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

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Message-ID: <Ze62bqgWhbReg9wl@yzhao56-desk.sh.intel.com>
Date: Mon, 11 Mar 2024 15:44:46 +0800
From: Yan Zhao <yan.y.zhao@...el.com>
To: Sean Christopherson <seanjc@...gle.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 Fri, Mar 08, 2024 at 05:09:25PM -0800, Sean Christopherson wrote:
> index a67c28a56417..05490b9d8a43 100644
> --- a/arch/x86/kvm/mtrr.c
> +++ b/arch/x86/kvm/mtrr.c
> @@ -19,33 +19,21 @@
>  #include <asm/mtrr.h>
>  
>  #include "cpuid.h"
"cpuid.h" is not required either.

> -#include "mmu.h"
>
..

> 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.

>  	 */
> -
>  	if (is_mmio)
>  		return MTRR_TYPE_UNCACHABLE << VMX_EPT_MT_EPTE_SHIFT;
>