On 16.03.2022 15:00, Andrew Cooper wrote:
> STIBP and PSFD are slightly weird bits, because they're both implied by other
> bits in MSR_SPEC_CTRL. Add fine grain controls for them, and take the
> implications into account when setting IBRS/SSBD.
>
> Rearrange the IBPB text/variables/logic to keep all the MSR_SPEC_CTRL bits
> together, for consistency.
>
> However, AMD have a hardware hint CPUID bit recommending that STIBP be set
> uniaterally. This is advertised on Zen3, so follow the recommendation. This
> is the only default change.
>
> Signed-off-by: Andrew Cooper <andrew.coop...@citrix.com>
In principle
Reviewed-by: Jan Beulich <jbeul...@suse.com>
but I have two comments:
> @@ -170,12 +174,18 @@ static int __init cf_check parse_spec_ctrl(const char
> *s)
> else
> rc = -EINVAL;
> }
> +
> else if ( (val = parse_boolean("ibrs", s, ss)) >= 0 )
> opt_ibrs = val;
> - else if ( (val = parse_boolean("ibpb", s, ss)) >= 0 )
> - opt_ibpb = val;
> + else if ( (val = parse_boolean("stibp", s, ss)) >= 0 )
> + opt_stibp = val;
> else if ( (val = parse_boolean("ssbd", s, ss)) >= 0 )
> opt_ssbd = val;
> + else if ( (val = parse_boolean("psfd", s, ss)) >= 0 )
> + opt_psfd = val;
> +
> + else if ( (val = parse_boolean("ibpb", s, ss)) >= 0 )
> + opt_ibpb = val;
> else if ( (val = parse_boolean("eager-fpu", s, ss)) >= 0 )
> opt_eager_fpu = val;
> else if ( (val = parse_boolean("l1d-flush", s, ss)) >= 0 )
Personally I find blank lines ahead of "else if" misleading. Could I
talk you into moving ibrs+stibp and ssbd+psfd close to the end of this
(immediately ahead of "else"), prefixing each pair with a comment about
one feature implying the other (and hence the comments replacing the
blank lines)?
Otoh I notice that we already have blank lines elsewhere in the middle
if this block of code, but at least there they're accompanied by a
comment making more obvious why there is such separation. Which means
as an intermediate approach I'd be okay with no re-ordering, but with
comments added.
> @@ -1070,12 +1083,50 @@ void __init init_speculation_mitigations(void)
>
> /* If we have IBRS available, see whether we should use it. */
> if ( has_spec_ctrl && ibrs )
> + {
> + /* IBRS implies STIBP. */
> + if ( opt_stibp == -1 )
> + opt_stibp = 1;
> +
> default_xen_spec_ctrl |= SPEC_CTRL_IBRS;
> + }
> +
> + /* Use STIBP by default if the hardware hint is set. */
> + if ( opt_stibp == -1 && boot_cpu_has(X86_FEATURE_STIBP_ALWAYS) )
> + opt_stibp = 1;
> +
> + /*
> + * Otherwise, don't use STIBP by default. It has some severe performance
> + * implications on older hardware.
> + */
> + if ( opt_stibp == -1 )
> + opt_stibp = 0;
I'd find this easier to read if written along the lines of
if ( opt_stibp == -1 )
{
/*
* Use STIBP by default if the hardware hint is set. Otherwise,
* don't use STIBP by default. It has some severe performance
* implications on older hardware.
*/
opt_stibp = !!boot_cpu_has(X86_FEATURE_STIBP_ALWAYS);
}
FTAOD I'm not going to insist on either adjustment.
Jan
