linux-kernel - Re: [PATCH v19 023/130] KVM: TDX: Initialize the TDX module when loading the KVM intel kernel module

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Date: Mon, 29 Apr 2024 13:06:52 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Kai Huang <kai.huang@...el.com>
Cc: Tina Zhang <tina.zhang@...el.com>, Hang Yuan <hang.yuan@...el.com>, 
	Bo2 Chen <chen.bo@...el.com>, "sagis@...gle.com" <sagis@...gle.com>, 
	"isaku.yamahata@...il.com" <isaku.yamahata@...il.com>, 
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, Erdem Aktas <erdemaktas@...gle.com>, 
	"kvm@...r.kernel.org" <kvm@...r.kernel.org>, "pbonzini@...hat.com" <pbonzini@...hat.com>, 
	Isaku Yamahata <isaku.yamahata@...el.com>, 
	"isaku.yamahata@...ux.intel.com" <isaku.yamahata@...ux.intel.com>
Subject: Re: [PATCH v19 023/130] KVM: TDX: Initialize the TDX module when
 loading the KVM intel kernel module

On Mon, Apr 29, 2024, Kai Huang wrote:
> On Thu, 2024-04-25 at 15:43 -0700, Sean Christopherson wrote:
> > > And the odd is currently the common SEAMCALL functions, a.k.a,
> > > __seamcall() and seamcall() (the latter is a mocro actually), both return
> > > u64, so if we want to have such CR4.VMX check code in the common code, we
> > > need to invent a new error code for it.
> > 
> > Oh, I wasn't thinking that we'd check CR4.VMXE before *every* SEAMCALL, just
> > before the TDH.SYS.LP.INIT call, i.e. before the one that is most likely to fail
> > due to a software bug that results in the CPU not doing VMXON before enabling
> > TDX.
> > 
> > Again, my intent is to add a simple, cheap, and targeted sanity check to help
> > deal with potential failures in code that historically has been less than rock
> > solid, and in function that has a big fat assumption that the caller has done
> > VMXON on the CPU.
> 
> I see.
> 
> (To be fair, personally I don't recall that we ever had any bug due to
> "cpu not in post-VMXON before SEAMCALL", but maybe it's just me. :-).)
> 
> But if tdx_enable() doesn't call tdx_cpu_enable() internally, then we will
> have two functions need to handle.

Why?  I assume there will be exactly one caller of TDH.SYS.LP.INIT.

> For tdx_enable(), given it's still good idea to disable CPU hotplug around
> it, we can still do some check for all online cpus at the beginning, like:
> 
> 	on_each_cpu(check_cr4_vmx(), &err, 1);

If it gets to that point, just omit the check.  I really think you're making much
ado about nothing.  My suggestion is essentially "throw in a CR4.VMXE check before
TDH.SYS.LP.INIT if it's easy".  If it's not easy for some reason, then don't do
it.

> Btw, please also see my last reply to Chao why I don't like calling
> tdx_cpu_enable() inside tdx_enable():
> 
> https://lore.kernel.org/lkml/1fd17c931d5c2effcf1105b63deac8e3fb1664bc.camel@intel.com/
> 
> That being said, I can try to add additional patch(es) to do CR4.VMX check
> if you want, but personally I found hard to have a strong justification to
> do so.
>