This blog entry is intended as a follow-up to the original entry in
January regarding Spectre/Meltdown and the proposed changes to address
them in the upcoming 2.11.1 release.

This entry is meant to accompany the 2.11.1 release (planned for
2018-02-14) and document how to make use of the new options for
various architectures.

Cc: Eduardo Habkost <>
Cc: Paolo Bonzini <>
Cc: Peter Maydell <>
Cc: Suraj Jitindar Singh <>
Cc: David Gibson <>
Cc: Christian Borntraeger <>
Cc: Cornelia Huck <>
Cc: Thomas Huth <>
Signed-off-by: Michael Roth <>

The pseries/s390 bits have gotten some initial review (thanks Suraj/Christian),
but it can definitely use some additional review on the x86 side of things.

Also, Peter if think anything extra should to be mentioned on the ARM side just
let me know what to add.

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+layout: post
+title:  "QEMU 2.11.1 and making use of Spectre/Meltdown mitigation for KVM 
+date: 2018-02-14 10:35:44 -0600
+author: Michael Roth
+categories: [meltdown, spectre, security, x86, ppc, s390, releases, 'qemu 
+In a [previous post]( it was
+detailed how QEMU/KVM might be affected by Spectre/Meltdown attacks, and what
+the plan was to mitigate them in QEMU 2.11.1 (and eventually QEMU 2.12).
+QEMU 2.11.1 is now available, and contains the aforementioned mitigations for
+x86 guests, along with additional mitigation functionality for pseries and
+s390 guests (ARM guests do not currently require additional QEMU patches).
+However, enabling this functionality requires additional configuration beyond
+just updating QEMU, which we hope to address with this post.
+Please note that, as mentioned in the previous blog post, QEMU/KVM generally
+has the same requirements as other unpriviledged processes running on the
+host WRT Spectre/Meltdown mitigation. What is being addressed here is
+enabling a guest operating system to enable the same (or similar) mitigations
+to protect itself from unpriviledged guest processes. Thus, the
+patches/requirements listed here are specific to that goal and should not be
+regarded as the full set of requirements to enable mitigations on the host
+side (though in some cases there is some overlap between the two WRT required
+Also please note that this is a best-effort from the QEMU/KVM community, and
+these mitigations rely on a mix of additional kernel/firmware/microcode
+updates that are in some cases not available publically, or may not yet be
+implemented in some distros, so users are highly encouraged to consult with
+their respective vendors/distros to confirm whether all the required
+components are in place. We do our best to highlight the requirements here,
+but this may not be an exhaustive list.
+## enabling mitigations for x86 KVM guests
+For x86 guests there are 2 additional CPU flags associated with
+Spectre/Meltdown mitigation: **spec-ctrl**, and **ibpb**. These flags
+expose additional functionality made available through new microcode
+updates for certain Intel/AMD processors that can be used to mitigate
+various attack vectors related to Spectre. (Meltdown mitigation via KPTI
+does not require additional CPU functionality or microcode, and does not
+require an updated QEMU, only the related guest/host kernel patches).
+These CPU flags:
+* spec-ctrl: exposes Indirect Branch Restricted Speculation (IBRS)
+* ibpb: exposes Indirect Branch Prediction Barriers
+are both features requiring guest/host kernel updates, as well as
+microcode updates for Intel and recent AMD processors. The status of
+these kernel patches upstream is still in flux, but most supported
+distros have some form of the patches that is sufficient to make use
+of the features. The current status/availability of microcode updates
+depends on your CPU architecture/model. Please check with your
+vendor/distro to confirm these prerequisites are available/installed.
+Generally, for Intel CPUs with updated microcode, **spec-ctrl** will
+enable both IBRS and IBPB functionality. For AMD EPYC processors,
+**ibpb** can be used to enable IBPB specifically, and is thought to
+be sufficient by itself that particular architecture.
+These flags can be set in a similar manner as other CPU flags, i.e.:
+    qemu-system-x86_64 -cpu qemu64,+spec-ctrl,... ...
+    qemu-system-x86_64 -cpu IvyBridge,+spec-ctrl,... ...
+    qemu-system-x86_64 -cpu EPYC,+ibpb
+    etc...
+Additionally, for management stacks that lack support for setting
+specific CPU flags, a set of new CPU types have been added which
+enable the appropriate CPU flags automatically:
+    qemu-system-x86_64 -cpu Nehalem-IBRS ...
+    qemu-system-x86_64 -cpu Westmere-IBRS ...
+    qemu-system-x86_64 -cpu SandyBridge-IBRS ...
+    qemu-system-x86_64 -cpu IvyBridge-IBRS ...
+    qemu-system-x86_64 -cpu Haswell-IBRS ...
+    qemu-system-x86_64 -cpu Haswell-noTSX-IBRS ...
+    qemu-system-x86_64 -cpu Broadwell-IBRS ...
+    qemu-system-x86_64 -cpu Broadwell-noTSX-IBRS ...
+    qemu-system-x86_64 -cpu Skylake-Client-IBRS ...
+    qemu-system-x86_64 -cpu Skylake-Server-IBRS ...
+    qemu-system-x86_64 -cpu EPYC-IBPB ...
+With these settings enabled, guests may still require additional
+configuration to enable IBRS/IBPB, which may vary somewhat from one
+distro to another. For RHEL guests, the following resource may be
+WRT migration compatibility, **spec-ctrl**/**ibrs** (or the corresponding
+CPU type) should be set the same on both source/target to maintain
+compatibility. Thus, guests will need to be rebooted to make use of
+the new features.
+## enabling mitigations for pseries KVM guests
+For pseries guests there are 3 tri-state -machine options/capabilities
+relating to Spectre/Meltdown mitigation: **cap-cfpc**, **cap-sbbc**,
+**cap-ibs**, which each correspond to a set of host machine capabilities
+advertised by the KVM kernel module in new/patched host kernels that can
+be used to mitigate various aspects of Spectre/Meltdown:
+* cap-cfpc: Cache Flush on Privilege Change
+* cap-sbbc: Speculation Barrier Bounds Checking
+* cap-ibs: Indirect Branch Serialisation
+Each option can be set to one of "broken", "workaround", or "fixed", which
+correspond, respectively, to instructing the guest whether the host is
+vulnerable, has OS-level workarounds available, or has hardware/firmware
+that does not require OS-level workarounds. Based on these options, QEMU
+will perform checks to validate whether the specified settings are available
+on the current host and pass these settings on to the guest kernel. At a
+minimum, any setting other than "broken" will require a host kernel that has
+some form of the following patches:
+    commit 3214d01f139b7544e870fc0b7fcce8da13c1cb51
+    KVM: PPC: Book3S: Provide information about hardware/firmware CVE 
+    commit 191eccb1580939fb0d47deb405b82a85b0379070
+    powerpc/pseries: Add H_GET_CPU_CHARACTERISTICS flags & wrapper
+and whether a host will support "workaround" and "fixed" settings for each
+option will depend on the hardware/firmware level of the host system.
+In turn, to make use of "workaround" or "fixed" settings for each option,
+the guest kernel will require at least the following set of patches:
+These are available upstream and have been backported to a number of stable
+kernels. Please check with your vendor/distro to confirm the required
+hardware/firmware and guest kernel patches are available/installed.
+By default, all three options, **cap-cfpc**, **cap-sbbc**, and **cap-ibs**
+default to "broken" to maintain compatibility with previous versions of QEMU
+and unpatched host kernels. To enable them you must start QEMU with the
+desired mitigation strategy specified explicitly. For example:
+    qemu-system-ppc64 ... \
+      -machine 
+WRT migration compatibility, setting any of these features to a value other
+than "broken" will require an identical setting for that option on the
+source/destination guest. To enable these settings your guests will need to
+be rebooted at some point.
+## enabling mitigations for s390 KVM guests
+For s390 guests there are 2 CPU options relating to Spectre/Meltdown:
+* bpb: Branch prediction blocking
+* ppa15: PPA15 is installed
+**bpb** requires a host kernel patched with:
+    commit 35b3fde6203b932b2b1a5b53b3d8808abc9c4f60
+    KVM: s390: wire up bpb feature
+and both **bpb** and **ppa15** require a firmware with the appropriate support
+level as well as guest kernel patches to enable the functionality within
+guests. Please check with your distro/vendor to confirm.
+Both **bpb** and **ppa15** are enabled by default with newer/patched host
+kernels, and can also be set manually. For example:
+    qemu-system-s390x -M s390-ccw-virtio-2.11 ... \
+      -cpu zEC12,bpb=on,ppa15=on 
+WRT to migration, enabling **bpb** requires the source/target also have **bpb**
+enabled. Since this is enabled by default, you must ensure that **bpb**=off if
+you wish to maintain migration compatibility with existing guests, or take
+steps to reboot guests with **bpb** enabled prior to migrating them.

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