Attemping to use hwpmc from a Xen guest on a Core i7 running in 64 bit mode, I got the following crash on bootup:

xbd0: 20480MB <Virtual Block Device> at device/vbd/768 on xenbusb_front0
xbd0: attaching as ad0
Timecounter "TSC" frequency 2000036316 Hz quality 800


Fatal trap 12: page fault while in kernel mode
cpuid = 0; apic id = 00
fault virtual address	= 0x0
fault code		= supervisor read instruction, page not present
instruction pointer	= 0x20:0x0
stack pointer	        = 0x28:0xffffffff81985b90
frame pointer	        = 0x28:0xffffffff81985c10
code segment		= base rx0, limit 0xfffff, type 0x1b
			= DPL 0, pres 1, long 1, def32 0, gran 1
processor eflags	= interrupt enabled, resume, IOPL = 0
current process		= 0 (swapper)
[ thread pid 0 tid 100000 ]
Stopped at      0:      *** error reading from address 0 ***
db> where
Tracing pid 0 tid 100000 td 0xffffffff8154f8b0
uart_sab82532_class() at 0
load() at load+0x39
syscall_module_handler() at syscall_module_handler+0xe6
module_register_init() at module_register_init+0xb0
mi_startup() at mi_startup+0x139
btext() at btext+0x2c

The problem was that Xen changes the cpuid that guests see.  Specifically, the result of do_cpuid(CORE_CPUID_REQUEST, cpuid) is changed to indicate that performance counters are not supported.  Hence, pmc_core_initialize() returns EPROGMISMATCH.  Eventually, that causes pmc_initialize() to execute a null function pointer at line 4692.  Really, the root cause of the problem is that hwpmc isn't written very defensively.  I've attached a patch that makes the panic go away, but doesn't make PMC work for my processor.

Fix: 

==== sys/dev/hwpmc/hwpmc_mod.c#2 (text) ====

@@ -4689,7 +4689,15 @@
                if (md->pmd_pcpu_init)
                        error = md->pmd_pcpu_init(md, cpu);
                for (n = 0; error == 0 && n < md->pmd_nclass; n++)
-                       error = md->pmd_classdep[n].pcd_pcpu_init(md, cpu);
+                       if (md->pmd_classdep[n].pcd_pcpu_init != NULL)
+                               error = md->pmd_classdep[n].pcd_pcpu_init(md,
+                                   cpu);
+                       else
+                               printf("hwpmc: md->pmd_classdep[%d]."
+                                   "pcd_pcpu_init was not initialized.  "
+                                   "This is a bug.\n"
+                                   "\tMost likely your CPUID is not "
+                                   "recognized by hwpmc.\n", n);
        }
        pmc_restore_cpu_binding(&pb);
 
@@ -4882,7 +4890,15 @@
                                continue;
                        pmc_select_cpu(cpu);
                        for (c = 0; c < md->pmd_nclass; c++)
-                               md->pmd_classdep[c].pcd_pcpu_fini(md, cpu);
+                               if (md->pmd_classdep[c].pcd_pcpu_fini != NULL)
+                                       md->pmd_classdep[c].pcd_pcpu_fini(md,
+                                           cpu);
+                               else
+                                       printf("hwpmc: md->pmd_classdep[%d]."
+                                           "pcd_pcpu_fini was not initialized."
+                                           "  This is a bug.\n"
+                                           "\tMost likely your CPUID is not "
+                                           "recognized by hwpmc.\n", c);
                        if (md->pmd_pcpu_fini)
                                md->pmd_pcpu_fini(md, cpu);
                }
How-To-Repeat: Compile an amd64 Xen domU kernel with the below lines in your configuration file and boot it:
options    HWPMC_HOOKS
device     hwpmc