Re: [PATCH] x86/asm: Replace __force_order with memory clobber

[Nathan Chancellor]

On Sun, Aug 23, 2020 at 05:25:50PM -0400, Arvind Sankar wrote:
> The CRn accessor functions use __force_order as a dummy operand to
> prevent the compiler from reordering the inline asm.
> 
> The fact that the asm is volatile should be enough to prevent this
> already, however older versions of GCC had a bug that could sometimes
> result in reordering. This was fixed in 8.1, 7.3 and 6.5.
> 
> There are some issues with __force_order as implemented:
> - It is used only as an input operand for the write functions, and hence
>   doesn't do anything additional to prevent reordering writes.
> - It allows memory accesses to be cached/reordered across write
>   functions, but CRn writes affect the semantics of memory accesses, so
>   this could be dangerous.
> - __force_order is not actually defined in the kernel proper, but the
>   LLVM toolchain can in some cases require a definition: LLVM (as well
>   as GCC 4.9) requires it for PIE code, which is why the compressed
>   kernel has a definition, but also the clang integrated assembler may
>   consider the address of __force_order to be significant, resulting in
>   a reference that requires a definition.
> 
> Fix this by:
> - Using a memory clobber for the write functions to additionally prevent
>   caching/reordering memory accesses across CRn writes.
> - Using a dummy input operand with an arbitrary constant address for the
>   read functions, instead of a global variable. This will prevent reads
>   from being reordered across writes, while allowing memory loads to be
>   cached/reordered across CRn reads, which should be safe.
> 
> Signed-off-by: Arvind Sankar <nivedita@xxxxxxxxxxxx>
> Link: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=82602

I applied this patch to v5.9-rc2 and next-20200824 and built several
different configurations with clang + GNU as and some with clang +
LLVM's integrated assembler and saw no build failures.

Tested-by: Nathan Chancellor <natechancellor@xxxxxxxxx>

> ---
>  arch/x86/boot/compressed/pgtable_64.c |  9 ---------
>  arch/x86/include/asm/special_insns.h  | 27 ++++++++++++++-------------
>  arch/x86/kernel/cpu/common.c          |  4 ++--
>  3 files changed, 16 insertions(+), 24 deletions(-)
> 
> diff --git a/arch/x86/boot/compressed/pgtable_64.c b/arch/x86/boot/compressed/pgtable_64.c
> index c8862696a47b..7d0394f4ebf9 100644
> --- a/arch/x86/boot/compressed/pgtable_64.c
> +++ b/arch/x86/boot/compressed/pgtable_64.c
> @@ -5,15 +5,6 @@
>  #include "pgtable.h"
>  #include "../string.h"
>  
> -/*
> - * __force_order is used by special_insns.h asm code to force instruction
> - * serialization.
> - *
> - * It is not referenced from the code, but GCC < 5 with -fPIE would fail
> - * due to an undefined symbol. Define it to make these ancient GCCs work.
> - */
> -unsigned long __force_order;
> -
>  #define BIOS_START_MIN		0x20000U	/* 128K, less than this is insane */
>  #define BIOS_START_MAX		0x9f000U	/* 640K, absolute maximum */
>  
> diff --git a/arch/x86/include/asm/special_insns.h b/arch/x86/include/asm/special_insns.h
> index 59a3e13204c3..8f7791217ef4 100644
> --- a/arch/x86/include/asm/special_insns.h
> +++ b/arch/x86/include/asm/special_insns.h
> @@ -11,45 +11,46 @@
>  #include <linux/jump_label.h>
>  
>  /*
> - * Volatile isn't enough to prevent the compiler from reordering the
> - * read/write functions for the control registers and messing everything up.
> - * A memory clobber would solve the problem, but would prevent reordering of
> - * all loads stores around it, which can hurt performance. Solution is to
> - * use a variable and mimic reads and writes to it to enforce serialization
> + * The compiler should not reorder volatile asm, however older versions of GCC
> + * had a bug (which was fixed in 8.1, 7.3 and 6.5) where they could sometimes
> + * reorder volatile asm. The write functions are not a problem since they have
> + * memory clobbers preventing reordering. To prevent reads from being reordered
> + * with respect to writes, use a dummy memory operand.
>   */
> -extern unsigned long __force_order;
> +
> +#define __FORCE_ORDER "m"(*(unsigned int *)0x1000UL)
>  
>  void native_write_cr0(unsigned long val);
>  
>  static inline unsigned long native_read_cr0(void)
>  {
>  	unsigned long val;
> -	asm volatile("mov %%cr0,%0\n\t" : "=r" (val), "=m" (__force_order));
> +	asm volatile("mov %%cr0,%0\n\t" : "=r" (val) : __FORCE_ORDER);
>  	return val;
>  }
>  
>  static __always_inline unsigned long native_read_cr2(void)
>  {
>  	unsigned long val;
> -	asm volatile("mov %%cr2,%0\n\t" : "=r" (val), "=m" (__force_order));
> +	asm volatile("mov %%cr2,%0\n\t" : "=r" (val) : __FORCE_ORDER);
>  	return val;
>  }
>  
>  static __always_inline void native_write_cr2(unsigned long val)
>  {
> -	asm volatile("mov %0,%%cr2": : "r" (val), "m" (__force_order));
> +	asm volatile("mov %0,%%cr2": : "r" (val) : "memory");
>  }
>  
>  static inline unsigned long __native_read_cr3(void)
>  {
>  	unsigned long val;
> -	asm volatile("mov %%cr3,%0\n\t" : "=r" (val), "=m" (__force_order));
> +	asm volatile("mov %%cr3,%0\n\t" : "=r" (val) : __FORCE_ORDER);
>  	return val;
>  }
>  
>  static inline void native_write_cr3(unsigned long val)
>  {
> -	asm volatile("mov %0,%%cr3": : "r" (val), "m" (__force_order));
> +	asm volatile("mov %0,%%cr3": : "r" (val) : "memory");
>  }
>  
>  static inline unsigned long native_read_cr4(void)
> @@ -64,10 +65,10 @@ static inline unsigned long native_read_cr4(void)
>  	asm volatile("1: mov %%cr4, %0\n"
>  		     "2:\n"
>  		     _ASM_EXTABLE(1b, 2b)
> -		     : "=r" (val), "=m" (__force_order) : "0" (0));
> +		     : "=r" (val) : "0" (0), __FORCE_ORDER);
>  #else
>  	/* CR4 always exists on x86_64. */
> -	asm volatile("mov %%cr4,%0\n\t" : "=r" (val), "=m" (__force_order));
> +	asm volatile("mov %%cr4,%0\n\t" : "=r" (val) : __FORCE_ORDER);
>  #endif
>  	return val;
>  }
> diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
> index c5d6f17d9b9d..178499f90366 100644
> --- a/arch/x86/kernel/cpu/common.c
> +++ b/arch/x86/kernel/cpu/common.c
> @@ -359,7 +359,7 @@ void native_write_cr0(unsigned long val)
>  	unsigned long bits_missing = 0;
>  
>  set_register:
> -	asm volatile("mov %0,%%cr0": "+r" (val), "+m" (__force_order));
> +	asm volatile("mov %0,%%cr0": "+r" (val) : : "memory");
>  
>  	if (static_branch_likely(&cr_pinning)) {
>  		if (unlikely((val & X86_CR0_WP) != X86_CR0_WP)) {
> @@ -378,7 +378,7 @@ void native_write_cr4(unsigned long val)
>  	unsigned long bits_changed = 0;
>  
>  set_register:
> -	asm volatile("mov %0,%%cr4": "+r" (val), "+m" (cr4_pinned_bits));
> +	asm volatile("mov %0,%%cr4": "+r" (val) : : "memory");
>  
>  	if (static_branch_likely(&cr_pinning)) {
>  		if (unlikely((val & cr4_pinned_mask) != cr4_pinned_bits)) {
> -- 
> 2.26.2
> 
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