Public bug reported: BugLink: https://bugs.launchpad.net/bugs/2093146
[Impact] The CPUID instruction is particularly slow on newer generation processors, with Intel's Emerald Rapids processor taking significantly longer to execute CPUID than Skylake or Icelake. This introduces significant latency into the KVM subsystem, as it frequently calls CPUID when recomputing XSTATE offsets, and especially XSAVE values, as they need to call CPUID twice for each XSAVE call. CPUID.0xD.[1..n] are constant and do not change during runtime, as they don't depend on XCR0 or XSS values, and can be saved and cached for future usage. By caching CPUID.0xD.[1..n] at kvm.ko module load, latency decreases by up to 400%. For a round trip transition between VM-Enter and VM-Exit figures from the commit log are: Skylake 11650 Icelake 22350 Emerald 28850 When you add the caching in: Skylake 6850 Icelake 9000 Emerald 7900 That's a saving of 170% for Skylake, 248% for Icelake and 365% for Emerald Rapids. [Fix] The fix is part of a 5 patch series. We will only SRU patch 1 for the moment, as it is the only one in mainline, and provides a 400% latency improvement, doing the brunt of the work. Patches 2-5 are refactors and smaller performance improvements, not yet mainlined due to needing rework, and only account for about 2.5% latency improvement, quite small, compared to what patch 1 does. The fix is: commit 1201f226c863b7da739f7420ddba818cedf372fc Author: Sean Christopherson <sea...@google.com> Date: Tue Dec 10 17:32:58 2024 -0800 Subject: KVM: x86: Cache CPUID.0xD XSTATE offsets+sizes during module init Link: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=1201f226c863b7da739f7420ddba818cedf372fc This applies cleanly to noble, oracular. For jammy, it requires the below dependency, and a small backport to fix some minor context mismatches. commit cc04b6a21d431359eceeec0d812b492088b04af5 Author: Jing Liu <jing2....@intel.com> Date: Wed Jan 5 04:35:14 2022 -0800 Subject: kvm: x86: Fix xstate_required_size() to follow XSTATE alignment rule Link: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=cc04b6a21d431359eceeec0d812b492088b04af5 [Testcase] 1) Install KVM Stack on Baremetal host $ sudo apt-get install qemu-kvm libvirt-daemon-system libvirt-clients bridge-utils 2) Enable nested virt $ vim /etc/modprobe.d/kvm.conf options kvm-intel nested=1 3) Start a VM. 4) Install KVM Stack in Guest $ sudo apt-get install qemu-kvm libvirt-daemon-system libvirt-clients bridge-utils 5) Start a VM. 6) Install kvm-unit-tests and run x86/vmexit/cpuid testcase. https://gitlab.com/kvm-unit-tests/kvm-unit-tests (To be expanded upon). [Where problems could occur] This fix is related to nested virtualisation in the KVM subsystem. We are adding a new function, called on KVM module load, which caches the CPUID instead of fetching it every time XSAVE needs to be recomputed, which can be multiple times on VM-Enter and VM-Exit on nested guests. CPUID is static and should never change, so there should be no issues in saving a value and reusing it later. If a regression were to occur, it would affect all KVM users, and there would be no workarounds. [Other info] Full mailing list series: https://lore.kernel.org/kvm/20241211013302.1347853-1-sea...@google.com/T/#u ** Affects: linux (Ubuntu) Importance: Undecided Status: Fix Released ** Affects: linux (Ubuntu Jammy) Importance: Medium Assignee: Matthew Ruffell (mruffell) Status: In Progress ** Affects: linux (Ubuntu Noble) Importance: Medium Assignee: Matthew Ruffell (mruffell) Status: In Progress ** Affects: linux (Ubuntu Oracular) Importance: Medium Assignee: Matthew Ruffell (mruffell) Status: In Progress ** Tags: sts ** Also affects: linux (Ubuntu Noble) Importance: Undecided Status: New ** Also affects: linux (Ubuntu Jammy) Importance: Undecided Status: New ** Also affects: linux (Ubuntu Oracular) Importance: Undecided Status: New ** Changed in: linux (Ubuntu) Status: New => Fix Released ** Changed in: linux (Ubuntu Jammy) Status: New => In Progress ** Changed in: linux (Ubuntu Noble) Status: New => In Progress ** Changed in: linux (Ubuntu Oracular) Status: New => In Progress ** Changed in: linux (Ubuntu Jammy) Importance: Undecided => Medium ** Changed in: linux (Ubuntu Jammy) Assignee: (unassigned) => Matthew Ruffell (mruffell) ** Changed in: linux (Ubuntu Oracular) Assignee: (unassigned) => Matthew Ruffell (mruffell) ** Changed in: linux (Ubuntu Noble) Assignee: (unassigned) => Matthew Ruffell (mruffell) ** Changed in: linux (Ubuntu Noble) Importance: Undecided => Medium ** Changed in: linux (Ubuntu Oracular) Importance: Undecided => Medium ** Description changed: - BugLink: https://bugs.launchpad.net/bugs/ + BugLink: https://bugs.launchpad.net/bugs/2093146 [Impact] The CPUID instruction is particularly slow on newer generation processors, with Intel's Emerald Rapids processor taking significantly longer to execute CPUID than Skylake or Icelake. This introduces significant latency into the KVM subsystem, as it frequently calls CPUID when recomputing XSTATE offsets, and especially XSAVE values, as they need to call CPUID twice for each XSAVE call. CPUID.0xD.[1..n] are constant and do not change during runtime, as they don't depend on XCR0 or XSS values, and can be saved and cached for future usage. By caching CPUID.0xD.[1..n] at kvm.ko module load, latency decreases by up to 400%. For a round trip transition between VM-Enter and VM-Exit figures from the commit log are: Skylake 11650 Icelake 22350 Emerald 28850 When you add the caching in: Skylake 6850 Icelake 9000 Emerald 7900 That's a saving of 170% for Skylake, 248% for Icelake and 365% for Emerald Rapids. [Fix] The fix is part of a 5 patch series. We will only SRU patch 1 for the moment, as it is the only one in mainline, and provides a 400% latency improvement, doing the brunt of the work. Patches 2-5 are refactors and smaller performance improvements, not yet mainlined due to needing rework, and only account for about 2.5% latency improvement, quite small, compared to what patch 1 does. The fix is: commit 1201f226c863b7da739f7420ddba818cedf372fc Author: Sean Christopherson <sea...@google.com> Date: Tue Dec 10 17:32:58 2024 -0800 Subject: KVM: x86: Cache CPUID.0xD XSTATE offsets+sizes during module init Link: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=1201f226c863b7da739f7420ddba818cedf372fc This applies cleanly to noble, oracular. For jammy, it requires the below dependency, and a small backport to fix some minor context mismatches. - + commit cc04b6a21d431359eceeec0d812b492088b04af5 Author: Jing Liu <jing2....@intel.com> Date: Wed Jan 5 04:35:14 2022 -0800 Subject: kvm: x86: Fix xstate_required_size() to follow XSTATE alignment rule Link: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=cc04b6a21d431359eceeec0d812b492088b04af5 - + [Testcase] 1) Install KVM Stack on Baremetal host $ sudo apt-get install qemu-kvm libvirt-daemon-system libvirt-clients bridge-utils 2) Enable nested virt $ vim /etc/modprobe.d/kvm.conf options kvm-intel nested=1 3) Start a VM. 4) Install KVM Stack in Guest $ sudo apt-get install qemu-kvm libvirt-daemon-system libvirt-clients bridge-utils 5) Start a VM. 6) Install kvm-unit-tests and run x86/vmexit/cpuid testcase. https://gitlab.com/kvm-unit-tests/kvm-unit-tests (To be expanded upon). [Where problems could occur] This fix is related to nested virtualisation in the KVM subsystem. We are adding a new function, called on KVM module load, which caches the CPUID instead of fetching it every time XSAVE needs to be recomputed, which can be multiple times on VM-Enter and VM-Exit on nested guests. CPUID is static and should never change, so there should be no issues in saving - a value and reusing it later. + a value and reusing it later. If a regression were to occur, it would affect all KVM users, and there would be no workarounds. [Other info] Full mailing list series: https://lore.kernel.org/kvm/20241211013302.1347853-1-sea...@google.com/T/#u ** Tags added: sts -- You received this bug notification because you are a member of Kernel Packages, which is subscribed to linux in Ubuntu. https://bugs.launchpad.net/bugs/2093146 Title: KVM: Cache CPUID at KVM.ko module init to reduce latency of VM-Enter and VM-Exit Status in linux package in Ubuntu: Fix Released Status in linux source package in Jammy: In Progress Status in linux source package in Noble: In Progress Status in linux source package in Oracular: In Progress Bug description: BugLink: https://bugs.launchpad.net/bugs/2093146 [Impact] The CPUID instruction is particularly slow on newer generation processors, with Intel's Emerald Rapids processor taking significantly longer to execute CPUID than Skylake or Icelake. This introduces significant latency into the KVM subsystem, as it frequently calls CPUID when recomputing XSTATE offsets, and especially XSAVE values, as they need to call CPUID twice for each XSAVE call. CPUID.0xD.[1..n] are constant and do not change during runtime, as they don't depend on XCR0 or XSS values, and can be saved and cached for future usage. By caching CPUID.0xD.[1..n] at kvm.ko module load, latency decreases by up to 400%. For a round trip transition between VM-Enter and VM-Exit figures from the commit log are: Skylake 11650 Icelake 22350 Emerald 28850 When you add the caching in: Skylake 6850 Icelake 9000 Emerald 7900 That's a saving of 170% for Skylake, 248% for Icelake and 365% for Emerald Rapids. [Fix] The fix is part of a 5 patch series. We will only SRU patch 1 for the moment, as it is the only one in mainline, and provides a 400% latency improvement, doing the brunt of the work. Patches 2-5 are refactors and smaller performance improvements, not yet mainlined due to needing rework, and only account for about 2.5% latency improvement, quite small, compared to what patch 1 does. The fix is: commit 1201f226c863b7da739f7420ddba818cedf372fc Author: Sean Christopherson <sea...@google.com> Date: Tue Dec 10 17:32:58 2024 -0800 Subject: KVM: x86: Cache CPUID.0xD XSTATE offsets+sizes during module init Link: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=1201f226c863b7da739f7420ddba818cedf372fc This applies cleanly to noble, oracular. For jammy, it requires the below dependency, and a small backport to fix some minor context mismatches. commit cc04b6a21d431359eceeec0d812b492088b04af5 Author: Jing Liu <jing2....@intel.com> Date: Wed Jan 5 04:35:14 2022 -0800 Subject: kvm: x86: Fix xstate_required_size() to follow XSTATE alignment rule Link: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=cc04b6a21d431359eceeec0d812b492088b04af5 [Testcase] 1) Install KVM Stack on Baremetal host $ sudo apt-get install qemu-kvm libvirt-daemon-system libvirt-clients bridge-utils 2) Enable nested virt $ vim /etc/modprobe.d/kvm.conf options kvm-intel nested=1 3) Start a VM. 4) Install KVM Stack in Guest $ sudo apt-get install qemu-kvm libvirt-daemon-system libvirt-clients bridge-utils 5) Start a VM. 6) Install kvm-unit-tests and run x86/vmexit/cpuid testcase. https://gitlab.com/kvm-unit-tests/kvm-unit-tests (To be expanded upon). [Where problems could occur] This fix is related to nested virtualisation in the KVM subsystem. We are adding a new function, called on KVM module load, which caches the CPUID instead of fetching it every time XSAVE needs to be recomputed, which can be multiple times on VM-Enter and VM-Exit on nested guests. CPUID is static and should never change, so there should be no issues in saving a value and reusing it later. If a regression were to occur, it would affect all KVM users, and there would be no workarounds. [Other info] Full mailing list series: https://lore.kernel.org/kvm/20241211013302.1347853-1-sea...@google.com/T/#u To manage notifications about this bug go to: https://bugs.launchpad.net/ubuntu/+source/linux/+bug/2093146/+subscriptions -- Mailing list: https://launchpad.net/~kernel-packages Post to : kernel-packages@lists.launchpad.net Unsubscribe : https://launchpad.net/~kernel-packages More help : https://help.launchpad.net/ListHelp
