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Stefano Stabellini
9782e13b10
Introduce a new document about big.LITTLE and update the documentation of hmp-unsafe. Also update the warning messages to point users to the docs. Signed-off-by: Stefano Stabellini <sstabellini@kernel.org> Acked-by: Julien Grall <julien.grall@arm.com> CC: jbeulich@suse.com CC: konrad.wilk@oracle.com CC: tim@xen.org CC: wei.liu2@citrix.com CC: andrew.cooper3@citrix.com CC: George.Dunlap@eu.citrix.com CC: ian.jackson@eu.citrix.com
47 lines
1.9 KiB
Plaintext
47 lines
1.9 KiB
Plaintext
big.LITTLE is a form of heterogeneous computing that comes with two
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types of general purpose cpu cores: big cores, more powerful and with a
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higher power consumption rate, and LITTLE cores, less powerful and
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cheaper to run. For example, Cortex A53 and Cortex A57 cpus. Typically,
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big cores are only recommended for burst activity, especially in
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battery powered environments. Please note that Xen doesn't not use any
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board specific power management techniques at the moment, it only uses
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WFI. It is recommended to check the vendor's big.LITTLE and power
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management documentation before using it in a Xen environment.
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big and LITTLE cores are fully compatible in terms of instruction sets,
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but can differ in many subtle ways. For example, their cacheline sizes
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might differ. For this reason, vcpu migration between big and LITTLE
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cores can lead to data corruptions.
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Today, the Xen scheduler does not have support for big.LITTLE,
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therefore, it might unknowingly move any vcpus between big and LITTLE
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cores, potentially leading to breakages. To avoid this kind of issues,
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at boot time Xen disables all cpus that differ from the boot cpu.
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Expert users can enable all big.LITTLE cores by passing hmp-unsafe=true
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to the Xen command line [1]. Given the lack of big.LITTLE support in the
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scheduler, it is only safe if the cpu affinity of all domains is
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manually specified, so that the scheduler is not allowed to switch a
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vcpu from big to LITTLE or vice versa.
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In the case of dom0, dom0_vcpus_pin needs to be added to the Xen command
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line options [1]. For DomUs, the `cpus' option should be added to all VM
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config files [2].
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For example, if the first 4 cpus are big and the last 4 are LITTLE, the
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following options run all domain vcpus on either big or LITTLE cores
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(not both):
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cpus = "0-3"
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cpus = "4-7"
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The following option runs one domain vcpu as big and one as LITTLE:
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cpus = ["0-3", "4-7"]
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[1] docs/misc/xen-command-line.markdown
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[2] docs/man/xl.cfg.pod.5
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