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Revert the chunk of commit01b4f510b9("kvm: x86: ensure pv_cpuid.features is initialized when enabling cap") that forced a PV features cache refresh during KVM_CAP_ENFORCE_PV_FEATURE_CPUID, as whatever ioctl() ordering issue it alleged to have fixed never existed upstream, and likely never existed in any kernel. At the time of the commit, there was a tangentially related ioctl() ordering issue, as toggling KVM_X86_DISABLE_EXITS_HLT after KVM_SET_CPUID2 would have resulted in KVM potentially leaving KVM_FEATURE_PV_UNHALT set. But (a) that bug affected the entire guest CPUID, not just the cache, (b) commit01b4f510b9didn't address that bug, it only refreshed the cache (with the bad CPUID), and (c) setting KVM_X86_DISABLE_EXITS_HLT after vCPU creation is completely broken as KVM configures HLT-exiting only during vCPU creation, which is why KVM_CAP_X86_DISABLE_EXITS is now disallowed if vCPUs have been created. Another tangentially related bug was KVM's failure to clear the cache when handling KVM_SET_CPUID2, but again commit01b4f510b9did nothing to fix that bug. The most plausible explanation for the what commit01b4f510b9was trying to fix is a bug that existed in Google's internal kernel that was the source of commit01b4f510b9. At the time, Google's internal kernel had not yet picked up commit0d3b2ba16b("KVM: X86: Go on updating other CPUID leaves when leaf 1 is absent"), i.e. KVM would not initialize the PV features cache if KVM_SET_CPUID2 was called without a CPUID.0x1 entry. Of course, no sane real world VMM would omit CPUID.0x1, including the KVM selftest added by commitac4a4d6de2("selftests: kvm: test enforcement of paravirtual cpuid features"). And the test didn't actually try to verify multiple orderings, nor did the selftest enter the guest without doing KVM_SET_CPUID2, so who knows what motivated the change. Regardless of why commit01b4f510b9("kvm: x86: ensure pv_cpuid.features is initialized when enabling cap") was added, refreshing the cache during KVM_CAP_ENFORCE_PV_FEATURE_CPUID isn't necessary. Cc: Oliver Upton <oliver.upton@linux.dev> Reviewed-by: Maxim Levitsky <mlevitsk@redhat.com> Reviewed-by: Binbin Wu <binbin.wu@linux.intel.com> Reviewed-by: Xiaoyao Li <xiaoyao.li@intel.com> Link: https://lore.kernel.org/r/20241128013424.4096668-20-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.