Re: [PATCH 11/13] x86_64: Remove pointless set_64bit() usage

[Nathan Chancellor <nathan@kernel.org>]

Hi Peter,

On Sat, Oct 22, 2022 at 01:14:14PM +0200, Peter Zijlstra wrote:
> The use of set_64bit() in X86_64 only code is pretty pointless, seeing
> how it's a direct assignment. Remove all this nonsense.
> 
> Additionally, since x86_64 unconditionally has HAVE_CMPXCHG_DOUBLE,
> there is no point in even having that fallback.
> 
> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
> ---
>  arch/um/include/asm/pgtable-3level.h |    8 --------
>  arch/x86/include/asm/cmpxchg_64.h    |    5 -----
>  drivers/iommu/intel/irq_remapping.c  |   10 ++--------
>  3 files changed, 2 insertions(+), 21 deletions(-)
> 
> --- a/arch/um/include/asm/pgtable-3level.h
> +++ b/arch/um/include/asm/pgtable-3level.h
> @@ -58,11 +58,7 @@
>  #define pud_populate(mm, pud, pmd) \
>  	set_pud(pud, __pud(_PAGE_TABLE + __pa(pmd)))
>  
> -#ifdef CONFIG_64BIT
> -#define set_pud(pudptr, pudval) set_64bit((u64 *) (pudptr), pud_val(pudval))
> -#else
>  #define set_pud(pudptr, pudval) (*(pudptr) = (pudval))
> -#endif
>  
>  static inline int pgd_newpage(pgd_t pgd)
>  {
> @@ -71,11 +67,7 @@ static inline int pgd_newpage(pgd_t pgd)
>  
>  static inline void pgd_mkuptodate(pgd_t pgd) { pgd_val(pgd) &= ~_PAGE_NEWPAGE; }
>  
> -#ifdef CONFIG_64BIT
> -#define set_pmd(pmdptr, pmdval) set_64bit((u64 *) (pmdptr), pmd_val(pmdval))
> -#else
>  #define set_pmd(pmdptr, pmdval) (*(pmdptr) = (pmdval))
> -#endif
>  
>  static inline void pud_clear (pud_t *pud)
>  {
> --- a/arch/x86/include/asm/cmpxchg_64.h
> +++ b/arch/x86/include/asm/cmpxchg_64.h
> @@ -2,11 +2,6 @@
>  #ifndef _ASM_X86_CMPXCHG_64_H
>  #define _ASM_X86_CMPXCHG_64_H
>  
> -static inline void set_64bit(volatile u64 *ptr, u64 val)
> -{
> -	*ptr = val;
> -}
> -
>  #define arch_cmpxchg64(ptr, o, n)					\
>  ({									\
>  	BUILD_BUG_ON(sizeof(*(ptr)) != 8);				\
> --- a/drivers/iommu/intel/irq_remapping.c
> +++ b/drivers/iommu/intel/irq_remapping.c
> @@ -173,7 +173,6 @@ static int modify_irte(struct irq_2_iomm
>  	index = irq_iommu->irte_index + irq_iommu->sub_handle;
>  	irte = &iommu->ir_table->base[index];
>  
> -#if defined(CONFIG_HAVE_CMPXCHG_DOUBLE)
>  	if ((irte->pst == 1) || (irte_modified->pst == 1)) {
>  		bool ret;
>  
> @@ -187,11 +186,6 @@ static int modify_irte(struct irq_2_iomm
>  		 * same as the old value.
>  		 */
>  		WARN_ON(!ret);
> -	} else
> -#endif
> -	{
> -		set_64bit(&irte->low, irte_modified->low);
> -		set_64bit(&irte->high, irte_modified->high);
>  	}
>  	__iommu_flush_cache(iommu, irte, sizeof(*irte));
>  
> @@ -249,8 +243,8 @@ static int clear_entries(struct irq_2_io
>  	end = start + (1 << irq_iommu->irte_mask);
>  
>  	for (entry = start; entry < end; entry++) {
> -		set_64bit(&entry->low, 0);
> -		set_64bit(&entry->high, 0);
> +		WRITE_ONCE(entry->low, 0);
> +		WRITE_ONCE(entry->high, 0);
>  	}
>  	bitmap_release_region(iommu->ir_table->bitmap, index,
>  			      irq_iommu->irte_mask);
> 
> 

This commit is now in -next as commit 0475a2d10fc7 ("x86_64: Remove
pointless set_64bit() usage") and I just bisect a boot failure on my
Intel test desktop to it.

# bad: [81214a573d19ae2fa5b528286ba23cd1cb17feec] Add linux-next specific files for 20221103
# good: [8e5423e991e8cd0988d0c4a3f4ac4ca1af7d148a] Merge tag 'parisc-for-6.1-2' of git://git.kernel.org/pub/scm/linux/kernel/git/deller/parisc-linux
git bisect start '81214a573d19ae2fa5b528286ba23cd1cb17feec' '8e5423e991e8cd0988d0c4a3f4ac4ca1af7d148a'
# good: [8c13089d26d070fef87a64b48191cb7ae6dfbdb2] Merge branch 'master' of git://git.kernel.org/pub/scm/linux/kernel/git/herbert/cryptodev-2.6.git
git bisect good 8c13089d26d070fef87a64b48191cb7ae6dfbdb2
# bad: [1bba8e9d15551d2f1c304d8f9d5c647a5b54bfc0] Merge branch 'master' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip.git
git bisect bad 1bba8e9d15551d2f1c304d8f9d5c647a5b54bfc0
# good: [748c419c7ade509684ce5bcf74f50e13e0447afd] Merge branch 'for-next' of git://git.kernel.org/pub/scm/linux/kernel/git/broonie/sound.git
git bisect good 748c419c7ade509684ce5bcf74f50e13e0447afd
# good: [0acc81a3bf9f875c5ef03037ff5431d37f536f05] Merge branch 'for-next' of git://git.kernel.org/pub/scm/linux/kernel/git/robh/linux.git
git bisect good 0acc81a3bf9f875c5ef03037ff5431d37f536f05
# bad: [c0fb84e0698d2ce57f9391c7f4112f6e17676f99] Merge branch into tip/master: 'x86/cleanups'
git bisect bad c0fb84e0698d2ce57f9391c7f4112f6e17676f99
# good: [7212c34aac1ec6abadf8b439824c8307ef0dd338] Merge branch 'x86/core' into x86/paravirt, to resolve conflicts
git bisect good 7212c34aac1ec6abadf8b439824c8307ef0dd338
# good: [e1f2ac1d285d963a783a027a1b109420b07f30c1] Merge branch into tip/master: 'x86/cpu'
git bisect good e1f2ac1d285d963a783a027a1b109420b07f30c1
# good: [306b75edbf25b86fe8189a4f96c217e49483f8ae] Merge branch into tip/master: 'x86/cleanups'
git bisect good 306b75edbf25b86fe8189a4f96c217e49483f8ae
# good: [8f28b415703e1935457a4bf0be7f03dc5471d09f] mm: Rename GUP_GET_PTE_LOW_HIGH
git bisect good 8f28b415703e1935457a4bf0be7f03dc5471d09f
# bad: [0475a2d10fc7ced3268cd0f0551390b5858f90cd] x86_64: Remove pointless set_64bit() usage
git bisect bad 0475a2d10fc7ced3268cd0f0551390b5858f90cd
# good: [a677802d5b0258f93f54620e1cd181b56547c36c] x86/mm/pae: Don't (ab)use atomic64
git bisect good a677802d5b0258f93f54620e1cd181b56547c36c
# good: [533627610ae7709572a4fac1393fb61153e2a5b3] x86/mm/pae: Be consistent with pXXp_get_and_clear()
git bisect good 533627610ae7709572a4fac1393fb61153e2a5b3
# first bad commit: [0475a2d10fc7ced3268cd0f0551390b5858f90cd] x86_64: Remove pointless set_64bit() usage
# good: [533627610ae7709572a4fac1393fb61153e2a5b3] x86/mm/pae: Be consistent with pXXp_get_and_clear()
git bisect good 533627610ae7709572a4fac1393fb61153e2a5b3
# first bad commit: [0475a2d10fc7ced3268cd0f0551390b5858f90cd] x86_64: Remove pointless set_64bit() usage

Unfortunately, I see no output on the screen it is attached to so I
assume it is happening pretty early during the boot sequence, which will
probably make getting logs somewhat hard. I can provide information
about the system if that would help reveal anything. If there is
anything I can test, I am more than happy to do so.

Cheers,
Nathan