[Xen-changelog] [xen master] x86: Don't increase ApicIdCoreSize past 7

[patchbot@xxxxxxx]

commit 8c79c129a6db2220c1089e0ce5fa49e7298b1d3e
Author:     George Dunlap <george.dunlap@xxxxxxxxxx>
AuthorDate: Tue Nov 26 10:32:42 2019 +0000
Commit:     George Dunlap <george.dunlap@xxxxxxxxxx>
CommitDate: Tue Nov 26 10:33:52 2019 +0000

    x86: Don't increase ApicIdCoreSize past 7
    
    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>
    Acked-by: Jan Beulich <jbeulich@xxxxxxxx>
    Release-acked-by: 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;
--
generated by git-patchbot for /home/xen/git/xen.git#master

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