Re: [PATCH] x86/spec: fix reporting of BHB clearing usage from guest entry points

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Thu, Apr 18, 2024 at 12:44:26PM +0200, Jan Beulich wrote:
> On 15.04.2024 16:17, Roger Pau Monne wrote:
> > --- a/xen/arch/x86/spec_ctrl.c
> > +++ b/xen/arch/x86/spec_ctrl.c
> > @@ -643,7 +643,7 @@ static void __init print_details(enum ind_thunk thunk)
> >             opt_eager_fpu                             ? " EAGER_FPU"     : 
> > "",
> >             opt_verw_hvm                              ? " VERW"          : 
> > "",
> >             boot_cpu_has(X86_FEATURE_IBPB_ENTRY_HVM)  ? " IBPB-entry"    : 
> > "",
> > -           cpu_has_bhb_seq                           ? " BHB-entry"     : 
> > "");
> > +           opt_bhb_entry_hvm                         ? " BHB-entry"     : 
> > "");
> >  
> >  #endif
> >  #ifdef CONFIG_PV
> > @@ -658,7 +658,7 @@ static void __init print_details(enum ind_thunk thunk)
> >             opt_eager_fpu                             ? " EAGER_FPU"     : 
> > "",
> >             opt_verw_pv                               ? " VERW"          : 
> > "",
> >             boot_cpu_has(X86_FEATURE_IBPB_ENTRY_PV)   ? " IBPB-entry"    : 
> > "",
> > -           cpu_has_bhb_seq                           ? " BHB-entry"     : 
> > "");
> > +           opt_bhb_entry_pv                          ? " BHB-entry"     : 
> > "");
> >  
> >      printk("  XPTI (64-bit PV only): Dom0 %s, DomU %s (with%s PCID)\n",
> >             opt_xpti_hwdom ? "enabled" : "disabled",
> 
> For one the surrounding if() conditions need similar adjustment.

Oh, I see, you mean the printing of "None" if no options are active.

> But then I wonder whether the further uses of cpu_has_bhb_seq don't
> need switching, too (when determining whether to force
> X86_SPEC_NO_LFENCE_ENTRY_*). At which point the question is why
> cpu_has_bhb_seq exists in the first place: It in particular does not
> affect the default calculation of opt_bhb_entry_*, afaics.

Hm, yes, that's indeed not very helpful.  The current code is overly
restrictive, as the lfence won't be elided even when the BHB sequence
is not being used by that specific guest type, as long as some
sequence is configured.  Will add a further patch to fix that and then
remove cpu_has_bhb_seq.

Thanks, Roger.