No Alpha, PA-RISC or SH4 system supports PCI Express.
The older Marvell SoCs supported by iop32x and ixp4xx don't, but the
newer SoCs do. ARM Versatile doesn't support it and I'm pretty sure
QEMU won't let you add it, but will leave versatile alone for now.
Most supported MIPS platforms don't, but Octeon does.
I don't think PowerPC SPE systems have either PCI or PCI Express, but
I won't touch that configuration now.
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