Re: [Xen-devel] [QEMU] Correct PAGE_MASK definition

[Ian Jackson <Ian.Jackson@xxxxxxxxxxxxx>]

Han, Weidong writes ("[Xen-devel] [QEMU] Correct PAGE_MASK definition"):
> In tools/ioemu/hw/iommu.c, I found #define PAGE_MASK  (PAGE_SIZE - 1), I
> think it's not correct.

Well, yes, but just changing the definition isn't correct either.
This particular definition is used in only one place, in
iommu_translate_pa:

    pa = ((pa & IOPTE_PAGE) << 4) + (addr & PAGE_MASK);

So here it is used as `the part of the address which specifies the
location within the page' rather than `which specifies the page'.
This is both confusing and inconsistent with most of the other things
called *PAGE_MASK.

grepping xen-unstable shows similar confusion here:
 extras/mini-os/include/ia64/efi.h:#define EFI_PAGE_MASK   0xFFF
 tools/libxc/xc_ptrace.h:#define BSD_PAGE_MASK (PAGE_SIZE-1)

This confusion seems to be present in linux-2.6.18-xen.hg too (and I
imagine these came from upstream):
 arch/mips/boot/addinitrd.c:#define MIPS_PAGE_MASK     (MIPS_PAGE_SIZE-1)
 drivers/media/dvb/b2c2/flexcop-usb.h:#define V8_MEMORY_PAGE_MASK     0x7FFF
 drivers/mtd/nand/nand_base.c:#define BBT_PAGE_MASK    0xffffff3f
 drivers/w1/slaves/w1_ds2433.c:#define W1_PAGE_MASK            0x1F

Under these circumstances it may be best to just leave things be
rather than try to make this consistent.  In any case if you change
the definition you must change the point of use too.

Ian.

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