Re: [PATCH v5 1/2] ACPI: Rename get_acpi_id_for_cpu() to acpi_get_cpu_acpi_id() on non-x86

From: Andrew Cooper <andrew.cooper3@citrix.com>
To: Bjorn Helgaas <helgaas@kernel.org>,
	Chengwen Feng <fengchengwen@huawei.com>
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Subject: Re: [PATCH v5 1/2] ACPI: Rename get_acpi_id_for_cpu() to acpi_get_cpu_acpi_id() on non-x86
Date: Tue, 10 Mar 2026 22:39:48 +0000	[thread overview]
Message-ID: <89f34cfe-7b77-4f66-b3ed-8104c5b50eaf@citrix.com> (raw)
In-Reply-To: <20260310175305.GA730372@bhelgaas>

On 10/03/2026 5:53 pm, Bjorn Helgaas wrote:
>   - Split the x86 part to a separate patch and maybe (a tangent, but
>     looks dubious to me) figure out whether/why xen needs xen_vcpu_id
>     to be ACPI CPU IDs

This is an unfortunate mess made decades ago with no good solution. 
It's to do with how Xen and dom0 share responsibility for the system.

Xen PV dom0 (only, but still widely in use; ring-deprivileging and
predates hardware virt) is OSPM for the whole system, seeing the real
ACPI tables but with a number of vCPUs generally less than the whole system.

This causes Linux to have a split idea of what CPUs are, held together
by hope and duct tape.

Xen PVH dom0 (hardware virt) is OSPM, but has an MADT matching it's vCPU
layout.  It still sees the system DSDT/etc, so while this is better from
Linux's point of view, it's still not great.

Regular unprivileged VMs get a fully coherent set of CPUID + ACPI tables.

~Andrew

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