New: Nested virt on AMD (and probably older Intel) doesn't work with ignore_msrs=Y on L0 (fails with UMWAIT error)
https://bugzilla.kernel.org/show_bug.cgi?id=206877
Bug ID: 206877
Summary: Nested virt on AMD (and probably older Intel) doesn't
work with ignore_msrs=Y on L0 (fails with UMWAIT
error)
Product: Virtualization
Version: unspecified
Kernel Version: 5.6.0-rc6
Hardware: i386
OS: Linux
Tree: Mainline
Status: NEW
Severity: normal
Priority: P1
Component: kvm
Assignee: [email protected]
Reporter: [email protected]
Regression: NoI'm not sure if this is a supported configuration, but since the commit mentioned below nested virtualization on AMD when the host has 'ignore_msrs=Y' is broken. QEMU fails with:
kvm: error: failed to set MSR 0xe1 to 0x0
kvm: /qemu/target/i386/kvm.c:2947: kvm_put_msrs: Assertion `ret == cpu->kvm_msr_buf->nmsrs' failed.
If this is supposed to work, it's a regression from 6e3ba4abcea5 ("KVM: vmx:
Emulate MSR IA32_UMWAIT_CONTROL"), I can confirm that reverting this commit for the guest kernel makes everything work again. Ignoring UMWAIT in QEMU (kvm_get_supported_msrs) does the trick too.
I *think* this happens since MSR_IA32_UMWAIT_CONTROL (in msrs_to_save_all) is added to the guest CPUID with the only condition being that 'rdmsr_safe' in 'kvm_init_msr_list' succeeds - which it does, since the host ignores it. However, since the CPU doesn't actually support UMWAIT (in my case since it's an AMD chip, but I suppose the same happens on older Intel ones) the MSR set for the L2 guest fails.
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[Bug 206877] Nested virt on AMD (and probably older Intel) doesn't work with ignore_msrs=Y on L0 (fails with UMWAIT error)
https://bugzilla.kernel.org/show_bug.cgi?id=206877
Maxim Levitsky ([email protected]) changed:
What |Removed |Added
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CC| |[email protected]
--- Comment #1 from Maxim Levitsky ([email protected]) ---
I fixed this upstream, the patch should be in 5.8 kernel I think
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