linux-kernel - Re: [PATCH 0/2] Kexec enabling in TDX guest

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Date:   Thu, 16 Feb 2023 10:32:47 -0800
From:   Dave Hansen <dave.hansen@...el.com>
To:     "Kirill A. Shutemov" <kirill@...temov.name>
Cc:     "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Borislav Petkov <bp@...en8.de>,
        Kuppuswamy Sathyanarayanan 
        <sathyanarayanan.kuppuswamy@...ux.intel.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Isaku Yamahata <isaku.yamahata@...el.com>, x86@...nel.org,
        linux-coco@...ts.linux.dev, kexec@...ts.infradead.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 0/2] Kexec enabling in TDX guest

On 2/16/23 10:12, Kirill A. Shutemov wrote:
> On Thu, Feb 16, 2023 at 09:50:32AM -0800, Dave Hansen wrote:
>> On 2/13/23 15:48, Kirill A. Shutemov wrote:
>>> The patch brings basic enabling of kexec in TDX guests.
>>>
>>> By "basic enabling" I mean, kexec in the guests with a single CPU.
>>> TDX guests use ACPI MADT MPWK to bring up secondary CPUs. The mechanism
>>> doesn't allow to put a CPU back offline if it has woken up.
>>>
>>> We are looking into this, but it might take time.
>> This is simple enough.  But, nobody will _actually_ use this code as-is,
>> right?  What's the point of applying it now?
> Why nobody? Single CPU VMs are not that uncommon.

Here's one data point: the only "General Purpose" ones I see AWS
offering are Haswell era:

	https://aws.amazon.com/ec2/instance-types/

That _might_ be because of concerns about SMT side-channel exposure on
anything newer.

So, we can argue about what "uncommon" means.  But, a minority of folks
care about 1-cpu VMs.  Also, a separate minority of folks care about
kexec().  I'm worried that the overlap between the two will be an
*OVERWHELMING* minority of folks.  In other words, so few people will
use this code that it'll just bitrot.

I'm looking for compelling arguments why mainline should carry this.