linux-kernel - Re: [PATCH 1/5] KVM: x86: Remove VMX support for virtualizing guest MTRR memtypes

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Date: Mon, 11 Mar 2024 17:08:42 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Yan Zhao <yan.y.zhao@...el.com>
Cc: Paolo Bonzini <pbonzini@...hat.com>, Lai Jiangshan <jiangshanlai@...il.com>, 
	"Paul E. McKenney" <paulmck@...nel.org>, Josh Triplett <josh@...htriplett.org>, kvm@...r.kernel.org, 
	rcu@...r.kernel.org, linux-kernel@...r.kernel.org, 
	Kevin Tian <kevin.tian@...el.com>, Yiwei Zhang <zzyiwei@...gle.com>
Subject: Re: [PATCH 1/5] KVM: x86: Remove VMX support for virtualizing guest
 MTRR memtypes

On Mon, Mar 11, 2024, Yan Zhao wrote:
> On Fri, Mar 08, 2024 at 05:09:25PM -0800, Sean Christopherson wrote:
> > diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> > index 7a74388f9ecf..66bf79decdad 100644
> > --- a/arch/x86/kvm/vmx/vmx.c
> > +++ b/arch/x86/kvm/vmx/vmx.c
> > @@ -7596,39 +7596,27 @@ static int vmx_vm_init(struct kvm *kvm)
> >  
> >  static u8 vmx_get_mt_mask(struct kvm_vcpu *vcpu, gfn_t gfn, bool is_mmio)
> >  {
> > -	/* We wanted to honor guest CD/MTRR/PAT, but doing so could result in
> > -	 * memory aliases with conflicting memory types and sometimes MCEs.
> > -	 * We have to be careful as to what are honored and when.
> > -	 *
> > -	 * For MMIO, guest CD/MTRR are ignored.  The EPT memory type is set to
> > -	 * UC.  The effective memory type is UC or WC depending on guest PAT.
> > -	 * This was historically the source of MCEs and we want to be
> > -	 * conservative.
> > -	 *
> > -	 * When there is no need to deal with noncoherent DMA (e.g., no VT-d
> > -	 * or VT-d has snoop control), guest CD/MTRR/PAT are all ignored.  The
> > -	 * EPT memory type is set to WB.  The effective memory type is forced
> > -	 * WB.
> > -	 *
> > -	 * Otherwise, we trust guest.  Guest CD/MTRR/PAT are all honored.  The
> > -	 * EPT memory type is used to emulate guest CD/MTRR.
> > +	/*
> > +	 * Force UC for host MMIO regions, as allowing the guest to access MMIO
> > +	 * with cacheable accesses will result in Machine Checks.
> This does not always force UC. If guest PAT is WC, the effecitve one is WC.

Doh, right, I keep forgetting that KVM doesn't ignore guest PAT for MMIO.