linux-kernel - Re: [PATCH v10 035/108] KVM: x86/mmu: Track shadow MMIO value on a per-VM basis

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Date:   Wed, 9 Nov 2022 12:27:33 +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 035/108] KVM: x86/mmu: Track shadow MMIO value on a
 per-VM basis

On Sat, 2022-10-29 at 23:22 -0700, isaku.yamahata@...el.com wrote:
> From: Isaku Yamahata <isaku.yamahata@...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.  To untangle the logic to initialize
> shadow_mmio_access_mask, introduce a separate setter function.
> 

It's weird to mention "shadow_mmio_access_mask" here (and this doesn't stand
anymore anyway as we have changed to only make mmio_value per-vm).  Just say
something like "introduce a setter function to set mmio_value for the given
guest so that TDX guest can override later".

Also, seems it's worth to mention the same mmio_mask is used for both TDX guest
and VMX guest so we can still use global shadow_mmio_mask.

[...]

>  void kvm_mmu_set_mmio_spte_mask(u64 mmio_value, u64 mmio_mask, u64 access_mask)
>  {
>  	BUG_ON((u64)(unsigned)access_mask != access_mask);
> +
>  	WARN_ON(mmio_value & shadow_nonpresent_or_rsvd_lower_gfn_mask);

Looks unnecessary change.