linux-kernel - [PATCH 0/5] KVM: VMX: Drop MTRR virtualization, honor guest PAT

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Date: Fri,  8 Mar 2024 17:09:24 -0800
From: Sean Christopherson <seanjc@...gle.com>
To: Paolo Bonzini <pbonzini@...hat.com>, Sean Christopherson <seanjc@...gle.com>, 
	Lai Jiangshan <jiangshanlai@...il.com>, "Paul E. McKenney" <paulmck@...nel.org>, 
	Josh Triplett <josh@...htriplett.org>
Cc: kvm@...r.kernel.org, rcu@...r.kernel.org, linux-kernel@...r.kernel.org, 
	Kevin Tian <kevin.tian@...el.com>, Yan Zhao <yan.y.zhao@...el.com>, 
	Yiwei Zhang <zzyiwei@...gle.com>
Subject: [PATCH 0/5] KVM: VMX: Drop MTRR virtualization, honor guest PAT

First, rip out KVM's support for virtualizing guest MTRRs on VMX.  The
code is costly to main, a drag on guest boot performance, imperfect, and
not required for functional correctness with modern guest kernels.  Many
details in patch 1's changelog.

With MTRR virtualization gone, always honor guest PAT on Intel CPUs that
support self-snoop, as such CPUs are guaranteed to maintain coherency
even if the guest is aliasing memtypes, e.g. if the host is using WB but
the guest is using WC.  Honoring guest PAT is desirable for use cases
where the guest must use WC when accessing memory that is DMA'd from a
non-coherent device that does NOT bounce through VFIO, e.g. for mediated
virtual GPUs.

The SRCU patch adds an API that is effectively documentation for the
memory barrier in srcu_read_lock().  Intel CPUs with self-snoop require
a memory barrier after VM-Exit to ensure coherency, and KVM always does
a srcu_read_lock() before reading guest memory after VM-Exit.  Relying
on SRCU to provide the barrier allows KVM to avoid emitting a redundant
barrier of its own.

This series needs a _lot_ more testing; I arguably should have tagged it
RFC, but I'm feeling lucky.

Sean Christopherson (3):
  KVM: x86: Remove VMX support for virtualizing guest MTRR memtypes
  KVM: VMX: Drop support for forcing UC memory when guest CR0.CD=1
  KVM: VMX: Always honor guest PAT on CPUs that support self-snoop

Yan Zhao (2):
  srcu: Add an API for a memory barrier after SRCU read lock
  KVM: x86: Ensure a full memory barrier is emitted in the VM-Exit path

 Documentation/virt/kvm/api.rst        |   6 +-
 Documentation/virt/kvm/x86/errata.rst |  18 +
 arch/x86/include/asm/kvm_host.h       |  15 +-
 arch/x86/kvm/mmu.h                    |   7 +-
 arch/x86/kvm/mmu/mmu.c                |  35 +-
 arch/x86/kvm/mtrr.c                   | 644 ++------------------------
 arch/x86/kvm/vmx/vmx.c                |  40 +-
 arch/x86/kvm/x86.c                    |  24 +-
 arch/x86/kvm/x86.h                    |   4 -
 include/linux/srcu.h                  |  14 +
 10 files changed, 105 insertions(+), 702 deletions(-)

base-commit: 964d0c614c7f71917305a5afdca9178fe8231434
-- 
2.44.0.278.ge034bb2e1d-goog