On 14/03/2024 14:22, Henry Wang wrote:
Hi Julien,
Hi,
On 3/14/2024 9:27 PM, Julien Grall wrote:
Hi Henry,
On 28/02/2024 01:58, Henry Wang wrote:
diff --git a/xen/arch/arm/smpboot.c b/xen/arch/arm/smpboot.c
index a84e706d77..d9ebd55d4a 100644
--- a/xen/arch/arm/smpboot.c
+++ b/xen/arch/arm/smpboot.c
@@ -66,7 +66,6 @@ static bool cpu_is_dead;
/* ID of the PCPU we're running on */
DEFINE_PER_CPU(unsigned int, cpu_id);
-/* XXX these seem awfully x86ish... */
:). I guess at the time we didn't realize that MT was supported on
Arm. It is at least part of the spec, but as Michal pointed out it
doesn't look like a lot of processors supports it.
Yep. Do you think changing the content of this line to something like
"Although multithread is part of the Arm spec, there are not many
processors support multithread and current Xen on Arm assumes there is
no multithread" makes sense to you?
/* representing HT siblings of each logical CPU */
DEFINE_PER_CPU_READ_MOSTLY(cpumask_var_t, cpu_sibling_mask);
/* representing HT and core siblings of each logical CPU */
@@ -89,6 +88,10 @@ static int setup_cpu_sibling_map(int cpu)
cpumask_set_cpu(cpu, per_cpu(cpu_sibling_mask, cpu));
cpumask_set_cpu(cpu, per_cpu(cpu_core_mask, cpu));
+ /* Currently we assume there is no multithread. */
I am not very familiar with the scheduling in Xen. Do you know what's
the consequence of not properly supporting MT? One thing I can think
of is security (I know there were plenty of security issues with SMT).
Unfortunately me neither, so adding George to this thread as I think he
can bring us some insights on this topic from the scheduler perspective.
+Juergen as he worked on co-scheduling.
Depending on the answer, I would consider to print a warning and maybe
add it in SUPPORT.MD in a separate patch.
To be honest, as discussed in v1. I think I am quite tempted to add an
ASSERT(system_cpuinfo.mpidr.mt == 0) to make sure we catch the
multithread support stuff earlier.
ASSERT(...) is not the right solution. You may have user using a Xen
shipped by distros where this would be a NOP.
We could try to hide MT completely from the scheduler. If that's too
difficult, then we could add use warning_add() to throw a warning (see
how we dealt with opt_hmp_unsafe).
I don't really know what will happen
if running current Xen on top of a multithread-implemented processor,
probably it will be fine, but probably some strange behavior will happen
after the boot time which I think will be difficult to debug...
I am not sure what you mean by "strange behavior". AFAIK, you may see
different performance characteristics and more importantly this is a
nest for security issue. But I don't expect any difficult to debug.
Also, looking at the v1 discussion, it sounds like even
cpu_sibling_mask would not be correct. So I think it would make sense
to move the comment on top of setup_cpu_sibling_map.
Sounds good. I will move it in v3.
+ cpumask_or(per_cpu(cpu_core_mask, cpu),
+ per_cpu(cpu_core_mask, cpu), &cpu_possible_map);
AFIACT, per_cpu(cpu_core_mask, ...) will now be equal to
cpu_possible_map. In that case, wouldn't it be better to simply copy
the cpumask?
You mean cpumask_copy(per_cpu(cpu_core_mask, cpu), &cpu_possible_map)?
Yes.
Cheers,
--
Julien Grall
