On Tue, 2020-07-21 at 12:05 -0700, Palmer Dabbelt wrote: > > * We waste vmalloc space on 32-bit systems, where there isn't a lot of it. > * On 64-bit systems the VA space around the kernel is precious because it's the > only place we can place text (modules, BPF, whatever). Why ? Branch distance limits ? You can't use trampolines ? > If we start putting > the kernel in the vmalloc space then we either have to pre-allocate a bunch > of space around it (essentially making it a fixed mapping anyway) or it > becomes likely that we won't be able to find space for modules as they're > loaded into running systems. I dislike the kernel being in the vmalloc space (see my other email) but I don't understand the specific issue with modules. > * Relying on a relocatable kernel for sv48 support introduces a fairly large > performance hit. Out of curiosity why would relocatable kernels introduce a significant hit ? Where about do you see the overhead coming from ? > Roughly, my proposal would be to: > > * Leave the 32-bit memory map alone. On 32-bit systems we can load modules > anywhere and we only have one VA width, so we're not really solving any > problems with these changes. > * Staticly allocate a 2GiB portion of the VA space for all our text, as its own > region. We'd link/relocate the kernel here instead of around PAGE_OFFSET, > which would decouple the kernel from the physical memory layout of the system. > This would have the side effect of sorting out a bunch of bootloader headaches > that we currently have. > * Sort out how to maintain a linear map as the canonical hole moves around > between the VA widths without adding a bunch of overhead to the virt2phys and > friends. This is probably going to be the trickiest part, but I think if we > just change the page table code to essentially lie about VAs when an sv39 > system runs an sv48+sv39 kernel we could make it work -- there'd be some > logical complexity involved, but it would remain fast. > > This doesn't solve the problem of virtually relocatable kernels, but it does > let us decouple that from the sv48 stuff. It also lets us stop relying on a > fixed physical address the kernel is loaded into, which is another thing I > don't like. > > I know this may be a more complicated approach, but there aren't any sv48 > systems around right now so I just don't see the rush to support them, > particularly when there's a cost to what already exists (for those who haven't > been watching, so far all the sv48 patch sets have imposed a significant > performance penalty on all systems).
