linux-kernel - Re: [RFC PATCH v1 11/38] KVM: arm64: CCA: register host tsm platform device

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Message-ID: <d5144c99d7c04e4ad09ed9965fa3512c203b5694.camel@HansenPartnership.com>
Date: Wed, 15 Oct 2025 11:19:41 -0400
From: James Bottomley <James.Bottomley@...senPartnership.com>
To: Jason Gunthorpe <jgg@...pe.ca>, Greg KH <gregkh@...uxfoundation.org>
Cc: "Aneesh Kumar K.V" <aneesh.kumar@...nel.org>, Jeremy Linton
 <jeremy.linton@....com>, Jonathan Cameron <Jonathan.Cameron@...wei.com>,
 Dan Williams <dan.j.williams@...el.com>, linux-coco@...ts.linux.dev,
 kvmarm@...ts.linux.dev,  linux-pci@...r.kernel.org,
 linux-kernel@...r.kernel.org, aik@....com,  lukas@...ner.de, Samuel Ortiz
 <sameo@...osinc.com>, Xu Yilun <yilun.xu@...ux.intel.com>, Suzuki K Poulose
 <Suzuki.Poulose@....com>,  Steven Price <steven.price@....com>, Catalin
 Marinas <catalin.marinas@....com>, Marc Zyngier <maz@...nel.org>,  Will
 Deacon <will@...nel.org>, Oliver Upton <oliver.upton@...ux.dev>
Subject: Re: [RFC PATCH v1 11/38] KVM: arm64: CCA: register host tsm
 platform device

On Wed, 2025-10-15 at 08:50 -0300, Jason Gunthorpe wrote:
> On Wed, Oct 15, 2025 at 11:58:25AM +0200, Greg KH wrote:
[...]
> > The real device that has the resources you wish to share access
> > to.  Are there physical resources here you are sharing?  If so,
> > that device is the parent.  If there is no such thing, then just
> > make a bunch of faux devices and be done with it :)
> 
> At the very bottom of the stack it looks like the PSCI interface is
> discovered first through DT/ACPI. The PSCI interface has RPCs that
> are then used to discover if SMC/etc/etc are present and along the
> way it makes platform devices to plug in subsystems to it based on
> what it can discover.
> 
> It is just not sharing "resources" in the traditional sense, PSCI has
> no registers or interrupts, yet it is a service provided by the
> platform firmare.
> 
> Again faux devices don't serve the need here to load modules and do
> driver binding.

This came up for the SVSM as well: we want to expose things that can be
virtual devices or other resources that the guest discovers.  Our
conclusion was we either needed to share one of the virtual busses
(like virtio) or do our own svsm bus.  The agreement was to implement
our own bus, but we still haven't got around to it.

Regards,

James