arm64/kpti: Move DAIF masking to C code

We really don't want to take an exception while replacing TTBR1 so we mask
DAIF during the actual update. Currently this is done in the assembly
function idmap_cpu_replace_ttbr1() but it could equally be done in the only
caller of that function, cpu_replace_ttbr1(). This simplifies the assembly
code slightly and means that when working with the code around masking DAIF
flags there is one less piece of assembly code which needs to be considered.

While we're at it add a comment which makes explicit why we are masking
DAIF in this code.

There should be no functional effect.
Signed-off-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20221123180209.634650-2-broonie@kernel.orgSigned-off-by: Will Deacon <will@kernel.org>

arch/arm64/include/asm/mmu_context.h

@@ -18,6 +18,7 @@
 
 #include <asm/cacheflush.h>
 #include <asm/cpufeature.h>
+#include <asm/daifflags.h>
 #include <asm/proc-fns.h>
 #include <asm-generic/mm_hooks.h>
 #include <asm/cputype.h>
@@ -152,6 +153,7 @@ static inline void cpu_replace_ttbr1(pgd_t *pgdp, pgd_t *idmap)
 	typedef void (ttbr_replace_func)(phys_addr_t);
 	extern ttbr_replace_func idmap_cpu_replace_ttbr1;
 	ttbr_replace_func *replace_phys;
+	unsigned long daif;
 
 	/* phys_to_ttbr() zeros lower 2 bits of ttbr with 52-bit PA */
 	phys_addr_t ttbr1 = phys_to_ttbr(virt_to_phys(pgdp));
@@ -171,7 +173,15 @@ static inline void cpu_replace_ttbr1(pgd_t *pgdp, pgd_t *idmap)
 	replace_phys = (void *)__pa_symbol(idmap_cpu_replace_ttbr1);
 
 	__cpu_install_idmap(idmap);
+
+	/*
+	 * We really don't want to take *any* exceptions while TTBR1 is
+	 * in the process of being replaced so mask everything.
+	 */
+	daif = local_daif_save();
 	replace_phys(ttbr1);
+	local_daif_restore(daif);
+
 	cpu_uninstall_idmap();
 }
 

arch/arm64/mm/proc.S

@@ -189,16 +189,12 @@ SYM_FUNC_END(cpu_do_resume)
  * called by anything else. It can only be executed from a TTBR0 mapping.
  */
 SYM_TYPED_FUNC_START(idmap_cpu_replace_ttbr1)
-	save_and_disable_daif flags=x2
-
 	__idmap_cpu_set_reserved_ttbr1 x1, x3
 
 	offset_ttbr1 x0, x3
 	msr	ttbr1_el1, x0
 	isb
 
-	restore_daif x2
-
 	ret
 SYM_FUNC_END(idmap_cpu_replace_ttbr1)
 	.popsection