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75c69c82f2
Move KVM's swapping of XFEATURE masks, i.e. XCR0 and XSS, out of the fastpath loop now that the guts of the #MC handler runs in task context, i.e. won't invoke schedule() with preemption disabled and clobber state (or crash the kernel) due to trying to context switch XSTATE with a mix of host and guest state. For all intents and purposes, this reverts commit1811d979c7("x86/kvm: move kvm_load/put_guest_xcr0 into atomic context"), which papered over an egregious bug/flaw in the #MC handler where it would do schedule() even though IRQs are disabled. E.g. the call stack from the commit: kvm_load_guest_xcr0 ... kvm_x86_ops->run(vcpu) vmx_vcpu_run vmx_complete_atomic_exit kvm_machine_check do_machine_check do_memory_failure memory_failure lock_page Commit1811d979c7"fixed" the immediate issue of XRSTORS exploding, but completely ignored that scheduling out a vCPU task while IRQs and preemption is wildly broken. Thankfully, commit5567d11c21("x86/mce: Send #MC singal from task work") (somewhat incidentally?) fixed that flaw by pushing the meat of the work to the user-return path, i.e. to task context. KVM has also hardened itself against #MC goofs by moving #MC forwarding to kvm_x86_ops.handle_exit_irqoff(), i.e. out of the fastpath. While that's by no means a robust fix, restoring as much state as possible before handling the #MC will hopefully provide some measure of protection in the event that #MC handling goes off the rails again. Note, KVM always intercepts XCR0 writes for vCPUs without protected state, e.g. there's no risk of consuming a stale XCR0 when determining if a PKRU update is needed; kvm_load_host_xfeatures() only reads, and never writes, vcpu->arch.xcr0. Deferring the XCR0 and XSS loads shaves ~300 cycles off the fastpath for Intel, and ~500 cycles for AMD. E.g. using INVD in KVM-Unit-Test's vmexit.c, which an extra hack to enable CR4.OXSAVE, latency numbers for AMD Turin go from ~2000 => 1500, and for Intel Emerald Rapids, go from ~1300 => ~1000. Cc: Jon Kohler <jon@nutanix.com> Reviewed-by: Rick Edgecombe <rick.p.edgecombe@intel.com> Reviewed-by: Jon Kohler <jon@nutanix.com> Link: https://patch.msgid.link/20251118222328.2265758-4-seanjc@google.com Signed-off-by: Sean Christopherson <seanjc@google.com>
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.