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Sean Christopherson b965edb019 KVM: x86: Cache CPUID.0xD XSTATE offsets+sizes during module init 
commit 1201f226c8 upstream.

Snapshot the output of CPUID.0xD.[1..n] during kvm.ko initiliaization to
avoid the overead of CPUID during runtime.  The offset, size, and metadata
for CPUID.0xD.[1..n] sub-leaves does not depend on XCR0 or XSS values, i.e.
is constant for a given CPU, and thus can be cached during module load.

On Intel's Emerald Rapids, CPUID is *wildly* expensive, to the point where
recomputing XSAVE offsets and sizes results in a 4x increase in latency of
nested VM-Enter and VM-Exit (nested transitions can trigger
xstate_required_size() multiple times per transition), relative to using
cached values.  The issue is easily visible by running `perf top` while
triggering nested transitions: kvm_update_cpuid_runtime() shows up at a
whopping 50%.

As measured via RDTSC from L2 (using KVM-Unit-Test's CPUID VM-Exit test
and a slightly modified L1 KVM to handle CPUID in the fastpath), a nested
roundtrip to emulate CPUID on Skylake (SKX), Icelake (ICX), and Emerald
Rapids (EMR) takes:

  SKX 11650
  ICX 22350
  EMR 28850

Using cached values, the latency drops to:

  SKX 6850
  ICX 9000
  EMR 7900

The underlying issue is that CPUID itself is slow on ICX, and comically
slow on EMR.  The problem is exacerbated on CPUs which support XSAVES
and/or XSAVEC, as KVM invokes xstate_required_size() twice on each
runtime CPUID update, and because there are more supported XSAVE features
(CPUID for supported XSAVE feature sub-leafs is significantly slower).

 SKX:
  CPUID.0xD.2  = 348 cycles
  CPUID.0xD.3  = 400 cycles
  CPUID.0xD.4  = 276 cycles
  CPUID.0xD.5  = 236 cycles
  <other sub-leaves are similar>

 EMR:
  CPUID.0xD.2  = 1138 cycles
  CPUID.0xD.3  = 1362 cycles
  CPUID.0xD.4  = 1068 cycles
  CPUID.0xD.5  = 910 cycles
  CPUID.0xD.6  = 914 cycles
  CPUID.0xD.7  = 1350 cycles
  CPUID.0xD.8  = 734 cycles
  CPUID.0xD.9  = 766 cycles
  CPUID.0xD.10 = 732 cycles
  CPUID.0xD.11 = 718 cycles
  CPUID.0xD.12 = 734 cycles
  CPUID.0xD.13 = 1700 cycles
  CPUID.0xD.14 = 1126 cycles
  CPUID.0xD.15 = 898 cycles
  CPUID.0xD.16 = 716 cycles
  CPUID.0xD.17 = 748 cycles
  CPUID.0xD.18 = 776 cycles

Note, updating runtime CPUID information multiple times per nested
transition is itself a flaw, especially since CPUID is a mandotory
intercept on both Intel and AMD.  E.g. KVM doesn't need to ensure emulated
CPUID state is up-to-date while running L2.  That flaw will be fixed in a
future patch, as deferring runtime CPUID updates is more subtle than it
appears at first glance, the benefits aren't super critical to have once
the XSAVE issue is resolved, and caching CPUID output is desirable even if
KVM's updates are deferred.

Cc: Jim Mattson <jmattson@google.com>
Cc: stable@vger.kernel.org
Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-ID: <20241211013302.1347853-2-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
History 2024-12-27 14:02:07 +01:00
arch KVM: x86: Cache CPUID.0xD XSTATE offsets+sizes during module init 2024-12-27 14:02:07 +01:00
block block: Fix potential deadlock while freezing queue and acquiring sysfs_lock 2024-12-19 18:13:22 +01:00
certs sign-file,extract-cert: use pkcs11 provider for OPENSSL MAJOR >= 3 2024-09-20 19:52:48 +03:00
crypto crypto: ecdsa - Avoid signed integer overflow on signature decoding 2024-12-14 20:03:36 +01:00
Documentation Documentation: networking: Add a caveat to nexthop_compat_mode sysctl 2024-12-19 18:13:16 +01:00
drivers EDAC/amd64: Simplify ECC check on unified memory controllers 2024-12-27 14:02:06 +01:00
fs ksmbd: fix broken transfers when exceeding max simultaneous operations 2024-12-27 14:02:03 +01:00
include sched/dlserver: Fix dlserver double enqueue 2024-12-27 14:01:59 +01:00
init Compiler Attributes: disable __counted_by for clang < 19.1.3 2024-12-05 14:02:45 +01:00
io_uring io_uring/tctx: work around xa_store() allocation error issue 2024-12-14 20:04:10 +01:00
ipc ipc: fix memleak if msg_init_ns failed in create_ipc_ns 2024-12-05 14:03:02 +01:00
kernel sched/dlserver: Fix dlserver time accounting 2024-12-27 14:01:59 +01:00
lib timekeeping: Remove CONFIG_DEBUG_TIMEKEEPING 2024-12-14 20:04:16 +01:00
LICENSES LICENSES: add 0BSD license text 2024-09-01 20:43:24 -07:00
mm memcg: slub: fix SUnreclaim for post charged objects 2024-12-19 18:13:01 +01:00
net psample: adjust size if rate_as_probability is set 2024-12-27 14:02:06 +01:00
rust rust: kbuild: set bindgen's Rust target version 2024-12-19 18:13:22 +01:00
samples samples/bpf: Fix a resource leak 2024-12-14 20:03:46 +01:00
scripts setlocalversion: work around "git describe" performance 2024-12-14 20:04:10 +01:00
security apparmor: test: Fix memory leak for aa_unpack_strdup() 2024-12-05 14:02:46 +01:00
sound firmware: arm_scmi: Fix i.MX build dependency 2024-12-27 14:01:57 +01:00
tools selftests: openvswitch: fix tcpdump execution 2024-12-27 14:02:05 +01:00
usr initramfs: shorten cmd_initfs in usr/Makefile 2024-07-16 01:07:52 +09:00
virt KVM: x86: switch hugepage recovery thread to vhost_task 2024-12-05 14:02:43 +01:00
.clang-format clang-format: Update with v6.11-rc1's for_each macro list 2024-08-02 13:20:31 +02:00
.cocciconfig scripts: add Linux .cocciconfig for coccinelle 2016-07-22 12:13:39 +02:00
.editorconfig .editorconfig: remove trim_trailing_whitespace option 2024-06-13 16:47:52 +02:00
.get_maintainer.ignore Add Jeff Kirsher to .get_maintainer.ignore 2024-03-08 11:36:54 +00:00
.gitattributes .gitattributes: set diff driver for Rust source code files 2023-05-31 17:48:25 +02:00
.gitignore Kbuild updates for v6.12 2024-09-24 13:02:06 -07:00
.mailmap mailmap: add entry for Thorsten Blum 2024-11-07 14:14:59 -08:00
.rustfmt.toml rust: add .rustfmt.toml 2022-09-28 09:02:20 +02:00
COPYING COPYING: state that all contributions really are covered by this file 2020-02-10 13:32:20 -08:00
CREDITS MAINTAINERS: Remove self from DSA entry 2024-11-03 12:52:38 -08:00
Kbuild Kbuild updates for v6.1 2022-10-10 12:00:45 -07:00
Kconfig kbuild: ensure full rebuild when the compiler is updated 2020-05-12 13:28:33 +09:00
MAINTAINERS MAINTAINERS: update location of media main tree 2024-12-05 14:02:42 +01:00
Makefile Linux 6.12.6 2024-12-19 18:13:24 +01:00
README README: Fix spelling 2024-03-18 03:36:32 -06:00

README 

Linux kernel
============

There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.

In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``.  The formatted documentation can also be read online at:

    https://www.kernel.org/doc/html/latest/

There are various text files in the Documentation/ subdirectory,
several of them using the reStructuredText markup notation.

Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.