arm64: drop unnecessary cache+tlb maintenance
Mark Rutland
reviewerPATCH 1/1 arm64: drop unnecessary cache+tlb maintenance
In paging_init, we call flush_cache_all, but this is backed by Set/Way operations which may not achieve anything in the presence of cache line migration and/or system caches. If the caches are already in an inconsistent state at this point, there is nothing we can do (short of flushing the entire physical address space by VA) to empty architected and system caches. As such, flush_cache_all only serves to mask other potential bugs. Hence, this patch removes the boot-time call to flush_cache_all.
Immediately after the cache maintenance we flush the TLBs, but this is also unnecessary. Before enabling the MMU, the TLBs are invalidated, and thus are initially clean. When changing the contents of active tables (e.g. in fixup_executable() for DEBUG_RODATA) we perform the required TLB maintenance following the update, and therefore no additional maintenance is required to ensure the new table entries are in effect. Since activating the MMU we will not have modified system register fields permitted to be cached in a TLB, and therefore do not need maintenance for any cached system register fields. Hence, the TLB flush is unnecessary.
Shortly after the unnecessary TLB flush, we update TTBR0 to point to an empty zero page rather than the idmap, and flush the TLBs. This maintenance is necessary to remove the global idmap entries from the TLBs (as they would conflict with userspace mappings), and is retained.
--- arch/arm64/mm/mmu.c | 7 ------- 1 file changed, 7 deletions(-)
arch/arm64/mm/mmu.c +0 -7
@@ -431,13 +431,6 @@ void __init paging_init(void)map_mem ();fixup_executable (); -/* -* Finally flush the caches and tlb to ensure that we're in a -* consistent state. -*/ -flush_cache_all (); -flush_tlb_all (); -/* allocate the zero page. */ zero_page =early_alloc (PAGE_SIZE );
Signature · 1 line
1.9.1
PATCH 1/1 arm64: drop unnecessary cache+tlb maintenance
Quoted 25 lines · Mark Rutland
In
paging_init, we callflush_cache_all, but this is backed by Set/Way operations which may not achieve anything in the presence of cache line migration and/or system caches. If the caches are already in an inconsistent state at this point, there is nothing we can do (short of flushing the entire physical address space by VA) to empty architected and system caches. As such,flush_cache_allonly serves to mask other potential bugs. Hence, this patch removes the boot-time call toflush_cache_all.Immediately after the cache maintenance we flush the TLBs, but this is also unnecessary. Before enabling the MMU, the TLBs are invalidated, and thus are initially clean. When changing the contents of active tables (e.g. in
fixup_executable()forDEBUG_RODATA) we perform the required TLB maintenance following the update, and therefore no additional maintenance is required to ensure the new table entries are in effect. Since activating the MMU we will not have modified system register fields permitted to be cached in a TLB, and therefore do not need maintenance for any cached system register fields. Hence, the TLB flush is unnecessary.Shortly after the unnecessary TLB flush, we update TTBR0 to point to an empty zero page rather than the idmap, and flush the TLBs. This maintenance is necessary to remove the global idmap entries from the TLBs (as they would conflict with userspace mappings), and is retained.
Quoted 5 lines
Signed-off-by Mark Rutland <[email protected]>Acked-by Marc Zyngier <[email protected]>Cc Catalin Marinas <[email protected]>Cc Steve Capper <[email protected]>Cc Will Deacon <[email protected]>
Hi Mark,
This looks reasonable to me.
Please feel free to add:
Cheers,
Signature · 1 line
Steve
Quoted 25 lines
--- arch/arm64/mm/mmu.c | 7 ------- 1 file changed, 7 deletions(-)arch/arm64/mm/mmu.c +0
-7@@ -431,13 +431,6 @@ void __init paging_init(void)map_mem ();fixup_executable (); -/* -* Finally flush the caches and tlb to ensure that we're in a -* consistent state. -*/ -flush_cache_all (); -flush_tlb_all (); -/* allocate the zero page. */ zero_page =early_alloc (PAGE_SIZE );Signature · 1 line
1.9.1