1================ 2RAPL MSR support 3================ 4 5The RAPL interface (Running Average Power Limit) is advertising the accumulated 6energy consumption of various power domains (e.g. CPU packages, DRAM, etc.). 7 8The consumption is reported via MSRs (model specific registers) like 9MSR_PKG_ENERGY_STATUS for the CPU package power domain. These MSRs are 64 bits 10registers that represent the accumulated energy consumption in micro Joules. 11 12Thanks to the MSR Filtering patch [#a]_ not all MSRs are handled by KVM. Some 13of them can now be handled by the userspace (QEMU). It uses a mechanism called 14"MSR filtering" where a list of MSRs is given at init time of a VM to KVM so 15that a callback is put in place. The design of this patch uses only this 16mechanism for handling the MSRs between guest/host. 17 18At the moment the following MSRs are involved: 19 20.. code:: C 21 22 #define MSR_RAPL_POWER_UNIT 0x00000606 23 #define MSR_PKG_POWER_LIMIT 0x00000610 24 #define MSR_PKG_ENERGY_STATUS 0x00000611 25 #define MSR_PKG_POWER_INFO 0x00000614 26 27The ``*_POWER_UNIT``, ``*_POWER_LIMIT``, ``*_POWER INFO`` are part of the RAPL 28spec and specify the power limit of the package, provide range of parameter(min 29power, max power,..) and also the information of the multiplier for the energy 30counter to calculate the power. Those MSRs are populated once at the beginning 31by reading the host CPU MSRs and are given back to the guest 1:1 when 32requested. 33 34The MSR_PKG_ENERGY_STATUS is a counter; it represents the total amount of 35energy consumed since the last time the register was cleared. If you multiply 36it with the UNIT provided above you'll get the power in micro-joules. This 37counter is always increasing and it increases more or less faster depending on 38the consumption of the package. This counter is supposed to overflow at some 39point. 40 41Each core belonging to the same Package reading the MSR_PKG_ENERGY_STATUS (i.e 42"rdmsr 0x611") will retrieve the same value. The value represents the energy 43for the whole package. Whatever Core reading it will get the same value and a 44core that belongs to PKG-0 will not be able to get the value of PKG-1 and 45vice-versa. 46 47High level implementation 48------------------------- 49 50In order to update the value of the virtual MSR, a QEMU thread is created. 51The thread is basically just an infinity loop that does: 52 531. Snapshot of the time metrics of all QEMU threads (Time spent scheduled in 54 Userspace and System) 55 562. Snapshot of the actual MSR_PKG_ENERGY_STATUS counter of all packages where 57 the QEMU threads are running on. 58 593. Sleep for 1 second - During this pause the vcpu and other non-vcpu threads 60 will do what they have to do and so the energy counter will increase. 61 624. Repeat 2. and 3. and calculate the delta of every metrics representing the 63 time spent scheduled for each QEMU thread *and* the energy spent by the 64 packages during the pause. 65 665. Filter the vcpu threads and the non-vcpu threads. 67 686. Retrieve the topology of the Virtual Machine. This helps identify which 69 vCPU is running on which virtual package. 70 717. The total energy spent by the non-vcpu threads is divided by the number 72 of vcpu threads so that each vcpu thread will get an equal part of the 73 energy spent by the QEMU workers. 74 758. Calculate the ratio of energy spent per vcpu threads. 76 779. Calculate the energy for each virtual package. 78 7910. The virtual MSRs are updated for each virtual package. Each vCPU that 80 belongs to the same package will return the same value when accessing the 81 the MSR. 82 8311. Loop back to 1. 84 85Ratio calculation 86----------------- 87 88In Linux, a process has an execution time associated with it. The scheduler is 89dividing the time in clock ticks. The number of clock ticks per second can be 90found by the sysconf system call. A typical value of clock ticks per second is 91100. So a core can run a process at the maximum of 100 ticks per second. If a 92package has 4 cores, 400 ticks maximum can be scheduled on all the cores 93of the package for a period of 1 second. 94 95The /proc/[pid]/stat [#b]_ is a sysfs file that can give the executed time of a 96process with the [pid] as the process ID. It gives the amount of ticks the 97process has been scheduled in userspace (utime) and kernel space (stime). 98 99By reading those metrics for a thread, one can calculate the ratio of time the 100package has spent executing the thread. 101 102Example: 103 104A 4 cores package can schedule a maximum of 400 ticks per second with 100 ticks 105per second per core. If a thread was scheduled for 100 ticks between a second 106on this package, that means my thread has been scheduled for 1/4 of the whole 107package. With that, the calculation of the energy spent by the thread on this 108package during this whole second is 1/4 of the total energy spent by the 109package. 110 111Usage 112----- 113 114Currently this feature is only working on an Intel CPU that has the RAPL driver 115mounted and available in the sysfs. if not, QEMU fails at start-up. 116 117This feature is activated with -accel 118kvm,rapl=true,rapl-helper-socket=/path/sock.sock 119 120It is important that the socket path is the same as the one 121:program:`qemu-vmsr-helper` is listening to. 122 123qemu-vmsr-helper 124---------------- 125 126The qemu-vmsr-helper is working very much like the qemu-pr-helper. Instead of 127making persistent reservation, qemu-vmsr-helper is here to overcome the 128CVE-2020-8694 which remove user access to the rapl msr attributes. 129 130A socket communication is established between QEMU processes that has the RAPL 131MSR support activated and the qemu-vmsr-helper. A systemd service and socket 132activation is provided in contrib/systemd/qemu-vmsr-helper.(service/socket). 133 134The systemd socket uses 600, like contrib/systemd/qemu-pr-helper.socket. The 135socket can be passed via SCM_RIGHTS by libvirt, or its permissions can be 136changed (e.g. 660 and root:kvm for a Debian system for example). Libvirt could 137also start a separate helper if needed. All in all, the policy is left to the 138user. 139 140See the qemu-pr-helper documentation or manpage for further details. 141 142Current Limitations 143------------------- 144 145- Works only on Intel host CPUs because AMD CPUs are using different MSR 146 addresses. 147 148- Only the Package Power-Plane (MSR_PKG_ENERGY_STATUS) is reported at the 149 moment. 150 151References 152---------- 153 154.. [#a] https://patchwork.kernel.org/project/kvm/patch/20200916202951.23760-7-graf@amazon.com/ 155.. [#b] https://man7.org/linux/man-pages/man5/proc.5.html 156