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

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Date: Wed,  5 Jun 2024 16:20:34 -0700
From: Sean Christopherson <seanjc@...gle.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, 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: Re: [PATCH 0/5] KVM: VMX: Drop MTRR virtualization, honor guest PAT

On Fri, 08 Mar 2024 17:09:24 -0800, Sean Christopherson wrote:
> 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.
> 
> [...]

Applied to kvm-x86 mtrrs, to get as much testing as possible before a potential
merge in 6.11.

Paul, if you can take a gander at patch 3, it would be much appreciated.

Thanks!

[1/5] KVM: x86: Remove VMX support for virtualizing guest MTRR memtypes
      https://github.com/kvm-x86/linux/commit/0a7b73559b39
[2/5] KVM: VMX: Drop support for forcing UC memory when guest CR0.CD=1
      https://github.com/kvm-x86/linux/commit/e1548088ff54
[3/5] srcu: Add an API for a memory barrier after SRCU read lock
      https://github.com/kvm-x86/linux/commit/fcfe671e0879
[4/5] KVM: x86: Ensure a full memory barrier is emitted in the VM-Exit path
      https://github.com/kvm-x86/linux/commit/eb8d8fc29286
[5/5] KVM: VMX: Always honor guest PAT on CPUs that support self-snoop
      https://github.com/kvm-x86/linux/commit/95200f24b862

--
https://github.com/kvm-x86/linux/tree/next