linux-kernel - Re: [PATCH v3 01/21] x86/virt/tdx: Detect SEAM

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Message-ID: <YmiMyk6T5zNhYeRB@google.com>
Date:   Wed, 27 Apr 2022 00:22:34 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Kai Huang <kai.huang@...el.com>
Cc:     Dave Hansen <dave.hansen@...el.com>, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org, pbonzini@...hat.com, len.brown@...el.com,
        tony.luck@...el.com, rafael.j.wysocki@...el.com,
        reinette.chatre@...el.com, dan.j.williams@...el.com,
        peterz@...radead.org, ak@...ux.intel.com,
        kirill.shutemov@...ux.intel.com,
        sathyanarayanan.kuppuswamy@...ux.intel.com,
        isaku.yamahata@...el.com
Subject: Re: [PATCH v3 01/21] x86/virt/tdx: Detect SEAM

On Wed, Apr 27, 2022, Kai Huang wrote:
> On Tue, 2022-04-26 at 16:28 -0700, Dave Hansen wrote:
> > On 4/26/22 16:12, Kai Huang wrote:
> > > Hi Dave,
> > > 
> > > Thanks for review!
> > > 
> > > On Tue, 2022-04-26 at 13:21 -0700, Dave Hansen wrote:
> > > > > +config INTEL_TDX_HOST
> > > > > +	bool "Intel Trust Domain Extensions (TDX) host support"
> > > > > +	default n
> > > > > +	depends on CPU_SUP_INTEL
> > > > > +	depends on X86_64
> > > > > +	help
> > > > > +	  Intel Trust Domain Extensions (TDX) protects guest VMs from
> > > > > malicious
> > > > > +	  host and certain physical attacks.  This option enables necessary
> > > > > TDX
> > > > > +	  support in host kernel to run protected VMs.
> > > > > +
> > > > > +	  If unsure, say N.
> > > > 
> > > > Nothing about KVM?
> > > 
> > > I'll add KVM into the context. How about below?
> > > 
> > > "Intel Trust Domain Extensions (TDX) protects guest VMs from malicious
> > > host and certain physical attacks.  This option enables necessary TDX
> > > support in host kernel to allow KVM to run protected VMs called Trust
> > > Domains (TD)."
> > 
> > What about a dependency?  Isn't this dead code without CONFIG_KVM=y/m?
> 
> Conceptually, KVM is one user of the TDX module, so it doesn't seem correct to
> make CONFIG_INTEL_TDX_HOST depend on CONFIG_KVM.  But so far KVM is the only
> user of TDX, so in practice the code is dead w/o KVM.
> 
> What's your opinion?

Take a dependency on CONFIG_KVM_INTEL, there's already precedence for this specific
case of a feature that can't possibly have an in-kernel user.  See
arch/x86/kernel/cpu/feat_ctl.c, which in the (very) unlikely event IA32_FEATURE_CONTROL
is left unlocked by BIOS, will deliberately disable VMX if CONFIG_KVM_INTEL=n.