* Kees Cook <keescook@xxxxxxxxxxxx> wrote: > On Thu, Feb 16, 2017 at 2:25 PM, Pavel Machek <pavel@xxxxxx> wrote: > > Hi! > > > >> > >> -config DEBUG_RODATA > >> +config STRICT_KERNEL_RWX > >> bool "Make kernel text and rodata read-only" if ARCH_OPTIONAL_KERNEL_RWX > >> depends on ARCH_HAS_STRICT_KERNEL_RWX > >> default !ARCH_OPTIONAL_KERNEL_RWX || > > > > Debug features are expected to have runtime cost, so kconfig help is > > silent about those. But there are runtime costs, right? It would be > > nice to mention them in the help text... > > It depends on the architecture. The prior help text for arm said: > > The tradeoff is that each region is padded to section-size (1MiB) > boundaries (because their permissions are different and splitting > the 1M pages into 4K ones causes TLB performance problems), which > can waste memory. > > parisc (somewhat inaccurately) said: > > This option may have a slight performance impact because a > portion of the kernel code won't be covered by a TLB anymore. > > IIUC, arm64 does what parisc is hinting at: mappings at the end are > broken down to PAGE_SIZE. On x86, IIUC, there's actually no change to > TLB performance due to how the mappings are already set up. BTW., a good strategy with RAM sizes above say 4GB would be to just round up to the next large-TLB boundary (2MB) and waste 0-2MB of RAM - which is only 0.05% of 4GB of RAM. On most workloads, especially with SSDs it's probably a positive RAM vs. performance trade-off. Thanks, Ingo -- To unsubscribe from this list: send the line "unsubscribe linux-doc" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
