[PATCH] PCI: dynamically map ECAM regions

[Arnd Bergmann]

On Thu, May 13, 2021 at 4:18 PM Russell King <rmk+kernel at armlinux.org.uk> wrote:
>
> Attempting to boot 32-bit ARM kernels under QEMU's 3.x virt models
> fails when we have more than 512M of RAM in the model as we run out
> of vmalloc space for the PCI ECAM regions. This failure will be
> silent when running libvirt, as the console in that situation is a
> PCI device.
>
> In this configuration, the kernel maps the whole ECAM, which QEMU
> sets up for 256 buses, even when maybe only seven buses are in use.
> Each bus uses 1M of ECAM space, and ioremap() adds an additional
> guard page between allocations. The kernel vmap allocator will
> align these regions to 512K, resulting in each mapping eating 1.5M
> of vmalloc space. This means we need 384M of vmalloc space just to
> map all of these, which is very wasteful of resources.
>
> Fix this by only mapping the ECAM for buses we are going to be using.
> In my setups, this is around seven buses in most guests, which is
> 10.5M of vmalloc space - way smaller than the 384M that would
> otherwise be required. This also means that the kernel can boot
> without forcing extra RAM into highmem with the vmalloc= argument,
> or decreasing the virtual RAM available to the guest.
>
> Suggested-by: Arnd Bergmann <arnd at arndb.de>
> Signed-off-by: Russell King <rmk+kernel at armlinux.org.uk>

Looks good to me. I wonder if we should actually mark this for stable
backports. It is a somewhat invasive change, so there is a regression
risk, but it's also likely that others will run into this problem on distro
kernels.

Reviewed-by: Arnd Bergmann <arnd at arndb.de>