[Xen-changelog] [xen staging] libxc/x86: avoid certain overflows in CPUID APIC ID adjustments

[patchbot@xxxxxxx]

commit df29d03f1d97bdde1bc0cea8ef8538d4f524b3ec
Author:     Jan Beulich <jbeulich@xxxxxxxx>
AuthorDate: Tue Sep 24 10:50:33 2019 +0200
Commit:     Jan Beulich <jbeulich@xxxxxxxx>
CommitDate: Tue Sep 24 10:50:33 2019 +0200

    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@xxxxxxxx>
    Reviewed-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
---
 tools/libxc/xc_cpuid_x86.c | 27 ++++++++++++++++++++-------
 1 file changed, 20 insertions(+), 7 deletions(-)

diff --git a/tools/libxc/xc_cpuid_x86.c b/tools/libxc/xc_cpuid_x86.c
index b829336082..b472fbc610 100644
--- a/tools/libxc/xc_cpuid_x86.c
+++ b/tools/libxc/xc_cpuid_x86.c
@@ -385,7 +385,7 @@ static void amd_xc_cpuid_policy(const struct 
cpuid_domain_info *info,
     {
     case 0x00000002:
     case 0x00000004:
-        regs[0] = regs[1] = regs[2] = 0;
+        regs[0] = regs[1] = regs[2] = regs[3] = 0;
         break;
 
     case 0x80000000:
@@ -395,11 +395,20 @@ static void amd_xc_cpuid_policy(const struct 
cpuid_domain_info *info,
 
     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).
          */
-        regs[2] = ((regs[2] + (1u << 12)) & 0xf000u) |
-                  ((regs[2] & 0xffu) << 1) | 1u;
+        if ( (regs[2] & 0x7fu) < 0x7fu )
+        {
+            if ( (regs[2] & 0xf000u) && (regs[2] & 0xf000u) != 0xf000u )
+                regs[2] = ((regs[2] + 0x1000u) & 0xf000u) | (regs[2] & 0xffu);
+            regs[2] = (regs[2] & 0xf000u) | ((regs[2] & 0x7fu) << 1) | 1u;
+        }
         break;
 
     case 0x8000000a: {
@@ -478,9 +487,13 @@ static void xc_cpuid_hvm_policy(const struct 
cpuid_domain_info *info,
     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)] |
--
generated by git-patchbot for /home/xen/git/xen.git#staging

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