Re: [Qemu-devel] [Qemu-ppc] [PATCH v2] spapr: quantify error messages regarding capability settings

[Daniel Black <daniel@linux.ibm.com>]

On Thu, 1 Aug 2019 12:41:59 +0200
Greg Kurz <groug@kaod.org> wrote:

> On Thu,  1 Aug 2019 13:38:19 +1000
> Daniel Black <daniel@linux.ibm.com> wrote:
> 
> > Its not immediately obvious how cap-X=Y setting need to be applied
> > to the command line so, for spapr capability error messages, this
> > has been clarified to:
> > 
...
> > index bbb001f84a..1c0222a081 100644
> > --- a/hw/ppc/spapr_caps.c
> > +++ b/hw/ppc/spapr_caps.c
> > @@ -37,6 +37,8 @@
> >  
> >  #include "hw/ppc/spapr.h"
> >  
> > +#define CAPABILITY_ERROR(X) "appending -machine " X  
> 
> I would make that:
> 
> #define CAPABILITY_HINT() "try appending -machine " X
> 
> because it is really an hint for the user, not an
> error,

Works for me. At the lowest layer it is a hint.

> and all original strings have "try",

True.

> except...


> > @@ -249,11 +255,13 @@ static void
> > cap_safe_cache_apply(SpaprMachineState *spapr, uint8_t val, if
> > (tcg_enabled() && val) { /* TCG only supports broken, allow other
> > values and print a warning */ error_setg(&local_err,
> > -                   "TCG doesn't support requested feature,
> > cap-cfpc=%s",
> > +                   "TCG doesn't support requested feature, "
> > +                   CAPABILITY_ERROR("cap-cfpc=%s"),  
> 
> ... this one, but it doesn't look like a hint to me. It just tells
> which is the unsupported cap.

This is one of 3 that local_error (commit
006e9d3618698eeef2f3e07628d22cb6f5c2a039) - intentionally just a
warning and to TLDR the commit/Suraj conversation; defaults apply
to all machine types; hardware security measures don't make sense in
TCG; hence warning.

For every function with CAPABILITY_[ERROR|HINT] its called by
spapr_caps_apply, has its errp as &error_fatal (intentionally - spoke
to Suraj - migrations to machines without capabilities need to fail and
defaults (kvm) should be secure unless explicitly disabled).

> >                     cap_cfpc_possible.vals[val]);
> >      } else if (kvm_enabled() && (val > kvm_val)) {
> >          error_setg(errp,
> > -"Requested safe cache capability level not supported by kvm, try
> > cap-cfpc=%s", +"Requested safe cache capability level not supported
> > by kvm, try "
> > +                   CAPABILITY_ERROR("cap-cfpc=%s"),
> >                     cap_cfpc_possible.vals[kvm_val]);  
> 
> Also, we have a dedicated API for hints, which are only printed under
> the monitor but ignored under QMP.

Ok.
 
> Not sure why it isn't used here but it should be something like:

If error_append_hint should be used for fatal errors (all that use
errp), then this patten should be applied further to
CAPABILITY_[HINT|ERROR] functions.

If error_append_hint needs to apply to warnings
cap_[cfpc/sbbc/ibs]_apply functions need to use it.

Would I be right in I'm assuming that the below pattern needs to apply
to both of these cases?

>         error_setg(errp, 
>                    "Requested safe cache capability level not
> supported by kvm");
>         error_append_hint(errp,
> CAPABILITY_HINT("cap-cfpc=%s") "\n", cap_cfpc_possible.vals[kvm_val]);

This is going a little beyond the scope of fixing a message, ok, but
lets not extend the scope too much more.