Re: [patch V2 09/29] x86/exceptions: Add structs for exception stacks

[Sean Christopherson]

On Fri, Apr 05, 2019 at 05:07:07PM +0200, Thomas Gleixner wrote:
> At the moment everything assumes a full linear mapping of the various
> exception stacks. Adding guard pages to the cpu entry area mapping of the
> exception stacks will break that assumption.
> 
> As a preparatory step convert both the real storage and the effective
> mapping in the cpu entry area from character arrays to structures.
> 
> To ensure that both arrays have the same ordering and the same size of the
> individual stacks fill the members with a macro. The guard size is the only
> difference between the two resulting structures. For now both have guard
> size 0 until the preparation of all usage sites is done.
> 
> Provide a couple of helper macros which are used in the following
> conversions.
> 
> Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> ---
>  arch/x86/include/asm/cpu_entry_area.h |   51 ++++++++++++++++++++++++++++++----
>  arch/x86/kernel/cpu/common.c          |    2 -
>  arch/x86/mm/cpu_entry_area.c          |    8 ++---
>  3 files changed, 50 insertions(+), 11 deletions(-)
> 
> --- a/arch/x86/include/asm/cpu_entry_area.h
> +++ b/arch/x86/include/asm/cpu_entry_area.h
> @@ -7,6 +7,50 @@
>  #include <asm/processor.h>
>  #include <asm/intel_ds.h>
>  
> +#ifdef CONFIG_X86_64
> +
> +/* Macro to enforce the same ordering and stack sizes */
> +#define ESTACKS_MEMBERS(guardsize)		\
> +	char	DF_stack[EXCEPTION_STKSZ];	\
> +	char	DF_stack_guard[guardsize];	\
> +	char	NMI_stack[EXCEPTION_STKSZ];	\
> +	char	NMI_stack_guard[guardsize];	\
> +	char	DB_stack[DEBUG_STKSZ];		\
> +	char	DB_stack_guard[guardsize];	\
> +	char	MCE_stack[EXCEPTION_STKSZ];	\
> +	char	MCE_stack_guard[guardsize];	\

Conceptually, shouldn't the stack guard precede its associated stack
since the stacks grow down?  And don't we want a guard page below the
DF_stack?  There could still be a guard page above MCE_stack,
e.g. IST_stack_guard or something.

E.g. the example in patch "Speedup in_exception_stack()" also suggests
that "guard page" is associated with the stack physical above it:

      --- top of DB_stack       <- Initial stack
      --- end of DB_stack
          guard page

      --- top of DB1_stack      <- Top of stack after entering first #DB
      --- end of DB1_stack
          guard page

      --- top of DB2_stack      <- Top of stack after entering second #DB
      --- end of DB2_stack
          guard page

> +
> +/* The exception stacks linear storage. No guard pages required */
> +struct exception_stacks {
> +	ESTACKS_MEMBERS(0)
> +};
> +
> +/*
> + * The effective cpu entry area mapping with guard pages. Guard size is
> + * zero until the code which makes assumptions about linear mapping is
> + * cleaned up.
> + */
> +struct cea_exception_stacks {
> +	ESTACKS_MEMBERS(0)
> +};
> +
> +#define CEA_ESTACK_TOP(ceastp, st)			\
> +	((unsigned long)&(ceastp)->st## _stack_guard)

IMO, using the stack guard to define the top of stack is unnecessarily
confusing and fragile, e.g. reordering the names of the stack guards
would break this macro.

What about:

#define CEA_ESTACK_TOP(ceastp, st)			\
	(CEA_ESTACK_BOT(ceastp, st) + CEA_ESTACK_SIZE(st))

> +#define CEA_ESTACK_BOT(ceastp, st)			\
> +	((unsigned long)&(ceastp)->st## _stack)
> +
> +#define CEA_ESTACK_OFFS(st)					\
> +	offsetof(struct cea_exception_stacks, st## _stack)
> +
> +#define CEA_ESTACK_SIZE(st)					\
> +	sizeof(((struct cea_exception_stacks *)0)->st## _stack)
> +
> +#define CEA_ESTACK_PAGES					\
> +	(sizeof(struct cea_exception_stacks) / PAGE_SIZE)
> +
> +#endif
> +
>  /*
>   * cpu_entry_area is a percpu region that contains things needed by the CPU
>   * and early entry/exit code.  Real types aren't used for all fields here
> @@ -32,12 +76,9 @@ struct cpu_entry_area {
>  
>  #ifdef CONFIG_X86_64
>  	/*
> -	 * Exception stacks used for IST entries.
> -	 *
> -	 * In the future, this should have a separate slot for each stack
> -	 * with guard pages between them.
> +	 * Exception stacks used for IST entries with guard pages.
>  	 */
> -	char exception_stacks[(N_EXCEPTION_STACKS - 1) * EXCEPTION_STKSZ + DEBUG_STKSZ];
> +	struct cea_exception_stacks estacks;
>  #endif
>  #ifdef CONFIG_CPU_SUP_INTEL
>  	/*
> --- a/arch/x86/kernel/cpu/common.c
> +++ b/arch/x86/kernel/cpu/common.c
> @@ -1754,7 +1754,7 @@ void cpu_init(void)
>  	 * set up and load the per-CPU TSS
>  	 */
>  	if (!oist->ist[0]) {
> -		char *estacks = get_cpu_entry_area(cpu)->exception_stacks;
> +		char *estacks = (char *)&get_cpu_entry_area(cpu)->estacks;
>  
>  		for (v = 0; v < N_EXCEPTION_STACKS; v++) {
>  			estacks += exception_stack_sizes[v];
> --- a/arch/x86/mm/cpu_entry_area.c
> +++ b/arch/x86/mm/cpu_entry_area.c
> @@ -13,8 +13,7 @@
>  static DEFINE_PER_CPU_PAGE_ALIGNED(struct entry_stack_page, entry_stack_storage);
>  
>  #ifdef CONFIG_X86_64
> -static DEFINE_PER_CPU_PAGE_ALIGNED(char, exception_stacks
> -	[(N_EXCEPTION_STACKS - 1) * EXCEPTION_STKSZ + DEBUG_STKSZ]);
> +static DEFINE_PER_CPU_PAGE_ALIGNED(struct exception_stacks, exception_stacks);
>  #endif
>  
>  struct cpu_entry_area *get_cpu_entry_area(int cpu)
> @@ -138,9 +137,8 @@ static void __init setup_cpu_entry_area(
>  #ifdef CONFIG_X86_64
>  	BUILD_BUG_ON(sizeof(exception_stacks) % PAGE_SIZE != 0);
>  	BUILD_BUG_ON(sizeof(exception_stacks) !=
> -		     sizeof(((struct cpu_entry_area *)0)->exception_stacks));
> -	cea_map_percpu_pages(&cea->exception_stacks,
> -			     &per_cpu(exception_stacks, cpu),
> +		     sizeof(((struct cpu_entry_area *)0)->estacks));
> +	cea_map_percpu_pages(&cea->estacks, &per_cpu(exception_stacks, cpu),
>  			     sizeof(exception_stacks) / PAGE_SIZE, PAGE_KERNEL);
>  #endif
>  	percpu_setup_debug_store(cpu);
> 
>