linux-kernel - [RFC PATCH 0/2] xapic: make sure x2APIC -> xapic transition correctly

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Date:   Mon,  9 Jan 2023 08:06:03 -0500
From:   Emanuele Giuseppe Esposito <eesposit@...hat.com>
To:     kvm@...r.kernel.org
Cc:     Sean Christopherson <seanjc@...gle.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
        "H. Peter Anvin" <hpa@...or.com>, Shuah Khan <shuah@...nel.org>,
        Gautam Menghani <gautammenghani201@...il.com>,
        Emanuele Giuseppe Esposito <eesposit@...hat.com>,
        Zeng Guang <guang.zeng@...el.com>,
        Krish Sadhukhan <krish.sadhukhan@...cle.com>,
        Jim Mattson <jmattson@...gle.com>,
        linux-kernel@...r.kernel.org, linux-kselftest@...r.kernel.org
Subject: [RFC PATCH 0/2] xapic: make sure x2APIC -> xapic transition correctly

The root cause is kvm_lapic_set_base() failing to handle x2APIC -> xapic ID
switch, which is addressed by patch 1.
Patch 2 provides a selftest to verify this behavior.

This serie is an RFC because I think that commit ef40757743b47 already tries to
fix one such effect of the error made in kvm_lapic_set_base, but I am not sure
how such error described in the commit message is triggered, nor how to
reproduce it using a selftest. I don't think one can enable/disable x2APIC using
KVM_SET_LAPIC, and kvm_lapic_set_base() in kvm_apic_set_state() just takes care
of updating apic->base_address, since value == old_value.
The test in patch 2 fails with the fix in ef40757743b47.

Thank you,
Emanuele

Emanuele Giuseppe Esposito (2):
  KVM: x86: update APIC_ID also when disabling x2APIC in
    kvm_lapic_set_base
  KVM: selftests: APIC_ID must be correctly updated when disabling
    x2apic

 arch/x86/kvm/lapic.c                          |  8 ++-
 .../selftests/kvm/x86_64/xapic_state_test.c   | 64 +++++++++++++++++++
 2 files changed, 70 insertions(+), 2 deletions(-)

-- 
2.31.1