinion either way (or yet
> another one) here:
Me too.


Juergen


_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxxxxxxxxx
https://lists.xenproject.org/mailman/listinfo/xen-devel