x86: Don't increase ApicIdCoreSize past 7

Changeset ca2eee92df ("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@crc.id.au>
Reported-by: Andreas Kinzler <hfp@posteo.de>
Signed-off-by: George Dunlap <george.dunlap@citrix.com>
Acked-by: Jan Beulich <jbeulich@suse.com>
master commit: 8c79c129a6
master date: 2019-11-26 10:33:52 +0000

tools/libxc/xc_cpuid_x86.c

@@ -414,10 +414,15 @@ static void amd_xc_cpuid_policy(const struct cpuid_domain_info *info,
          * - 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: In 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 ( (regs[2] & 0xffu) < 0x7fu )
         {
-            if ( (regs[2] & 0xf000u) && (regs[2] & 0xf000u) != 0xf000u )
+            if ( (regs[2] & 0xf000u) && (regs[2] & 0xf000u) < 0x7000u )
                 regs[2] = ((regs[2] + 0x1000u) & 0xf000u) | (regs[2] & 0xffu);
             regs[2] = (regs[2] & 0xf000u) | ((regs[2] & 0x7fu) << 1) | 1u;
         }