From: George Dunlap Date: Tue, 26 Nov 2019 10:32:42 +0000 (+0000) Subject: x86: Don't increase ApicIdCoreSize past 7 X-Git-Url: http://xenbits.xensource.com/gitweb?a=commitdiff_plain;h=8c79c129a6db2220c1089e0ce5fa49e7298b1d3e;p=people%2Fliuw%2Fxen.git 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 Reported-by: Andreas Kinzler Signed-off-by: George Dunlap Acked-by: Jan Beulich Release-acked-by: Juergen Gross --- 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;