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Only x86 implements them, as > out-of-line functions that warn about a shadow stack entry (Write=3D0, > Dirty=3D1) in a VMA without VM_SHADOW_STACK. > > The warning is a VM_WARN_ON_ONCE(), so without CONFIG_DEBUG_VM the > functions are empty, but the calls remain. With the SRSO "Safe RET" > mitigation the return of every such call mispredicts. > > The x86 helpers already keep such entries out of other VMAs. > pte_wrprotect(), pte_mkdirty(), pte_modify() and their PMD versions go > through SavedDirty, and only pte_mkwrite() and pmd_mkwrite() on a > VM_SHADOW_STACK VMA create the combination. > > Remove the checks, their callers and pud_shstk(), which has no other > user. > > In a KVM guest on Zen 4 with Safe RET, zapping 4K pages takes one branch > miss per page fewer. MADV_DONTNEED of 1 GiB gets 6% faster on anonymous > memory and 8% on shmem, munmap() 5%. THP is unchanged. Without Safe RET > the difference is within noise. Nice. > > Suggested-by: Dave Hansen > Signed-off-by: Kiryl Shutsemau (Meta) > --- > v2: > - Remove the checks instead of building them only with CONFIG_DEBUG_VM > (Dave Hansen). > - Drop the Fixes: tag (Rick Edgecombe). > - Note that without Safe RET the difference is within noise (Rick > Edgecombe). > - Drop Jose's Reviewed-by: the patch is new. > > v1: https://lore.kernel.org/all/20261102144001.342837-1-kas@kernel.org/ > Acked-by: Zi Yan --=20 Best Regards, Yan, Zi