Hi Arnd & Palmer, Sorry for the delayed reply, I'm working on the next version of the patch. On Fri, Jun 4, 2021 at 5:56 PM Arnd Bergmann <arnd@xxxxxxxx> wrote: > > On Thu, Jun 3, 2021 at 5:39 PM Palmer Dabbelt <palmer@xxxxxxxxxxx> wrote: > > On Wed, 02 Jun 2021 23:00:29 PDT (-0700), Anup Patel wrote: > > >> This implementation, which adds some Kconfig entries that control page table > > >> bits, definately isn't suitable for upstream. Allowing users to set arbitrary > > >> page table bits will eventually conflict with the standard, and is just going to > > >> be a mess. It'll also lead to kernels that are only compatible with specific > > >> designs, which we're trying very hard to avoid. At a bare minimum we'll need > > >> some way to detect systems with these page table bits before setting them, > > >> and some description of what the bits actually do so we can reason about > > >> them. > > > > > > Yes, vendor specific Kconfig options are strict NO NO. We can't give-up the > > > goal of unified kernel image for all platforms. Okay, Agree. Please help review the next version of the patch. > > > > I think this is just a phrasing issue, but just to be sure: > > > > IMO it's not that they're vendor-specific Kconfig options, it's that > > turning them on will conflict with standard systems (and other vendors). > > We've already got the ability to select sets of Kconfig settings that > > will only boot on one vendor's system, which is fine, as long as there > > remains a set of Kconfig settings that will boot on all systems. > > > > An example here would be the errata: every system has errata of some > > sort, so if we start flipping off various vendor's errata Kconfigs > > you'll end up with kernels that only function properly on some systems. > > That's fine with me, as long as it's possible to turn on all vendor's > > errata Kconfigs at the same time and the resulting kernel functions > > correctly on all systems. > > Yes, this is generally the goal, and it would be great to have that > working in a way where a 'defconfig' build just turns on all the options > that are needed to use any SoC specific features and drivers while > still working on all hardware. There are however limits you may run > into at some point, and other architectures usually only manage to span > some 10 to 15 years of hardware implementations with a single > kernel before it get really hard. I could follow the goal in the next version of the patchset. Please help review, thx. > > To give some common examples that make it break down: > > - 32-bit vs 64-bit already violates that rule on risc-v (as it does on > most other architectures) > > - architectures that support both big-endian and little-endian kernels > tend to have platforms that require one or the other (e.g. mips, > though not arm). Not an issue for you. > > - page table formats are the main cause of incompatibility: arm32 > and x86-32 require three-level tables for certain features, but those > are incompatible with older cores, arm64 supports three different > page sizes, but none of them works on all cores (4KB almost works > everywhere). > > - SMP-enabled ARMv7 kernels can be configured to run on either > ARMv6 or ARMv8, but not both, in this case because of incompatible > barrier instructions. > > - 32-bit Arm has a couple more remaining features that require building > a machine specific kernel if enabled because they hardcode physical > addresses: early printk (debug_ll, not the normal earlycon), NOMMU, > and XIP. > > Arnd -- Best Regards Guo Ren ML: https://lore.kernel.org/linux-csky/
