Re: [PATCH v2 0/6] x86/percpu: Share decrypted storage before guest setup

[Borislav Petkov <bp@alien8.de>]

On Tue, Sep 29, 2026 at 12:02:49AM -0400, Zack Rusin wrote:
> VMware publishes its per-CPU steal-time GPA without first sharing the
> storage in encrypted guests.

So this one liner is the only explanation why this patchset exists, AFAICT. So
I asked AI. I'm pasting what it said below, at the end.

Is any of it true?

If so, how much and is that the reason this patchset exists?

I.e., you want for vmware monitoring tools to work with CoCo guests.

Yes, no? Anything else?

And looking at your commit messages, they don't really talk about why the
patches exist.

And I don't think you know your audience - you're sending a bunch of patches
touching arch/x86/ and you throw all that virt gibberish around like it ain't
no tomorrow and you're thinking that tip people can follow. And I think we can
follow only bits and pieces but all of us will be mostly head-scratching here
and probably ignore the whole set.

So perhaps you should lay off your virt hat for a minute and try to explain in
an approachable way what you're trying to do so that even non-virt people can
follow.

Right now concepts are flying around left and right in that text and I have no
clue what's going on here.

Thx.

P.S., here's the AI gunk:

"In this context, “VMware per‑CPU steal‑time GPA” is referring to **Guest
Physical Address (GPA)**–based accounting of **CPU steal time per vCPU**
inside a VMware virtual machine.

Let’s break the components down:

- **Steal time**  

  In virtualization, “steal time” is the amount of time a guest CPU *wanted*
  to run but **was not scheduled by the hypervisor** because the physical CPU
  was busy running other VMs or host tasks.  

  Per‑CPU steal time means this is measured **for each vCPU** separately.

- **GPA (Guest Physical Address)**  
  GPA is the CPU’s view of “physical” memory inside the guest. It’s:
  - The address space the guest OS uses for RAM, MMIO, etc.
  - Translated by the hypervisor to real host physical memory (or nested translations with EPT/NPT).

- **Why GPA for steal time?**  
  VMware (and other hypervisors) can expose performance statistics (like steal time, idle time, run time) via:
  - Per‑vCPU counters/timers
  - Memory‑mapped (GPA‑mapped) structures that the guest can read
  - Paravirtual interfaces (e.g., hypercalls, PV drivers) which are often described or backed by GPA regions

So when someone says “VMware per‑CPU steal-time GPA,” they usually mean:

A VMware mechanism where **per‑vCPU steal‑time metrics** are exposed to the
guest via data structures located in **guest physical address space**, so the
guest OS or monitoring tools can read those counters and attribute lost CPU
time per vCPU."

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette