linux-kernel - Re: [PATCH v7 037/102] KVM: x86/mmu: Track shadow MMIO value/mask on a per-VM basis

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Date:   Thu, 30 Jun 2022 23:45:29 +1200
From:   Kai Huang <kai.huang@...el.com>
To:     isaku.yamahata@...el.com, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Cc:     isaku.yamahata@...il.com, Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>
Subject: Re: [PATCH v7 037/102] KVM: x86/mmu: Track shadow MMIO value/mask
 on a per-VM basis

On Mon, 2022-06-27 at 14:53 -0700, isaku.yamahata@...el.com wrote:
> From: Sean Christopherson <sean.j.christopherson@...el.com>
> 
> TDX will use a different shadow PTE entry value for MMIO from VMX.  Add
> members to kvm_arch and track value for MMIO per-VM instead of global
> variables.  By using the per-VM EPT entry value for MMIO, the existing VMX
> logic is kept working.
> 
> In the case of VMX VM case, the EPT entry for MMIO is non-present PTE
> (present bit cleared) without backing guest physical address (on EPT
> violation, KVM searches backing guest memory and it finds there is no
> backing guest page.) or the value to trigger EPT misconfiguration.  Once
> MMIO is triggered on the EPT entry, the EPT entry is updated to trigger EPT
> misconfiguration for the future MMIO on the same GPA.  It allows KVM to
> understand the memory access is for MMIO without searching backing guest
> pages.). And then KVM parses guest instruction to figure out
> address/value/width for MMIO.
> 
> In the case of the guest TD, the guest memory is protected so that VMM
> can't parse guest instruction to understand the value and access width for
> MMIO.  Instead, VMM sets up (Shared) EPT to trigger #VE by clearing
> the VE-suppress bit.  When the guest TD issues MMIO, #VE is injected.  Guest VE
> handler converts MMIO access into MMIO hypercall to pass
> address/value/width for MMIO to VMM. (or directly paravirtualize MMIO into
> hypercall.)  Then VMM can handle the MMIO hypercall without parsing guest
> instructions.

This is an infrastructural patch which enables per-VM MMIO caching.  Why not
putting this patch first so you don't need to do below changes (which are
introduced by your previous patches)?

[...]

>  
> -		if (!is_shadow_present_pte(spte) || is_mmio_spte(spte))
> +		if (!is_shadow_present_pte(spte) ||
> +		    is_mmio_spte(vcpu->kvm, spte))
>  			break;
>  
> 

[...]

> @@ -1032,7 +1032,7 @@ static int FNAME(sync_page)(struct kvm_vcpu *vcpu, struct kvm_mmu_page *sp)
>  		gfn_t gfn;
>  
>  		if (!is_shadow_present_pte(sp->spt[i]) &&
> -		    !is_mmio_spte(sp->spt[i]))
> +		    !is_mmio_spte(vcpu->kvm, sp->spt[i]))
>  			continue;