Re: [PATCH] x86/spec: fix BRANCH_HARDEN option to only be set when build-enabled

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

On Fri, Feb 23, 2024 at 10:26:15AM +0000, Andrew Cooper wrote:
> On 23/02/2024 10:17 am, Roger Pau Monné wrote:
> > On Fri, Feb 23, 2024 at 09:46:27AM +0000, Andrew Cooper wrote:
> >> On 23/02/2024 9:42 am, Roger Pau Monne wrote:
> >>> The current logic to handle the BRANCH_HARDEN option will report it as 
> >>> enabled
> >>> even when build-time disabled. Fix this by only allowing the option to be 
> >>> set
> >>> when support for it is built into Xen.
> >>>
> >>> Fixes: 2d6f36daa086 ('x86/nospec: Introduce 
> >>> CONFIG_SPECULATIVE_HARDEN_BRANCH')
> >>> Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
> >>> ---
> >>>  xen/arch/x86/spec_ctrl.c | 6 ++++--
> >>>  1 file changed, 4 insertions(+), 2 deletions(-)
> >>>
> >>> diff --git a/xen/arch/x86/spec_ctrl.c b/xen/arch/x86/spec_ctrl.c
> >>> index 421fe3f640df..e634c6b559b4 100644
> >>> --- a/xen/arch/x86/spec_ctrl.c
> >>> +++ b/xen/arch/x86/spec_ctrl.c
> >>> @@ -50,7 +50,8 @@ static int8_t __initdata opt_psfd = -1;
> >>>  int8_t __ro_after_init opt_ibpb_ctxt_switch = -1;
> >>>  int8_t __read_mostly opt_eager_fpu = -1;
> >>>  int8_t __read_mostly opt_l1d_flush = -1;
> >>> -static bool __initdata opt_branch_harden = true;
> >>> +static bool __initdata opt_branch_harden =
> >>> +    IS_ENABLED(CONFIG_SPECULATIVE_HARDEN_BRANCH);
> >>>  
> >>>  bool __initdata bsp_delay_spec_ctrl;
> >>>  uint8_t __read_mostly default_xen_spec_ctrl;
> >>> @@ -267,7 +268,8 @@ static int __init cf_check parse_spec_ctrl(const char 
> >>> *s)
> >>>              opt_eager_fpu = val;
> >>>          else if ( (val = parse_boolean("l1d-flush", s, ss)) >= 0 )
> >>>              opt_l1d_flush = val;
> >>> -        else if ( (val = parse_boolean("branch-harden", s, ss)) >= 0 )
> >>> +        else if ( IS_ENABLED(CONFIG_SPECULATIVE_HARDEN_BRANCH) &&
> >>> +                  (val = parse_boolean("branch-harden", s, ss)) >= 0 )
> >>>              opt_branch_harden = val;
> >> Yeah, we should definitely fix this, but could we use no_config_param()
> >> here for the compiled-out case ?
> >>
> >> See cet= for an example.  If we're going to ignore what the user asks,
> >> we should tell them why.
> > Maybe I'm missing something: I've looked into using no_config_param(),
> > but there's no difference really, because cmdline_parse() is called
> > before the console is initialized, so those messages seem to be
> > lost.
> 
> Look at `xl dmesg` rather than the console.  They also do appear on vga
> in some configurations.

Oh, my internal buffer was too small on those also got truncated, had
to bump it.

> There's a separate todo to get these out in a slightly nicer way, but
> they at least exist in logs.

I've created:

https://gitlab.com/xen-project/xen/-/issues/184

Thanks, Roger.