Hi Ard, On Tue, Jul 15, 2014 at 05:50:30PM +0100, Ard Biesheuvel wrote: > The EFI loader may load Image at any available offset. This means Image may > reside very close to or at the base of DRAM, in which case the relocation > done by efi_entry() results in Image being moved up in memory, which is > undesirable (memory below the kernel is currently not usable). Rather than complicating this path I would prefer that we fixed not being able to address memory below stext - TEXT_OFFSET. That would simplify the bootloader's job regardless of UEFI, as any loader can then simply place the kernel at a 2MB-aligned address + TEXT_OFFSET (the kernel would have to not span a 512MB boundary to keep the initial page tables simple, but that's still a weaker requirement than what we currently need). The approach taken in this patch seems sane for the current way the VA space is laid out, but if we're not currently wasting an absurd amount of memory I'd rather we got the VA layout fixed such that we can address memory below the kernel image. How much memory are we seeing wasted as a result of the existing relocation code, and how often? [...] > /* > + * Check whether the original allocation done by the EFI loader is more > + * favorable (i.e., closer to the base of DRAM) than the new one created > + * by efi_entry(). If so, reuse the original one. > + */ > + adr x3, 0f > + cmp x3, x0 > + bgt 1f // this image is loaded higher, so just proceed normally > + > + /* > + * Jump into the relocated image so we can move the original Image to > + * a 2 MB boundary + TEXT_OFFSET inside the original mapping without > + * overwriting ourselves. > + */ > + sub x3, x3, x22 // subtract current base offset > + add x3, x3, x0 // add base offset of relocated image > + br x3 // jump to next line, but in relocated image At this point I don't believe the necessary icache maintenance + isb have occurred to make the new image visible to the I-side and to ensure we don't have any stale prefetched instructions in the pipeline. That still seems to happen later. Have I missed something? > + > +0: mov x1, x0 // copy from relocated Image > + sub x0, x22, #1 // copy to base of original Image > + orr x0, x0, #(SZ_2M - 1) // .. rounded up to 2 MB > + ldr x3, =(TEXT_OFFSET + 1) // .. plus TEXT_OFFSET > + add x0, x0, x3 > + mov x2, x24 // copy 'size of Image' bytes > + bl memcpy > + add x21, x0, x23 // add stext_offset to entry point > +1: > + /* > * Flush dcache covering current runtime addresses > * of kernel text/data. Then flush all of icache. > */ > - adrp x1, _text > - add x1, x1, #:lo12:_text > - adrp x2, _edata > - add x2, x2, #:lo12:_edata > - sub x1, x2, x1 > - > + mov x1, x24 // size of Image > bl __flush_dcache_area > ic ialluis Cheers, Mark. -- To unsubscribe from this list: send the line "unsubscribe linux-efi" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
