linux-kernel - Re: [PATCH v13 057/113] KVM: TDX: MTRR: implement get_mt

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Date:   Wed, 29 Mar 2023 18:15:54 -0700
From:   Isaku Yamahata <isaku.yamahata@...il.com>
To:     "Huang, Kai" <kai.huang@...el.com>
Cc:     "isaku.yamahata@...il.com" <isaku.yamahata@...il.com>,
        "Christopherson,, Sean" <seanjc@...gle.com>,
        "Shahar, Sagi" <sagis@...gle.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "Aktas, Erdem" <erdemaktas@...gle.com>,
        "dmatlack@...gle.com" <dmatlack@...gle.com>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "zhi.wang.linux@...il.com" <zhi.wang.linux@...il.com>,
        "pbonzini@...hat.com" <pbonzini@...hat.com>,
        "Yamahata, Isaku" <isaku.yamahata@...el.com>
Subject: Re: [PATCH v13 057/113] KVM: TDX: MTRR: implement get_mt_mask() for
 TDX

On Mon, Mar 27, 2023 at 09:54:40AM +0000,
"Huang, Kai" <kai.huang@...el.com> wrote:

> > diff -u b/arch/x86/kvm/vmx/tdx.c b/arch/x86/kvm/vmx/tdx.c
> > --- b/arch/x86/kvm/vmx/tdx.c
> > +++ b/arch/x86/kvm/vmx/tdx.c
> > @@ -347,6 +347,25 @@
> >  	return 0;
> >  }
> >  
> > +u8 tdx_get_mt_mask(struct kvm_vcpu *vcpu, gfn_t gfn, bool is_mmio)
> > +{
> > +	/* TDX private GPA is always WB. */
> > +	if (!(gfn & kvm_gfn_shared_mask(vcpu->kvm))) {
> 
> Are you still passing a "raw" GFN?  Could you explain why you choose this way?
> 
> > +		/* MMIO is only for shared GPA. */
> > +		WARN_ON_ONCE(is_mmio);
> > +		return  MTRR_TYPE_WRBACK << VMX_EPT_MT_EPTE_SHIFT;
> 
> I guess it's better to include VMX_EPT_IPAT_BIT bit.

On second thought, there is no need to check it.  We can simply drop this check.

u8 tdx_get_mt_mask(struct kvm_vcpu *vcpu, gfn_t gfn, bool is_mmio)
{
	if (is_mmio)
		return MTRR_TYPE_UNCACHABLE << VMX_EPT_MT_EPTE_SHIFT;

	if (!kvm_arch_has_noncoherent_dma(vcpu->kvm))
		return (MTRR_TYPE_WRBACK << VMX_EPT_MT_EPTE_SHIFT) | VMX_EPT_IPAT_BIT;

	/* TDX enforces CR0.CD = 0 and KVM MTRR emulation enforces writeback. */
	return MTRR_TYPE_WRBACK << VMX_EPT_MT_EPTE_SHIFT;
}

-- 
Isaku Yamahata <isaku.yamahata@...il.com>