File 5d89d8d9-libxc-x86-avoid-overflow-in-CPUID-APIC... of Package xen.17648

Overview Repositories Revisions Requests Users Attributes Meta

File 5d89d8d9-libxc-x86-avoid-overflow-in-CPUID-APIC-ID.patch of Package xen.17648

References: bsc#1137717

# Commit df29d03f1d97bdde1bc0cea8ef8538d4f524b3ec
# Date 2019-09-24 10:50:33 +0200
# Author Jan Beulich <jbeulich@suse.com>
# Committer Jan Beulich <jbeulich@suse.com>
libxc/x86: avoid certain overflows in CPUID APIC ID adjustments

Recent AMD processors may report up to 128 logical processors in CPUID
leaf 1. Doubling this value produces 0 (which OSes sincerely dislike),
as the respective field is only 8 bits wide. Suppress doubling the value
(and its leaf 0x80000008 counterpart) in such a case.

Note that while there's a similar overflow in intel_xc_cpuid_policy(),
that one is being left alone for now.

Note further that while it was considered to suppress the multiplication
by 2 altogether if the host topology already provides at least one bit
of thread ID within APIC IDs, it was decided to avoid more change here
than really needed at this point.

Also zap leaf 4 (and at the same time leaf 2) EDX output for AMD, as it
should have been from the beginning.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>

# Commit c9c7ac508b3f65f7d5f9685893096a1b22d8b176
# Date 2019-09-25 15:50:58 +0200
# Author Jan Beulich <jbeulich@suse.com>
# Committer Jan Beulich <jbeulich@suse.com>
libxc/x86: correct overflow avoidance check in AMD CPUID handling

Commit df29d03f1d ("libxc/x86: avoid certain overflows in CPUID APIC ID
adjustments" introduced a one bit too narrow mask when checking whether
multiplying by 1 (in particular in leaf 1) would result in overflow.

Reported-by: Andrew Cooper <andrew.cooper3@citrix.com>
Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>

# Commit 8c79c129a6db2220c1089e0ce5fa49e7298b1d3e
# Date 2019-11-26 10:33:52 +0000
# Author George Dunlap <george.dunlap@citrix.com>
# Committer George Dunlap <george.dunlap@citrix.com>
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@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>

--- a/tools/libxc/xc_cpuid_x86.c
+++ b/tools/libxc/xc_cpuid_x86.c
@@ -335,7 +335,7 @@ static void amd_xc_cpuid_policy(xc_inter
     {
     case 0x00000002:
     case 0x00000004:
-        regs[0] = regs[1] = regs[2] = 0;
+        regs[0] = regs[1] = regs[2] = regs[3] = 0;
         break;
 
     case 0x80000000:
@@ -345,11 +345,24 @@ static void amd_xc_cpuid_policy(xc_inter
 
     case 0x80000008:
         /*
-         * ECX[15:12] is ApicIdCoreSize: ECX[7:0] is NumberOfCores (minus one).
-         * Update to reflect vLAPIC_ID = vCPU_ID * 2.
+         * ECX[15:12] is ApicIdCoreSize.
+         * ECX[7:0] is NumberOfCores (minus one).
+         * Update to reflect vLAPIC_ID = vCPU_ID * 2.  But make sure to avoid
+         * - overflow,
+         * - 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.
          */
-        regs[2] = ((regs[2] + (1u << 12)) & 0xf000u) |
-                  ((regs[2] & 0xffu) << 1) | 1u;
+        if ( (regs[2] & 0xffu) < 0x7fu )
+        {
+            if ( (regs[2] & 0xf000u) && (regs[2] & 0xf000u) < 0x7000u )
+                regs[2] = (regs[2] + 0x1000u) | (regs[2] & 0xffu);
+            regs[2] = (regs[2] & 0xf000u) | ((regs[2] & 0x7fu) << 1) | 1u;
+        }
         break;
 
     case 0x8000000a: {
@@ -430,9 +443,13 @@ static void xc_cpuid_hvm_policy(xc_inter
     case 0x00000001:
         /*
          * EBX[23:16] is Maximum Logical Processors Per Package.
-         * Update to reflect vLAPIC_ID = vCPU_ID * 2.
+         * Update to reflect vLAPIC_ID = vCPU_ID * 2, but make sure to avoid
+         * overflow.
          */
-        regs[1] = (regs[1] & 0x0000ffffu) | ((regs[1] & 0x007f0000u) << 1);
+        if ( !(regs[1] & 0x00800000u) )
+            regs[1] = (regs[1] & 0x0000ffffu) | ((regs[1] & 0x007f0000u) << 1);
+        else
+            regs[1] &= 0x00ffffffu;
 
         regs[2] = info->featureset[featureword_of(X86_FEATURE_SSE3)];
         regs[3] = (info->featureset[featureword_of(X86_FEATURE_FPU)] |

Places

File 5d89d8d9-libxc-x86-avoid-overflow-in-CPUID-APIC-ID.patch of Package xen.17648

Places