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The proto VSID is built using both the MMU context id and effective segment ID (ESID). We should not have overlapping bits between those. That could result in us having a VSID collision. With the current code we missed masking the top bits of the ESID. This implies for kernel address we ended up using the top 4 bits of the ESID as part of the proto VSID, which is wrong. The current code use the top 4 context values (0x7fffc - 0x7ffff) for the kernel. With those context IDs used for the kernel, we don't run into VSID collisions because we get the same proto VSID irrespective of whether we mask the ESID bits or not. eg: ea = 0xf000000000000000 context = 0x7ffff w/out masking: proto_vsid = (0x7ffff << 6 | 0xf000000000000000 >> 40) = (0x1ffffc0 | 0xf00000) = 0x1ffffc0 with masking: proto_vsid = (0x7ffff << 6 | ((0xf000000000000000 >> 40) & 0x3f)) = (0x1ffffc0 | (0xf00000 & 0x3f)) = 0x1ffffc0 | 0) = 0x1ffffc0 So although there is no bug, the code is still overly subtle, so fix it to save ourselves pain in future. Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com> Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Linux kernel ============ This file was moved to Documentation/admin-guide/README.rst Please notice that there are several guides for kernel developers and users. These guides can be rendered in a number of formats, like HTML and PDF. In order to build the documentation, use ``make htmldocs`` or ``make pdfdocs``. There are various text files in the Documentation/ subdirectory, several of them using the Restructured Text markup notation. See Documentation/00-INDEX for a list of what is contained in each file. Please read the Documentation/process/changes.rst file, as it contains the requirements for building and running the kernel, and information about the problems which may result by upgrading your kernel.