[Xen-devel] [PATCH RFC] x86: Don't increase ApicIdCoreSize past 7

[George Dunlap <george.dunlap@xxxxxxxxxx>]

Changeset ca2eee92df44 ("x86, hvm: Expose host core/HT topology to HVM
guests") attempted to "fake up" a topology which would induce guest
operating systems to not treat vcpus as sibling hyperthreads.  This
involved actually reporting hyperthreading as available, but giving
vcpus every other ApicId; which in turn led to doubling the ApicIds
per core by bumping the ApicIdCoreSize by one.  In particular, Ryzen
3xxx series processors, and reportedly EPYC "Rome" cpus -- have an
ApicIdCoreSize of 7; the "fake" topology increases this to 8.

Unfortunately, Windows running on modern AMD hardware -- including
Ryzen 3xxx series processors, and reportedly EPYC "Rome" cpus --
doesn't seem to cope with this value being higher than 7.  (Linux
guests have so far continued to cope.)

A "proper" fix is complicated and it's too late to fix it either for
4.13, or to backport to supported branches.  As a short-term fix,
limit this value to 7.

This does mean that a Linux guest, booted on such a system without
this change, and then migrating to a system with this change, with
more than 64 vcpus, would see an apparent topology change.  This is a
low enough risk in practice that enabling this limit unilaterally, to
allow other guests to boot without manual intervention, is worth it.

Reported-by: Steven Haigh <netwiz@xxxxxxxxx>
Reported-by: Andreas Kinzler <hfp@xxxxxxxxx>
Signed-off-by: George Dunlap <george.dunlap@xxxxxxxxxx>
---
CC: Ian Jackson <ian.jackson@xxxxxxxxxx>
CC: Wei Liu <wl@xxxxxxx>
CC: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
CC: Jan Beulich <jbeulich@xxxxxxxx>
CC: Juergen Gross <jgross@xxxxxxxx>
---
 tools/libxc/xc_cpuid_x86.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/tools/libxc/xc_cpuid_x86.c b/tools/libxc/xc_cpuid_x86.c
index 312c481f1e..519d6d8bd0 100644
--- a/tools/libxc/xc_cpuid_x86.c
+++ b/tools/libxc/xc_cpuid_x86.c
@@ -616,10 +616,15 @@ int xc_cpuid_apply_policy(xc_interface *xch, uint32_t 
domid,
              * - going out of sync with leaf 1 EBX[23:16],
              * - incrementing ApicIdCoreSize when it's zero (which changes the
              *   meaning of bits 7:0).
+             *
+             * UPDATE: I addition to avoiding overflow, some
+             * proprietary operating systems have trouble with
+             * apic_id_size values greater than 7.  Limit the value to
+             * 7 for now.
              */
             if ( p->extd.nc < 0x7f )
             {
-                if ( p->extd.apic_id_size != 0 && p->extd.apic_id_size != 0xf )
+                if ( p->extd.apic_id_size != 0 && p->extd.apic_id_size < 0x7 )
                     p->extd.apic_id_size++;
 
                 p->extd.nc = (p->extd.nc << 1) | 1;
-- 
2.24.0


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