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Sean Christopherson f3c6fa7ce9 Revert "KVM: VMX: Move LOAD_IA32_PERF_GLOBAL_CTRL errata handling out of setup_vmcs_config()" 
commit 85434c3c73 upstream.

Revert back to clearing VM_{ENTRY,EXIT}_LOAD_IA32_PERF_GLOBAL_CTRL in KVM's
golden VMCS config, as applying the workaround during vCPU creation is
pointless and broken.  KVM *unconditionally* clears the controls in the
values returned by vmx_vmentry_ctrl() and vmx_vmexit_ctrl(), as KVM loads
PERF_GLOBAL_CTRL if and only if its necessary to do so.  E.g. if KVM wants
to run the guest with the same PERF_GLOBAL_CTRL as the host, then there's
no need to re-load the MSR on entry and exit.

Even worse, the buggy commit failed to apply the erratum where it's
actually needed, add_atomic_switch_msr().  As a result, KVM completely
ignores the erratum for all intents and purposes, i.e. uses the flawed
VMCS controls to load PERF_GLOBAL_CTRL.

To top things off, the patch was intended to be dropped, as the premise
of an L1 VMM being able to pivot on FMS is flawed, and KVM can (and now
does) fully emulate the controls in software.  Simply revert the commit,
as all upstream supported kernels that have the buggy commit should also
have commit f4c93d1a0e ("KVM: nVMX: Always emulate PERF_GLOBAL_CTRL
VM-Entry/VM-Exit controls"), i.e. the (likely theoretical) live migration
concern is a complete non-issue.

Opportunistically drop the manual "kvm: " scope from the warning about
the erratum, as KVM now uses pr_fmt() to provide the correct scope (v6.1
kernels and earlier don't, but the erratum only applies to CPUs that are
15+ years old; it's not worth a separate patch).

This reverts commit 9d78d6fb18.

Link: https://lore.kernel.org/all/YtnZmCutdd5tpUmz@google.com
Fixes: 9d78d6fb18 ("KVM: VMX: Move LOAD_IA32_PERF_GLOBAL_CTRL errata handling out of setup_vmcs_config()")
Cc: stable@vger.kernel.org
Cc: Vitaly Kuznetsov <vkuznets@redhat.com>
Cc: Maxim Levitsky <mlevitsk@redhat.com>
Signed-off-by: Sean Christopherson <seanjc@google.com>
Reviewed-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Message-ID: <20241119011433.1797921-1-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
History 2024-12-05 14:02:44 +01:00
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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.