On Fri, Apr 25, 2008 at 02:38:15PM +1000, Simon Horman wrote:

Below is an updated version of the description. I think that the key point is that using a RID from the 0th small-block will protect memory inserted using that RID from access by domains.

The rest is just an explanation of how I chose an RID in the 0th small-block.

-- Horms

/* * According to xen/arch/ia64/xen/regionreg.c the RID space is broken up * into large-blocks. Each block belongs to a domain, except 0th block, * which is broken up into small-blocks. The small-blocks are used for * metaphysical mappings, again one per domain, except for the 0th * small-block which is unused other than very early on in the * hypervisor boot. * * By default each large-block is 18 bits wide, which is also the minimum * allowed width for a block. Each small-block is by default 1/64 the width * of a large-block, which is the maximum division allowed. In other words * each small-block is at least 12 bits wide. * * The portion of the 0th small-block that is used early on during * the hypervisor boot relates to IA64_REGION_ID_KERNEL, which is * used to form an RID using the following scheme which seems to be * have been inherited from Linux: * * a: bits 0-2: Region Number (0-7) * b: 3-N: IA64_REGION_ID_KERNEL (0) * c: N-23: reserved (0) * * N is defined by the platform. * * For EFI we use the following RID: * * a: bits 0-2: Region Number (0-7) * e: bits 3-N: IA64_REGION_ID_KERNEL (1) * f: bits N-53: reserved (0) * * + Only 0 is used as we only need one RID. Its not really important * what this number is, so long as its between 0 and 7. * * The nice thing about this is that we are only using 4 bits of RID * space, so it shouldn't have any chance of running into an adjacent * small-block since small-blocks are at least 12 bits wide. * * It would actually be possible to just use a IA64_REGION_ID_KERNEL * based RID for EFI use. The important thing is that it is in the 0th * small block, and thus not available to domains. But as we have * lots of space, its seems to be nice and clean to just use a separate * RID for EFI. * * This can be trivially changed by updating the definition of XEN_EFI_RID. * * For reference, the RID is used to produce the value inserted * in to a region register in the following way: * * A: bit 0: VHPT (0 = off, 1 = on) * B: bit 1: reserved (0) * C: bits 2-7: log 2 page_size * D: bits 8-N: RID * E: bits N-53: reserved (0) */