This bug is awaiting verification that the linux/6.8.0-53.55 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-noble-linux' to 'verification-done-noble-linux'. If the problem still exists, change the tag 'verification-needed-noble- linux' to 'verification-failed-noble-linux'.
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! ** Tags added: kernel-spammed-noble-linux-v2 verification-needed-noble-linux -- 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: Fix Committed Status in linux source package in Noble: Fix Committed Status in linux source package in Oracular: Fix Committed 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 landed in 6.13-rc3, and is currently queued up for upstream -stable 6.12, 6.6 and 6.1. 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 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 $ sudo reboot 3) Start a VM. 4) In guest, run: $ sudo apt-get install qemu-kvm libvirt-daemon-system libvirt-clients bridge-utils 5) In host and the guest, run: $ sudo apt install build-essential 6) Install kvm-unit-tests and run x86/vmexit/cpuid testcase. https://gitlab.com/kvm-unit-tests/kvm-unit-tests $ git clone https://gitlab.com/kvm-unit-tests/kvm-unit-tests.git $ ./configure $ make standalone $ cd tests $ sudo -s # ACCEL=kvm ./vmexit_cpuid BUILD_HEAD=0ed2cdf3 timeout -k 1s --foreground 90s /usr/bin/qemu-system-x86_64 --no-reboot -nodefaults -device pc-testdev -device isa-debug-exit,iobase=0xf4,iosize=0x4 -vnc none -serial stdio -device pci-testdev -machine accel=kvm -kernel /tmp/tmp.GMVjItBglu -smp 1 -append cpuid # -initrd /tmp/tmp.uaD4VVyIqc qemu-system-x86_64: warning: host doesn't support requested feature: CPUID.80000001H:ECX.svm [bit 2] enabling apic smp: waiting for 0 APs paging enabled cr0 = 80010011 cr3 = 1007000 cr4 = 20 pci-testdev at 0x10 membar febff000 iobar c000 cpuid 66485 PASS vmexit_cpuid The numbers next to cpuid, is the (t2 = rdtsc) - (t1 = rdtsc) count. Smaller is better. A test kernel is available in the following ppa: https://launchpad.net/~mruffell/+archive/ubuntu/sf403286-test Test data for the above test kernel is: Sapphire Rapids Intel(R) Xeon(R) Platinum 8468 24.04 LTS Noble 6.8.0-51-generic Outside VM cpuid 3527 cpuid 3384 cpuid 3467 cpuid 3300 cpuid 3544 Inside VM cpuid 68395 cpuid 58364 cpuid 65254 cpuid 68554 cpuid 66905 6.8.0-51-generic+TEST403286v20250107b1 Outside VM cpuid 3253 cpuid 3416 cpuid 3447 cpuid 3260 cpuid 3281 Inside VM cpuid 22852 cpuid 22890 cpuid 18168 cpuid 23462 cpuid 23281 The number of cycles of rdtsc in a nested VM is of the same order of magnitude smaller as we are expecting. [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
