>>>> We have seen a significant reduction in performance in our research DomU
>>>> OS kernel when running on Fedora 16 with Linux 3.4.2 vs. 3.3.7. We run
>>> a series of benchmarks which are DomU-kernel-space-CPU-heavy; many of
>>>> these run 10x slower when using the 3.4.2 Linux kernel as Dom0.

>>>> This is a little surprising---we've been tracking the Fedora kernels for
>>>> a long time with no problem like this. Did anyone else notice any changes?

>>> Just to verify.. both the 3.3.7 and 3.4.2 Linux kernel are 'release' 
>>> builds? 
>>> and not debug-versions from rawhide? 

>> Yes, they are the Fedora 16 release builds.
> The commits that went in (3.4) were:
> 1fd1443 xen/Kconfig: fix Kconfig layout
> 76a8df7 xen/pci: don't use PCI BIOS service for configuration space accesses
> b7e5ffe xen/pte: Fix crashes when trying to see non-existent PGD/PMD/PUD/PTEs
> 558daa2 xen/apic: Return the APIC ID (and version) for CPU 0.
> a7a97c6 drivers/video/xen-fbfront.c: add missing cleanup code
> 7eb7ce4 xen: correctly check for pending events when restoring irq flags
> b930fe5 xen/acpi: Workaround broken BIOSes exporting non-existing C-states.
> cf405ae xen/smp: Fix crash when booting with ACPI hotplug CPUs.
> 521394e xen: use the pirq number to check the pirq_eoi_map
> df88b2d xen/enlighten: Disable MWAIT_LEAF so that acpi-pad won't be loaded.
> cd74257 x86, acpi: Call acpi_enter_sleep_state via an asmlinkage C function 
> from assembler
> 2a14e54 ACPI: Convert wake_sleep_flags to a value instead of function
> 3d81acb Revert "xen/p2m: m2p_find_override: use list_for_each_entry_safe"
> 186bab1 xen/resume: Fix compile warnings.
> 3066616 xen/xenbus: Add quirk to deal with misconfigured backends.
> a71e23d xen/blkback: Fix warning error.
> b960d6c xen/p2m: m2p_find_override: use list_for_each_entry_safe
> e8e937b xen/gntdev: do not set VM_PFNMAP
> 6b5e7d9 xen/grant-table: add error-handling code on failure of gnttab_resume
> f09d843 xen/pcifront: avoid pci_frontend_enable_msix() falsely returning 
> success
> 0ee46ec xen/pciback: fix XEN_PCI_OP_enable_msix result
> e8c9e78 xen/smp: Remove unnecessary call to smp_processor_id()
> 2531d64 xen/x86: Workaround 'x86/ioapic: Add register level checks to detect 
> bogus io-apic entries'
> e95ae5a xen: only check xen_platform_pci_unplug if hvm
> 136d249 x86/ioapic: Add io_apic_ops driver layer to allow interception
> 3389bb8 xen/blkback: Make optional features be really optional.
> 4dae767 xen/blkback: Squash the discard support for 'file' and 'phy' type.
> df7a3ee xen/acpi: Fix Kconfig dependency on CPU_FREQ
> f132c5b Fix full_name_hash() behaviour when length is a multiple of 8
> b9136d2 xen: initialize platform-pci even if xen_emul_unplug=never
> 106b443 xen/smp: Fix bringup bug in AP code.
> 27257fc xen/acpi: Remove the WARN's as they just create noise.
> 8e6f7c2 xen/tmem: cleanup
> 9846ff1 xen: support pirq_eoi_map
> 102b208 xen/acpi-processor: Do not depend on CPU frequency scaling drivers.
> 48cdd82 xen/cpufreq: Disable the cpu frequency scaling drivers from loading.
> 448c8b1 provide disable_cpufreq() function to disable the API.
> 3467811 xen-blkfront: make blkif_io_lock spinlock per-device
> dad5cf6 xen/blkfront: don't put bdev right after getting it
> 34ae2e4 xen-blkfront: use bitmap_set() and bitmap_clear()
> b2167ba xen/blkback: Enable blkback on HVM guests
> 4f14faa xen/blkback: use grant-table.c hypercall wrappers
> 4bc25af xen kconfig: relax INPUT_XEN_KBDDEV_FRONTEND deps
> a7b422c provide disable_cpufreq() function to disable the API.
> 59a5680 xen/acpi-processor: C and P-state driver that uploads said data to 
> hypervisor.
> ead1d01 xen: constify all instances of "struct attribute_group"
> 42c46e6 xen/xenbus: ignore console/0
> cf8e019 hvc_xen: introduce HVC_XEN_FRONTEND
> 02e19f9 hvc_xen: implement multiconsole support
> eb5ef07 hvc_xen: support PV on HVM consoles
> bd0d5aa xenbus: don't free other end details too early
> a1f37788 tboot: Add return values for tboot_sleep
> 09f98a8 x86, acpi, tboot: Have a ACPI os prepare sleep instead of calling 
> tboot_sleep.
> 73c154c xen/enlighten: Expose MWAIT and MWAIT_LEAF if hypervisor OKs it.
> cc7335b xen/setup/pm/acpi: Remove the call to boot_option_idle_override.
> 5ac0800 xenbus: address compiler warnings
> 1160831 xen/pciback: Support pci_reset_function, aka FLR or D3 support.
> 6fbf9e7 PCI: Introduce __pci_reset_function_locked to be used when holding 
> device_lock.
> cf66f9d xen/netfront: add netconsole support.
> f3ff924 Remove useless get_driver()/put_driver() calls
> 2113f46 xen: use this_cpu_xxx replace percpu_xxx funcs
> cd9db80 xen/pciback: Support pci_reset_function, aka FLR or D3 support.
> a96d627 pci: Introduce __pci_reset_function_locked to be used when holding 
> device_lock.
> 8605c68 xen: Utilize the restore_msi_irqs hook.

I'm having trouble with the Dom0 performance of this kernel too (vs. its
effect on DomU performance). I submitted a bug with a reproducible
test case:



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