[Bug 206877] New: Nested virt on AMD (and probably older Intel) doesn't work with ignore_msrs=Y on L0 (fails with UMWAIT error)

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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: virtualization_kvm@kernel-bugs.osdl.org
          Reporter: s.reiter@proxmox.com
        Regression: No

I'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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