Re: [PATCH v8 13/15] hw/pci: Use UINT32_MAX as a default value for rombar

From: Markus Armbruster <armbru@redhat.com>
To: Akihiko Odaki <akihiko.odaki@daynix.com>
Cc: "Philippe Mathieu-Daudé" <philmd@linaro.org>,
	"Michael S. Tsirkin" <mst@redhat.com>,
	"Marcel Apfelbaum" <marcel.apfelbaum@gmail.com>,
	"Alex Williamson" <alex.williamson@redhat.com>,
	"Cédric Le Goater" <clg@redhat.com>,
	"Paolo Bonzini" <pbonzini@redhat.com>,
	"Daniel P. Berrangé" <berrange@redhat.com>,
	"Eduardo Habkost" <eduardo@habkost.net>,
	"Sriram Yagnaraman" <sriram.yagnaraman@ericsson.com>,
	"Jason Wang" <jasowang@redhat.com>,
	"Keith Busch" <kbusch@kernel.org>,
	"Klaus Jensen" <its@irrelevant.dk>,
	qemu-devel@nongnu.org, qemu-block@nongnu.org
Subject: Re: [PATCH v8 13/15] hw/pci: Use UINT32_MAX as a default value for rombar
Date: Wed, 28 Feb 2024 13:36:57 +0100	[thread overview]
Message-ID: <87zfvkvk7q.fsf@pond.sub.org> (raw)
In-Reply-To: <20240228-reuse-v8-13-282660281e60@daynix.com> (Akihiko Odaki's message of "Wed, 28 Feb 2024 20:33:24 +0900")

Akihiko Odaki <akihiko.odaki@daynix.com> writes:

> Currently there is no way to distinguish the case that rombar is
> explicitly specified as 1 and the case that rombar is not specified.
>
> Set rombar UINT32_MAX by default to distinguish these cases just as it
> is done for addr and romsize. It was confirmed that changing the default
> value to UINT32_MAX will not change the behavior by looking at
> occurences of rom_bar.
>
> $ git grep -w rom_bar
> hw/display/qxl.c:328:    QXLRom *rom = memory_region_get_ram_ptr(&d->rom_bar);
> hw/display/qxl.c:431:    qxl_set_dirty(&qxl->rom_bar, 0, qxl->rom_size);
> hw/display/qxl.c:1048:    QXLRom *rom = memory_region_get_ram_ptr(&qxl->rom_bar);
> hw/display/qxl.c:2131:    memory_region_init_rom(&qxl->rom_bar, OBJECT(qxl), "qxl.vrom",
> hw/display/qxl.c:2154: PCI_BASE_ADDRESS_SPACE_MEMORY, &qxl->rom_bar);
> hw/display/qxl.h:101:    MemoryRegion       rom_bar;
> hw/pci/pci.c:74:    DEFINE_PROP_UINT32("rombar",  PCIDevice, rom_bar, 1),
> hw/pci/pci.c:2329:    if (!pdev->rom_bar) {
> hw/vfio/pci.c:1019:    if (vdev->pdev.romfile || !vdev->pdev.rom_bar) {
> hw/xen/xen_pt_load_rom.c:29:    if (dev->romfile || !dev->rom_bar) {
> include/hw/pci/pci_device.h:150:    uint32_t rom_bar;
>
> rom_bar refers to a different variable in qxl. It is only tested if
> the value is 0 or not in the other places.
>
> If a user explicitly set UINT32_MAX, we still cannot distinguish that
> from the implicit default. However, it is unlikely to be a problem as
> nobody would type literal UINT32_MAX (0xffffffff or 4294967295) by
> chance.
>
> Signed-off-by: Akihiko Odaki <akihiko.odaki@daynix.com>

Not exactly elegant, but I believe we have similar "default value means
not set by user (good enough because the default value is sufficiently
odd)" logic elsewhere.

Reviewed-by: Markus Armbruster <armbru@redhat.com>