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id = 1595
title = "CPU boot sometimes fails on big.LITTLE CPUs with varying cache sizes"
state = "closed"
created_at = "2023-04-12T09:56:04.614Z"
closed_at = "2023-04-14T10:39:21.731Z"
labels = []
url = "https://gitlab.com/qemu-project/qemu/-/issues/1595"
host-os = "Debian sid"
host-arch = "AArch64"
qemu-version = "7.2.0, also tested 7.2.93 (i.e. 8.0.0-rc3)"
guest-os = "Linux"
guest-arch = "AArch64"
description = """The RK3588 SoC has three core clusters; one with A55 cores, and the other two have A76 cores. The big cores have more L2 cache than the little cores, so the value of `CCSIDR` depends on the core that it is read from.
In `write_list_to_kvmstate`, QEMU attempts to use `KVM_SET_ONE_REG` with an ID for `KVM_REG_ARM_DEMUX_ID_CCSIDR`, trying to set `CCSIDR` to a previously read value.
Normally, that works fine, but if the host kernel has moved QEMU from one core cluster to the other, then the value will be different and `demux_c15_set` will return `EINVAL`, causing the entire `arm_set_cpu_on` to fail, and the guest kernel to print an error.
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/arch/arm64/kvm/sys_regs.c?h=v6.2#n2827
I tried changing the condition for the `ok = false` line in `write_list_to_kvmstate` to `ret && r.id >> 8 != 0x60200000001100`. This causes all CPUs to initialize correctly in the guest, but obviously that's a hack.
I assume that `CCSIDR` not being uniform across all CPUs means that the guest's copy of `CCSIDR` may be wrong, and so cache maintenance operations may not act on the entire cache. I do not know whether that could actually cause problems. Will QEMU need to find the maximum cache size across all CPUs and present that to guests?"""
reproduce = """On a SoC where big and little cores have different cache sizes (e.g. RK3588):
```text
$ qemu-system-aarch64 -M virt -accel kvm -cpu host -smp 4 -nographic -kernel arch/arm64/boot/Image -append quiet
[ 0.001399][ T1] psci: failed to boot CPU1 (-22)
[ 0.001407][ T1] CPU1: failed to boot: -22
[ 0.001685][ T1] psci: failed to boot CPU2 (-22)
[ 0.001691][ T1] CPU2: failed to boot: -22
[ 0.001809][ T1] psci: failed to boot CPU3 (-22)
[ 0.001814][ T1] CPU3: failed to boot: -22
```
The error is not always printed, because it depends on which core cluster the processes are scheduled on.
Using `taskset -c 0-3` or `taskset -c 4-7` to force QEMU to stick to the little or big cores respectively makes the bug not reproduce."""
additional = "n/a"
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