linux-kernel - Re: [RFC PATCH v5 008/104] KVM: TDX: Add a function to initialize TDX module

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Message-ID: <20220331170303.GA2179440@ls.amr.corp.intel.com>
Date:   Thu, 31 Mar 2022 10:03:03 -0700
From:   Isaku Yamahata <isaku.yamahata@...il.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     Isaku Yamahata <isaku.yamahata@...il.com>,
        Paolo Bonzini <pbonzini@...hat.com>, isaku.yamahata@...el.com,
        kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        Jim Mattson <jmattson@...gle.com>, erdemaktas@...gle.com,
        Connor Kuehl <ckuehl@...hat.com>
Subject: Re: [RFC PATCH v5 008/104] KVM: TDX: Add a function to initialize
 TDX module

On Thu, Mar 31, 2022 at 12:03:15AM +0000,
Sean Christopherson <seanjc@...gle.com> wrote:

> On Mon, Mar 14, 2022, Isaku Yamahata wrote:
> > On Sun, Mar 13, 2022 at 03:03:40PM +0100,
> > Paolo Bonzini <pbonzini@...hat.com> wrote:
> > 
> > > On 3/4/22 20:48, isaku.yamahata@...el.com wrote:
> > > > +
> > > > +	if (!tdx_module_initialized) {
> > > > +		if (enable_tdx) {
> > > > +			ret = __tdx_module_setup();
> > > > +			if (ret)
> > > > +				enable_tdx = false;
> > > 
> > > "enable_tdx = false" isn't great to do only when a VM is created.  Does it
> > > make sense to anticipate this to the point when the kvm_intel.ko module is
> > > loaded?
> > 
> > It's possible.  I have the following two reasons to chose to defer TDX module
> > initialization until creating first TD.  Given those reasons, do you still want
> > the initialization at loading kvm_intel.ko module?  If yes, I'll change it.
> 
> Yes, TDX module setup needs to be done at load time.  The loss of memory is
> unfortunate, e.g. if the host is part of a pool that _might_ run TDX guests, but
> the alternatives are worse.  If TDX fails to initialize, e.g. due to low mem,
> then the host will be unable to run TDX guests despite saying "I support TDX".
> Or this gem :-)

Ok.


> > - memory over head: The initialization of TDX module requires to allocate
> > physically contiguous memory whose size is about 0.43% of the system memory.
> > If user don't use TD, it will be wasted.
> > 
> > - VMXON on all pCPUs: The TDX module initialization requires to enable VMX
> > (VMXON) on all present pCPUs.  vmx_hardware_enable() which is called on creating
> > guest does it.  It naturally fits with the TDX module initialization at creating
> > first TD.  I wanted to avoid code to enable VMXON on loading the kvm_intel.ko.
> 
> That's a solvable problem, though making it work without exporting hardware_enable_all()
> could get messy.

Could you please explain any reason why it's bad idea to export it?

-- 
Isaku Yamahata <isaku.yamahata@...il.com>