On Tue, Nov 16 2021 at 10:47, Ard Biesheuvel wrote:
> The kmap_local conversion broke the ARM architecture, because the new
> code assumes that all PTEs used for creating kmaps form a linear array
> in memory, and uses array indexing to look up the kmap PTE belonging to
> a certain kmap index.
>
> On ARM, this cannot work, not only because the PTE pages may be
> non-adjacent in memory, but also because ARM/!LPAE interleaves hardware
> entries and extended entries (carrying software-only bits) in a way that
> is not compatible with array indexing.
>
> Fortunately, this only seems to affect configurations with more than 8
> CPUs, due to the way the per-CPU kmap slots are organized in memory.
Ooops. I completely missed that detail. Sorry for the wreckage.
> Work around this by permitting an architecture to set a Kconfig symbol
> that signifies that the kmap PTEs do not form a lineary array in memory,
> and so the only way to locate the appropriate one is to walk the page
> tables.
> +static pte_t *kmap_get_pte(unsigned long vaddr, int idx)
> {
> + if (IS_ENABLED(CONFIG_KMAP_LOCAL_NON_LINEAR_PTE_ARRAY))
> + /*
> + * Set by the arch if __kmap_pte[-idx] does not produce
> + * the correct entry.
> + */
> + return virt_to_kpte(vaddr);
Nit. The above is not a one line statement (even if it is from a
compiler perspective).
/*
* Does the architecture have non-linear KMAP pte entries which
* cannot be accessed by index?
*/
if (IS_ENABLED(CONFIG_KMAP_LOCAL_NON_LINEAR_PTE_ARRAY))
return virt_to_kpte(vaddr);
or
if (IS_ENABLED(CONFIG_KMAP_LOCAL_NON_LINEAR_PTE_ARRAY)) {
/* PTE entries cannot be accessed by index. */
return virt_to_kpte(vaddr);
}
are readable but this is really breaking the brain OCR:
if (IS_ENABLED(CONFIG_KMAP_LOCAL_NON_LINEAR_PTE_ARRAY))
/*
* Set by the arch if __kmap_pte[-idx] does not produce
* the correct entry.
*/
return virt_to_kpte(vaddr);
if (!__kmap_pte)
Other than that.
Reviewed-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
