linux-kernel - Re: [PATCH v10 105/108] KVM: TDX: Add methods to ignore accesses to CPU state

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Date:   Wed, 14 Dec 2022 11:43:14 +0000
From:   "Huang, Kai" <kai.huang@...el.com>
To:     "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "Yamahata, Isaku" <isaku.yamahata@...el.com>
CC:     "sean.j.christopherson@...el.com" <sean.j.christopherson@...el.com>,
        "pbonzini@...hat.com" <pbonzini@...hat.com>,
        "Shahar, Sagi" <sagis@...gle.com>,
        "Aktas, Erdem" <erdemaktas@...gle.com>,
        "isaku.yamahata@...il.com" <isaku.yamahata@...il.com>,
        "dmatlack@...gle.com" <dmatlack@...gle.com>,
        "Christopherson,, Sean" <seanjc@...gle.com>
Subject: Re: [PATCH v10 105/108] KVM: TDX: Add methods to ignore accesses to
 CPU state

On Sat, 2022-10-29 at 23:23 -0700, isaku.yamahata@...el.com wrote:
> +static u8 vt_get_mt_mask(struct kvm_vcpu *vcpu, gfn_t gfn, bool is_mmio)
> +{
> +	if (is_td_vcpu(vcpu)) {
> +		if (is_mmio)
> +			return MTRR_TYPE_UNCACHABLE << VMX_EPT_MT_EPTE_SHIFT;
> +		return  MTRR_TYPE_WRBACK << VMX_EPT_MT_EPTE_SHIFT;
> +	}
> +
> +	return vmx_get_mt_mask(vcpu, gfn, is_mmio);
> +}

So you are returning WB for _ALL_ guest memory, including shared.  Wouldn't this
break MTRR handling for shared memory?  For instance, IIUC we can still support
assigning a device to a TDX guest while the VT-d doesn't support coherent
memory, in which case guest's MTRR/PAT are honored.  I think this should also
apply to TDX guest's shared memory?