Guests using IBRS incur a large performance penalty

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Hi Vincent, thanks for your report - really good research, even indicating a potential fix.
I manage to reproduce this issue using the following steps:

1) Be sure to run Ubuntu 16.04 on both guest and host, both
running kernel 4.4.0 (could be latest version of this kernel)

2) In the guest, install the package "msr-tools", like:
"sudo apt install msr-tools"

3) Still on guest, run: "sudo modprobe msr; sudo wrmsr 0x48 1"
This will enable the MSR bit for IBRS inside the guest

4) Now the host shows bad performance

If we run "sudo wrmsr 0x48 0" in the guest, host gets its performance back.

I'll investigate some commits upstream, including the one you suggested, and once we figure the exact fix for this, will request SRU to the kernel team.
Thanks,

Guilherme

Gavin Guo had provided a patch to fix this issue, it's a SRU candidate: https://lists.ubuntu.com/archives/kernel-team/2018-November/096844.html

I'll update this issue accordingly, thanks Gavin!
Cheers,

Guilherme

Vincent, I'll update the description to match the SRU requirements, but your original description will still be available.

The update to stable 4.4.169 will fix the majority of the issue.

This bug is awaiting verification that the kernel in -proposed solves the problem. Please test the kernel and update this bug with the results. If the problem is solved, change the tag 'verification-needed-xenial' to 'verification-done-xenial'. If the problem still exists, change the tag 'verification-needed-xenial' to 'verification-failed-xenial'.

If verification is not done by 5 working days from today, this fix will be dropped from the source code, and this bug will be closed.

See https://wiki.ubuntu.com/Testing/EnableProposed for documentation how to enable and use -proposed. Thank you!

Final reminder: We are at the end of the SRU Cycle and request that you please provide verification the kernel in proposed resolves the problem for which this bug was submitted. -Thank you!

I've managed to verify this on kernel 4.4.0-143-generic, the problem isn't reproducible anymore.

Worth noticing that when using the kernel 4.4.0-143 in both host and guest, we cannot change MSR bit anymore from within the guest, so the performance drop is not observed.

This bug was fixed in the package linux - 4.4.0-143.169

---------------
linux (4.4.0-143.169) xenial; urgency=medium

  * linux: 4.4.0-143.169 -proposed tracker (LP: #1814647)

  * x86/kvm: Backport fixup and missing commits (LP: #1811646)
    - KVM: x86: avoid vmalloc(0) in the KVM_SET_CPUID
    - kvm: nVMX: VMCLEAR an active shadow VMCS after last use
    - X86/nVMX: Properly set spec_ctrl and pred_cmd before merging MSRs
    - KVM/VMX: Optimize vmx_vcpu_run() and svm_vcpu_run() by marking the RDMSR
      path as unlikely()
    - kvm: x86: IA32_ARCH_CAPABILITIES is always supported
    - KVM: SVM: Add MSR-based feature support for serializing LFENCE
    - KVM: X86: Allow userspace to define the microcode version
    - KVM: x86: SVM: Call x86_spec_ctrl_set_guest/host() with interrupts disabled
    - KVM: VMX: fixes for vmentry_l1d_flush module parameter
    - kvm: svm: Ensure an IBPB on all affected CPUs when freeing a vmcb
    - kvm: vmx: Scrub hardware GPRs at VM-exit
    - SAUCE: [Fix] x86/KVM/VMX: Add L1D flush logic
    - SAUCE: KVM: Move code fragments, cleanup and re-indent

  * linux-buildinfo: pull out ABI information into its own package
    (LP: #1806380)
    - [Packaging] limit preparation to linux-libc-dev in headers
    - [Packaging] commonise debhelper invocation
    - [Packaging] ABI -- accumulate abi information at the end of the build
    - [Packaging] buildinfo -- add basic build information
    - [Packaging] buildinfo -- add firmware information to the flavour ABI
    - [Packaging] buildinfo -- add compiler information to the flavour ABI
    - [Packaging] buildinfo -- add buildinfo support to getabis
    - [Config] buildinfo -- add retpoline version markers
    - [Packaging] getabis -- handle all known package combinations
    - [Packaging] getabis -- support parsing a simple version

  * signing: only install a signed kernel (LP: #1764794)
    - [Packaging] update to Debian like control scripts
    - [Packaging] switch to triggers for postinst.d postrm.d handling
    - [Packaging] signing -- switch to raw-signing tarballs
    - [Packaging] signing -- switch to linux-image as signed when available
    - [Packaging] printenv -- add signing options
    - [Packaging] fix invocation of header postinst hooks
    - [Packaging] signing -- add support for signing Opal kernel binaries
    - [Debian] Use src_pkg_name when constructing udeb control files
    - [Debian] Dynamically determine linux udebs package name
    - [Packaging] handle both linux-lts* and linux-hwe* as backports
    - [Config] linux-source-* is in the primary linux namespace
    - [Packaging] lookup the upstream tag
    - [Packaging] zfs/spl -- enhance provides information
    - [Packaging] switch up to debhelper 9
    - [Packaging] autopkgtest -- disable d-i when dropping flavours
    - [debian] support for ship_extras_package=false
    - [Debian] do_common_tools should always be on
    - [debian] do not force do_tools_common
    - [Packaging] Add linux-tools-host package for VM host tools
    - [Packaging] signing should be conditional
    - [Packaging] skip cloud tools packaging when not building package
    - [Packaging] add acpidbg
    - [debian] prep linu...