Messages in this thread | | | Date | Fri, 4 Aug 2023 20:12:43 +0200 | From | Borislav Petkov <> | Subject | Re: [patch V3 02/40] x86/apic: Fake primary thread mask for XEN/PV |
| |
On Wed, Aug 02, 2023 at 12:21:01PM +0200, Thomas Gleixner wrote: > The SMT control mechanism got added as speculation attack vector > mitigation. The implemented logic relies on the primary thread mask to > be set up properly. > > This turns out to be an issue with XEN/PV guests because their CPU hotplug > mechanics do not enumerate APICs and therefore the mask is never correctly > populated. > > This went unnoticed so far because by chance XEN/PV ends up with > smp_num_siblings == 2. So smt_hot-plug_control stays at its default value
I think this means to say "cpu_smt_control". Committer, pls fix. :-)
> CPU_SMT_ENABLED and the primary thread mask is never evaluated in the > context of CPU hotplug. > > This stopped "working" with the upcoming overhaul of the topology > evaluation which legitimately provides a fake topology for XEN/PV. That > sets smp_num_siblings to 1, which causes the core CPU hot-plug core to > refuse to bring up the APs. > > This happens because smt_hotplug_control is set to CPU_SMT_NOT_SUPPORTED
Ditto.
> which causes cpu_smt_allowed() to evaluate the unpopulated primary thread > mask with the conclusion that all non-boot CPUs are not valid to be > plugged. > > The core code has already been made more robust against this kind of fail, > but the primary thread mask really wants to be populated to avoid other > issues all over the place. > > Just fake the mask by pretending that all XEN/PV vCPUs are primary threads, > which is consistent because all of XEN/PVs topology is fake or non-existent. > > Fixes: 6a4d2657e048 ("x86/smp: Provide topology_is_primary_thread()") > Fixes: f54d4434c281 ("x86/apic: Provide cpu_primary_thread mask") > Reported-by: Juergen Gross <jgross@suse.com> > Signed-off-by: Thomas Gleixner <tglx@linutronix.de> > --- > arch/x86/kernel/apic/apic.c | 11 +++++++++++ > 1 file changed, 11 insertions(+) > > --- a/arch/x86/kernel/apic/apic.c > +++ b/arch/x86/kernel/apic/apic.c > @@ -36,6 +36,8 @@ > #include <linux/smp.h> > #include <linux/mm.h> > > +#include <xen/xen.h> > + > #include <asm/trace/irq_vectors.h> > #include <asm/irq_remapping.h> > #include <asm/pc-conf-reg.h> > @@ -2344,6 +2346,15 @@ static int __init smp_init_primary_threa > { > unsigned int cpu; > > + /* > + * XEN/PV provides either none or useless topology information. > + * Pretend that all vCPUs are primary threads. > + */ > + if (xen_pv_domain()) { > + cpumask_copy(&__cpu_primary_thread_mask, cpu_possible_mask); > + return 0; > + }
Can this be somewhere in the Xen init code instead?
-- Regards/Gruss, Boris.
https://people.kernel.org/tglx/notes-about-netiquette
| |