Jeremy Fitzhardinge <jeremy@xxxxxxxx> writes: > Chuck Ebbert wrote: >> H. Peter Anvin wrote: >> >>> Andi Kleen wrote: >>> >>>> Then we would have seen reports surely? >>>> > > Yes, I would have thought so. It surprised me that such an obvious bug > could be there, apparently for a long time. But it's real, and > potentially affects everyone. It probably doesn't affect highly modular > distros much, since the kernel itself will be relatively small. > >> I never saw a description of the symptoms of encountering this bug. >> Does it just hang, or what? >> > > You get an early-fault message on-screen, assuming that's enabled; > otherwise it will just appear to hang. It happens in pagetable_init, > when it allocates a new pagetable above the head.S mapping (8M in my > case). It will only hit if the kernel size approaches a 4M boundary, > since it won't leave enough space mapped to construct the lowmem mappings. > > It only affects native booting, since under Xen all those mappings have > already been constructed. It happened to me with a paravirt kernel that > happened to Xen compiled into it, but that was irrelevent (though > misleading; the 40k difference in kernel size was enough to make it not > happen in a non-Xen kernel). I happened to be looking at this stretch of code and I have realized that this is quite simply the wrong fix. The problem is that it depends intimately on the details of alloc_bootmem_pages_low. Essentially the problem is that when we are setting up the identity mappings in paging_init we assume the identity mappings already exist. If there are holes in the memory map or someone changes the way pages are returned from alloc_bootmem_pages_low() this code will break again. The only way to ensure this will not happen is to do what we do on x86_64 and map the new page table page into our address space before we write to it. Assuming the page we allocate is already mapped is simply not robust. Eric _______________________________________________ Virtualization mailing list Virtualization@xxxxxxxxxxxxxxxxxxxxxxxxxx https://lists.linux-foundation.org/mailman/listinfo/virtualization
