Le 26/08/2024 à 08:55, Mike Rapoport a écrit :
From: "Mike Rapoport (Microsoft)" <rppt@xxxxxxxxxx>
Using large pages to map text areas reduces iTLB pressure and improves
performance.
Extend execmem_alloc() with an ability to use PMD_SIZE'ed pages with ROX
permissions as a cache for smaller allocations.
Why only PMD_SIZE ?
On power 8xx, PMD_SIZE is 4M and the 8xx doesn't have such a page size.
When you call vmalloc() with VM_ALLOW_HUGE_VMAP you get 16k pages or
512k pages depending on the size you ask for, see function
arch_vmap_pte_supported_shift()
To populate the cache, a writable large page is allocated from vmalloc with
VM_ALLOW_HUGE_VMAP, filled with invalid instructions and then remapped as
ROX.
Portions of that large page are handed out to execmem_alloc() callers
without any changes to the permissions.
When the memory is freed with execmem_free() it is invalidated again so
that it won't contain stale instructions.
The cache is enabled when an architecture sets EXECMEM_ROX_CACHE flag in
definition of an execmem_range.
Christophe
