linux-kernel - [GIT PULL] x86/kvm: Clean up KVM's guest/host async #PF ABI, for 6.9

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Message-ID: <20240223211621.3348855-3-seanjc@google.com>
Date: Fri, 23 Feb 2024 13:16:23 -0800
From: Sean Christopherson <seanjc@...gle.com>
To: Paolo Bonzini <pbonzini@...hat.com>
Cc: kvm@...r.kernel.org, linux-kernel@...r.kernel.org, 
	Sean Christopherson <seanjc@...gle.com>
Subject: [GIT PULL] x86/kvm: Clean up KVM's guest/host async #PF ABI, for 6.9

An early(ish) pull request for 6.9 (not 6.8!) to clean up KVM's async #PF
guest/host ABI.  Due to a goof many years ago, the structure shared between
the guest and host was expanded to 68 bytes, not the intended 64 bytes (to
fit in a cache line).

Rather than document the goof, just drop the problematic 4 bytes from the
ABI as KVM-the-host never actually used them.

The following changes since commit 41bccc98fb7931d63d03f326a746ac4d429c1dd3:

  Linux 6.8-rc2 (2024-01-28 17:01:12 -0800)

are available in the Git repository at:

  https://github.com/kvm-x86/linux.git tags/kvm-x86-asyncpf_abi-6.9

for you to fetch changes up to df01f0a1165c35e95b5f52c7ba25c19020352ff9:

  KVM: x86: Improve documentation of MSR_KVM_ASYNC_PF_EN (2024-02-06 10:59:52 -0800)

----------------------------------------------------------------
Guest-side KVM async #PF ABI cleanup for 6.9

Delete kvm_vcpu_pv_apf_data.enabled to fix a goof in KVM's async #PF ABI where
the enabled field pushes the size of "struct kvm_vcpu_pv_apf_data" from 64 to
68 bytes, i.e. beyond a single cache line.

The enabled field is purely a guest-side flag that Linux-as-a-guest uses to
track whether or not the guest has enabled async #PF support.  The actual flag
that is passed to the host, i.e. to KVM proper, is a single bit in a synthetic
MSR, MSR_KVM_ASYNC_PF_EN, i.e. is in a location completely unrelated to the
shared kvm_vcpu_pv_apf_data structure.

Simply drop the the field and use a dedicated guest-side per-CPU variable to
fix the ABI, as opposed to fixing the documentation to match reality.  KVM has
never consumed kvm_vcpu_pv_apf_data.enabled, so the odds of the ABI change
breaking anything are extremely low.

----------------------------------------------------------------
Xiaoyao Li (2):
      x86/kvm: Use separate percpu variable to track the enabling of asyncpf
      KVM: x86: Improve documentation of MSR_KVM_ASYNC_PF_EN

 Documentation/virt/kvm/x86/msr.rst   | 19 +++++++++----------
 arch/x86/include/uapi/asm/kvm_para.h |  1 -
 arch/x86/kernel/kvm.c                | 11 ++++++-----
 3 files changed, 15 insertions(+), 16 deletions(-)