On Tue, Feb 14, 2023 at 05:11:40PM +0100, Jan Beulich wrote:
> When the outgoing vCPU had IBPB issued and RSB overwritten upon entering
> Xen, then there's no need for a 2nd barrier during context switch.
>
> Note that SCF_entry_ibpb is always clear for the idle domain, so no
> explicit idle domain check is needed to augment the feature check
> (which is simply inapplicable to "idle").
>
> Signed-off-by: Jan Beulich <jbeul...@suse.com>
Acked-by: Roger Pau Monné <roger....@citrix.com>
> ---
> v4: Tighten the condition.
> v3: Fold into series.
> ---
> I think in principle we could limit the impact from finding the idle
> domain as "prevd", by having __context_switch() tell us what kind
> domain's vCPU was switched out (it could still be "idle", but in fewer
> cases).
>
> --- a/xen/arch/x86/domain.c
> +++ b/xen/arch/x86/domain.c
> @@ -2005,17 +2005,26 @@ void context_switch(struct vcpu *prev, s
> }
> else
> {
> + unsigned int feat_sc_rsb = X86_FEATURE_SC_RSB_HVM;
> +
> __context_switch();
>
> /* Re-enable interrupts before restoring state which may fault. */
> local_irq_enable();
>
> if ( is_pv_domain(nextd) )
> + {
> load_segments(next);
>
> + feat_sc_rsb = X86_FEATURE_SC_RSB_PV;
> + }
> +
> ctxt_switch_levelling(next);
>
> - if ( opt_ibpb_ctxt_switch && !is_idle_domain(nextd) )
> + if ( opt_ibpb_ctxt_switch && !is_idle_domain(nextd) &&
> + (!(prevd->arch.spec_ctrl_flags & SCF_entry_ibpb) ||
> + /* is_idle_domain(prevd) || */
I would rather add a comment to note that the idle domain always has
SCF_entry_ibpb clear, rather than leaving this commented check in the
condition.
> + !boot_cpu_has(feat_sc_rsb)) )
I do wonder if it would be more fail safe (and easier to expand going
forward) if we introduce a new cpu_info field to track the CPU state:
relevant here would be whether RSB has been overwritten and IBPB
executed. Such state would be cleared on each return from guest path.
Thanks, Roger.
