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

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Date: Fri, 22 Mar 2024 09:29:04 +0000
From: "Ma, Yongwei" <yongwei.ma@...el.com>
To: Sean Christopherson <seanjc@...gle.com>, Paolo Bonzini
	<pbonzini@...hat.com>, Lai Jiangshan <jiangshanlai@...il.com>, "Paul E.
 McKenney" <paulmck@...nel.org>, Josh Triplett <josh@...htriplett.org>
CC: "kvm@...r.kernel.org" <kvm@...r.kernel.org>, "rcu@...r.kernel.org"
	<rcu@...r.kernel.org>, "linux-kernel@...r.kernel.org"
	<linux-kernel@...r.kernel.org>, "Tian, Kevin" <kevin.tian@...el.com>, "Zhao,
 Yan Y" <yan.y.zhao@...el.com>, Yiwei Zhang <zzyiwei@...gle.com>
Subject: RE: [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
> 
Verified iGPU passthrough(GVT-d) on Intel platforms, TGL Core(TM) i5-1135G7/ADL Core(TM) i7-12700/RPL/MTL Ultra 7 + Ubuntu22.04 LTS.
Both Linux Ubuntu 22.04 VM and Windows10 VM could boot up successfully. 
3D benchmark GLmark2 can run as expected in the guest VM.

Tested-by: Yongwei Ma <yongwei.ma@...el.com>

Best Regards,
Yongwei Ma