linux-kernel - Re: [PATCH v5 07/22] x86/virt/tdx: Implement SEAMCALL function

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Date:   Mon, 27 Jun 2022 13:58:35 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     Kai Huang <kai.huang@...el.com>, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org
Cc:     seanjc@...gle.com, 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 v5 07/22] x86/virt/tdx: Implement SEAMCALL function

On 6/26/22 22:23, Kai Huang wrote:
> On Fri, 2022-06-24 at 11:38 -0700, Dave Hansen wrote:
>> On 6/22/22 04:16, Kai Huang wrote:
>>> SEAMCALL instruction causes #GP when SEAMRR isn't enabled, and #UD when
>>> CPU is not in VMX operation.  The TDX_MODULE_CALL macro doesn't handle
>>> SEAMCALL exceptions.  Leave to the caller to guarantee those conditions
>>> before calling __seamcall().
>>
>> I was trying to make the argument earlier that you don't need *ANY*
>> detection for TDX, other than the ability to make a SEAMCALL.
>> Basically, patch 01/22 could go away.
...
>> So what does patch 01/22 buy us?  One EXTABLE entry?
> 
> There are below pros if we can detect whether TDX is enabled by BIOS during boot
> before initializing the TDX Module:
> 
> 1) There are requirements from customers to report whether platform supports TDX
> and the TDX keyID numbers before initializing the TDX module so the userspace
> cloud software can use this information to do something.  Sorry I cannot find
> the lore link now.

<sigh>

Never listen to customers literally.  It'll just lead you down the wrong
path.  They told you, "we need $FOO in dmesg" and you ran with it
without understanding why.  The fact that you even *need* to find the
lore link is because you didn't bother to realize what they really needed.

dmesg is not ABI.  It's for humans.  If you need data out of the kernel,
do it with a *REAL* ABI.  Not dmesg.

> 2) As you can see, it can be used to handle ACPI CPU/memory hotplug and driver
> managed memory hotplug.  Kexec() support patch also can use it.
> 
> Particularly, in concept, ACPI CPU/memory hotplug is only related to whether TDX
> is enabled by BIOS, but not whether TDX module is loaded, or the result of
> initializing the TDX module.  So I think we should have some code to detect TDX
> during boot.

This is *EXACTLY* why our colleagues at Intel needs to tell us about
what the OS and firmware should do when TDX is in varying states of decay.

Does the mere presence of the TDX module prevent hotplug?  Or, if a
system has the TDX module loaded but no intent to ever use TDX, why
can't it just use hotplug like a normal system which is not addled with
the TDX albatross around its neck?

> Also, it seems adding EXTABLE to TDX_MODULE_CALL doesn't have significantly less
> code comparing to detecting TDX during boot:

It depends on a bunch of things.  It might only be a line or two of
assembly.

If you actually went and tried it, you might be able to convince me it's
a bad idea.