On 30.01.2024 10:13, Roger Pau Monne wrote:
> The CPUID feature bit signals the presence of the IPRED_DIS_{U,S} controls in
> SPEC_CTRL MSR.
>
> Note that those controls are not used by the hypervisor.
Despite this, ...
> --- a/xen/arch/x86/msr.c
> +++ b/xen/arch/x86/msr.c
> @@ -324,6 +324,9 @@ uint64_t msr_spec_ctrl_valid_bits(const struct cpu_policy
> *cp)
> return (SPEC_CTRL_IBRS | SPEC_CTRL_STIBP |
> (ssbd ? SPEC_CTRL_SSBD : 0) |
> (psfd ? SPEC_CTRL_PSFD : 0) |
> + (cp->feat.ipred_ctrl ? (SPEC_CTRL_IPRED_DIS_U |
> + SPEC_CTRL_IPRED_DIS_S)
> + : 0) |
> 0);
> }
... if I'm not mistaken exposing SPEC_CTRL bits to guests is independent
of whether we write SPEC_CTRL on entry to Xen. Therefore I think in the
description it wants clarifying why it is acceptable to run Xen with the
guest chosen settings for at least the DIS_S bit (assuming that it is
okay to do so). Likely (didn't look there yet) also applicable to the
further two patches.
Jan
