** Changed in: linux-aws (Ubuntu Noble)
Status: In Progress => Fix Committed
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https://bugs.launchpad.net/bugs/2080569
Title:
AWS: support Out-Of-Band pstate mode for Emerald Rapids
Status in linux-aws package in Ubuntu:
Fix Released
Status in linux-aws source package in Noble:
Fix Committed
Bug description:
SRU Justification:
[Impact]
In Emerald Rapids VMs, a stack trace is printed during boot on 6.8
based kernels, but not 6.5 ones. The stack traces look like:
[ 1.206658] intel_pstate: Intel P-state driver initializing
[ 1.207453] unchecked MSR access error: WRMSR to 0x199 (tried to write
0x0000000000000800) at rIP: 0xffffffffb94c3b24 (native_write_msr+0x4/0x40)
[ 1.208422] Call Trace:
[ 1.208422] <TASK>
[ 1.208422] ? show_stack_regs+0x23/0x40
[ 1.208422] ? ex_handler_msr+0x10a/0x180
[ 1.208422] ? fixup_exception+0x183/0x390
[ 1.208422] ? gp_try_fixup_and_notify+0x23/0xc0
[ 1.208422] ? exc_general_protection+0x15e/0x480
[ 1.208422] ? asm_exc_general_protection+0x27/0x30
[ 1.208422] ? __pfx___wrmsr_on_cpu+0x10/0x10
[ 1.208422] ? native_write_msr+0x4/0x40
[ 1.208422] ? __wrmsr_on_cpu+0x4b/0x90
[ 1.208422] ? __pfx___rdmsr_on_cpu+0x10/0x10
[ 1.208422] ? __pfx___wrmsr_on_cpu+0x10/0x10
[ 1.208422] generic_exec_single+0x7e/0x120
[ 1.208422] smp_call_function_single+0x103/0x140
[ 1.208422] ? __pfx___wrmsr_on_cpu+0x10/0x10
[ 1.208422] wrmsrl_on_cpu+0x57/0x80
[ 1.208422] intel_pstate_set_pstate+0x3e/0x80
[ 1.208422] intel_pstate_get_cpu_pstates.constprop.0+0xd7/0x190
[ 1.208422] intel_pstate_init_cpu+0x3f/0x140
[ 1.208422] intel_cpufreq_cpu_init+0x44/0x270
[ 1.208422] ? freq_qos_add_notifier+0x45/0x80
[ 1.208422] cpufreq_online+0x444/0xb80
[ 1.208422] cpufreq_add_dev+0x99/0xd0
[ 1.208422] subsys_interface_register+0x11c/0x140
[ 1.208422] cpufreq_register_driver+0x1b5/0x330
[ 1.208422] intel_pstate_register_driver+0x48/0xd0
[ 1.208422] intel_pstate_init+0x25c/0x810
[ 1.208422] ? __pfx_intel_pstate_init+0x10/0x10
[ 1.208422] do_one_initcall+0x5b/0x310
[ 1.208422] do_initcalls+0x104/0x210
[ 1.208422] ? __pfx_kernel_init+0x10/0x10
[ 1.208422] kernel_init_freeable+0x134/0x1f0
[ 1.208422] kernel_init+0x1b/0x200
[ 1.208422] ret_from_fork+0x44/0x70
[ 1.208422] ? __pfx_kernel_init+0x10/0x10
[ 1.208422] ret_from_fork_asm+0x1b/0x30
[ 1.208422] </TASK>
[Fix]
The reason that these traces are not found in the 6.5 kernel, but are
in the 6.8 kernel one had the traces is that in the 6.5 kernel the CPU
in the instance (Emerald Rapids) does not have the pstate support
added yet. The 6.8 kernel does have support though. That is why they
don't appear in the 6.5 kernel, but in the 6.8 kernel.
In the 6.11 kernel the patch
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=7e1c3f584ee78b0d0210fc424420d9529f3ca952
was added to prevent trying to set the p-states while in OOB (Out-of-
band) mode. So, rather than having the VM trying to set the p-states,
it is handled by the host OS/platform.
[Test Plan]
I have tested this.
[Where problems could occur]
In the 6.11 kernel tree where this change comes from, there has been some
significant refactoring of the code. If other changes need to be back-ported
to the AWS 6.8 kernel, other conflicts or issues may arise.
[Other]
SF #00395865
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