linux-kernel - Re: [RFC PATCH v5 042/104] KVM: x86/mmu: Track shadow MMIO value/mask on a per-VM basis

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Message-ID: <84d56339-4a8a-6ddb-17cb-12074588ba9c@redhat.com>
Date:   Tue, 5 Apr 2022 17:25:34 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     isaku.yamahata@...el.com, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Cc:     isaku.yamahata@...il.com, Jim Mattson <jmattson@...gle.com>,
        erdemaktas@...gle.com, Connor Kuehl <ckuehl@...hat.com>,
        Sean Christopherson <seanjc@...gle.com>
Subject: Re: [RFC PATCH v5 042/104] KVM: x86/mmu: Track shadow MMIO value/mask
 on a per-VM basis

On 3/4/22 20:48, isaku.yamahata@...el.com wrote:
> +	if (enable_ept) {
> +		const u64 init_value = enable_tdx ? VMX_EPT_SUPPRESS_VE_BIT : 0ull;
>   		kvm_mmu_set_ept_masks(enable_ept_ad_bits,
> -				      cpu_has_vmx_ept_execute_only());
> +				      cpu_has_vmx_ept_execute_only(), init_value);
> +		kvm_mmu_set_spte_init_value(init_value);
> +	}

I think kvm-intel.ko should use VMX_EPT_SUPPRESS_VE_BIT unconditionally 
as the init value.  The bit is ignored anyway if the "EPT-violation #VE" 
execution control is 0.  Otherwise looks good, but I have a couple more 
crazy ideas:

1) there could even be a test mode where KVM enables the execution 
control, traps #VE in the exception bitmap, and shouts loudly if it gets 
a #VE.  That might avoid hard-to-find bugs due to forgetting about 
VMX_EPT_SUPPRESS_VE_BIT.

2) or even, perhaps the init_value for the TDP MMU could set bit 63 
_unconditionally_, because KVM always sets the NX bit on AMD hardware. 
That would remove the whole infrastructure to keep shadow_init_value, 
because it would be constant 0 in mmu.c and constant BIT(63) in tdp_mmu.c.

Sean, what do you think?

Paolo