[Xen-devel] [PATCH for-4.8] libxc/x86: Report consistent initial APIC value for PV guests

[Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>]

Currently hypervisor provides PV guest's CPUID(1).EBX[31:24] (initial
APIC ID) with contents of that field on the processor that launched
the guest. This results in the guest reporting different initial
APIC IDs across runs.

We should be consistent in how this value is reported, let's set
it to 0 (which is also what Linux guests expect).

Signed-off-by: Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>
---

I think this should go to stable branches as well. This has been causing
problems lately in Linux with introduction of topology maps.


 tools/libxc/xc_cpuid_x86.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/tools/libxc/xc_cpuid_x86.c b/tools/libxc/xc_cpuid_x86.c
index d761805..1f26294 100644
--- a/tools/libxc/xc_cpuid_x86.c
+++ b/tools/libxc/xc_cpuid_x86.c
@@ -618,6 +618,12 @@ static void xc_cpuid_pv_policy(xc_interface *xch,
         /* Host topology exposed to PV guest.  Provide host value. */
         bool host_htt = regs[3] & bitmaskof(X86_FEATURE_HTT);
 
+        /*
+         * Don't pick host's Initial APIC ID which can change from run
+         * to run. 
+         */
+        regs[1] &= 0x00ffffffu;
+
         regs[2] = info->featureset[featureword_of(X86_FEATURE_SSE3)];
         regs[3] = (info->featureset[featureword_of(X86_FEATURE_FPU)] &
                    ~bitmaskof(X86_FEATURE_HTT));
-- 
2.7.4


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