linux-kernel - Re: PCI IO ECS access is no longer possible for AMD family 17h

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Message-ID: <CAJDH93uE+foFfRAXVJ48-PYvEUsbpEu_-BVoG-5HsDG66yY7AQ@mail.gmail.com>
Date: Mon, 3 Mar 2025 10:21:25 +0100
From: Rostyslav Khudolii <ros@...c.com>
To: Ingo Molnar <mingo@...nel.org>
Cc: Yazen Ghannam <yazen.ghannam@....com>, Borislav Petkov <bp@...en8.de>, linux-pci@...r.kernel.org, 
	linux-kernel@...r.kernel.org, x86@...nel.org, 
	"bhelgaas@...gle.com" <bhelgaas@...gle.com>, "tglx@...utronix.de" <tglx@...utronix.de>
Subject: Re: PCI IO ECS access is no longer possible for AMD family 17h

Hi,

> Rostyslav, I would like to ask you, do you have patches / updates for
> enabling the EnableCf8ExtCfg bit for AMD 17h+ family? I could try to
> adjust my lspci changes for new machines.

Pali, sorry for the late reply. Do I understand correctly, that even
though you have access to the ECS via
the MMCFG you still want the legacy (direct IO) to work for the
debugging purposes? I can prepare a
simple patch that will allow you to do so if that's the case.

>
> So what is the practical impact here? Do things start breaking
> unexpectedly if CONFIG_ACPI_MCFG and CONFIG_PCI_MMCONFIG are disabled?
> Then I'd suggest fixing that in the Kconfig space, either by adding a
> dependency on ACPI_MCFG && PCI_MMCONFIG, or by selecting those
> must-have pieces of infrastructure.
>

Ingo, thank you for the reply.

The way I understand the access to the PCI ECS (via raw_pci_ext_ops)
works, is the following:
1. If CONFIG_ACPI_MCFG or CONFIG_PCI_MMCONFIG are enabled - set the
raw_pci_ext_ops to use
    MMCFG to access ECS. See pci_mmcfg_early_init() / pci_mmcfg_late_init();
2. If CONFIG_ACPI_MCFG and CONFIG_PCI_MMCONFIG are disabled - set the
raw_pci_ext_ops to use
    the 'direct' access to ECS. See pci_direct_init(). The direct
access is conditional on the PCI_HAS_IO_ECS
    flag being set.

On AMD, the kernel enables the ECS IO access via the
amd_bus_cpu_online() and pci_enable_pci_io_ecs().
Except those functions have no desired effect on the AMD 17h+ family
because the register (EnableCf8ExtCfg),
they access, has been moved. What is important though, is that the
PCI_HAS_IO_ECS flag is set unconditionally.
See pci_io_ecs_init() in amd_bus.c

Therefore I was wondering whether we should add support for the 17h+
family in those functions to have
the direct access work for those families as well.

Regarding your suggestion to address this in the Kconfig space - I'm
not quite sure I follow, since right now the kernel
will use raw_pci_ext_ops whenever access beyond the first 256 bytes is
requested. Say we want to make that
conditional on CONFIG_ACPI_MCFG and CONFIG_PCI_MMCONFIG, does it also
mean then we want to drop support
for the 'direct' PCI IO ECS access altogether?

Best regards,
Rostyslav