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Reinitialize the xAPIC ID to the vCPU ID when userspace forces the APIC to transition directly from x2APIC to xAPIC mode, e.g. to emulate RESET. KVM already stuffs the xAPIC ID when the APIC is transitioned from DISABLED to xAPIC (commit49bd29ba1d("KVM: x86: reset APIC ID when enabling LAPIC")), i.e. userspace is conditioned to expect KVM to update the xAPIC ID, but KVM doesn't handle the architecturally-impossible case where userspace forces x2APIC=>xAPIC via KVM_SET_MSRS. On its own, the "bug" is benign, as userspace emulation of RESET will also stuff APIC registers via KVM_SET_LAPIC, i.e. will manually set the xAPIC ID. However, commit3743c2f025("KVM: x86: inhibit APICv/AVIC on changes to APIC ID or APIC base") introduced a bug, fixed by commit commitef40757743("KVM: x86: fix APICv/x2AVIC disabled when vm reboot by itself"), that caused KVM to fail to properly update the xAPIC ID when handling KVM_SET_LAPIC. Refresh the xAPIC ID even though it's not strictly necessary so that KVM provides consistent behavior. Note, KVM follows Intel architecture with regard to handling the xAPIC ID and x2APIC IDs across mode transitions. For the APIC DISABLED case (commit49bd29ba1d), Intel's SDM says the xAPIC ID _may_ be reinitialized 10.4.3 Enabling or Disabling the Local APIC When IA32_APIC_BASE[11] is set to 0, prior initialization to the APIC may be lost and the APIC may return to the state described in Section 10.4.7.1, “Local APIC State After Power-Up or Reset.” 10.4.7.1 Local APIC State After Power-Up or Reset ... The local APIC ID register is set to a unique APIC ID. ... i.e. KVM's behavior is legal as per Intel's architecture. In practice, Intel's behavior is N/A as modern Intel CPUs (since at least Haswell) make the xAPIC ID fully read-only. And for xAPIC => x2APIC transitions (commit257b9a5faa("KVM: x86: use correct APIC ID on x2APIC transition")), Intel's SDM says: Any APIC ID value written to the memory-mapped local APIC ID register is not preserved. AMD's APM says nothing (that I could find) about the xAPIC ID when the APIC is DISABLED, but testing on bare metal (Rome) shows that the xAPIC ID is preserved when the APIC is DISABLED and re-enabled in xAPIC mode. AMD also preserves the xAPIC ID when the APIC is transitioned from xAPIC to x2APIC, i.e. allows a backdoor write of the x2APIC ID, which is again not emulated by KVM. Signed-off-by: Emanuele Giuseppe Esposito <eesposit@redhat.com> Link: https://lore.kernel.org/all/20230109130605.2013555-2-eesposit@redhat.com [sean: rewrite changelog, set xAPIC ID iff APIC is enabled] 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 Restructured Text 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.