Re: [Xen-devel] [PATCH 4/4] x86/traps: honor EXT bit in error codes

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 10/11/15 17:40, Jan Beulich wrote:
> The specification does not explicitly limit the use of this bit to
> exceptions that can have selector style error codes, so to be on the
> safe side we should deal with it being set even on error codes formally
> documented to be always zero (if they're indeed always zero, the change
> is simply dead code in those cases).
>
> Introduce and use (where suitable) X86_XEC_* constants to make the code
> easier to read.
>
> To match the placement of the "hardware_trap" lable, the "hardware_gp"
> one gets moved slightly too.
>
> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
>
> --- a/xen/arch/x86/cpu/mcheck/mce.c
> +++ b/xen/arch/x86/cpu/mcheck/mce.c
> @@ -431,16 +431,16 @@ static enum mce_result mce_action(const
>  
>  /*
>   * Return:
> - * -1: if system can't be recovered
> + * 1: if system can't be recovered
>   * 0: Continue to next step
>   */
> -static int mce_urgent_action(const struct cpu_user_regs *regs,
> -                              mctelem_cookie_t mctc)
> +static bool_t mce_urgent_action(const struct cpu_user_regs *regs,
> +                                mctelem_cookie_t mctc)
>  {
>      uint64_t gstatus;
>  
> -    if ( mctc == NULL)
> -        return 0;
> +    if ( regs->error_code & X86_XEC_EXT )
> +        return 1;

#MC doesn't push an error code.  0 is pushed by the machine check handler.

>  
>      gstatus = mca_rdmsr(MSR_IA32_MCG_STATUS);
>  
> @@ -455,9 +455,9 @@ static int mce_urgent_action(const struc
>       */
>      if ( !(gstatus & MCG_STATUS_RIPV) &&
>           (!(gstatus & MCG_STATUS_EIPV) || !guest_mode(regs)) )
> -        return -1;
> +        return 1;
>  
> -    return mce_action(regs, mctc) == MCER_RESET ? -1 : 0;
> +    return mctc && mce_action(regs, mctc) == MCER_RESET;
>  }
>  
>  /* Shared #MC handler. */
> --- a/xen/arch/x86/traps.c
> +++ b/xen/arch/x86/traps.c
> @@ -618,6 +618,9 @@ static void do_trap(struct cpu_user_regs
>      unsigned int trapnr = regs->entry_vector;
>      unsigned long fixup;
>  
> +    if ( use_error_code && (regs->error_code & X86_XEC_EXT) )

In the case that use_error_code is 0, regs->error_code will be filled
with 0.

Looking at the code, the parameter is redundant and could be derived
from regs->entry_vector alone.

> +        goto hardware_trap;
> +
>      DEBUGGER_trap_entry(trapnr, regs);
>  
>      if ( guest_mode(regs) )
> @@ -644,6 +647,7 @@ static void do_trap(struct cpu_user_regs
>          return;
>      }
>  
> + hardware_trap:
>      DEBUGGER_trap_fatal(trapnr, regs);
>  
>      show_execution_state(regs);
> @@ -1265,13 +1269,14 @@ static int handle_gdt_ldt_mapping_fault(
>              tb = propagate_page_fault(curr->arch.pv_vcpu.ldt_base + offset,
>                                        regs->error_code);
>              if ( tb )
> -                tb->error_code = ((u16)offset & ~3) | 4;
> +                tb->error_code = (offset & ~(X86_XEC_EXT | X86_XEC_IDT)) |
> +                                 X86_XEC_TI;
>          }
>      }
>      else
>      {
>          /* GDT fault: handle the fault as #GP(selector). */
> -        regs->error_code = (u16)offset & ~7;
> +        regs->error_code = offset & ~(X86_XEC_EXT | X86_XEC_IDT | 
> X86_XEC_TI);
>          (void)do_general_protection(regs);
>      }
>  
> @@ -3231,7 +3236,7 @@ void do_general_protection(struct cpu_us
>  
>      DEBUGGER_trap_entry(TRAP_gp_fault, regs);
>  
> -    if ( regs->error_code & 1 )
> +    if ( regs->error_code & X86_XEC_EXT )
>          goto hardware_gp;
>  
>      if ( !guest_mode(regs) )
> @@ -3257,7 +3262,7 @@ void do_general_protection(struct cpu_us
>       * instruction. The DPL specified by the guest OS for these vectors is 
> NOT
>       * CHECKED!!
>       */
> -    if ( (regs->error_code & 3) == 2 )
> +    if ( regs->error_code & X86_XEC_IDT )

The code here has changed.  It is still technically correct because EXT
breaks earlier, but please do update the comment which currently talks
about the EXT check you have just removed.

~Andrew

>      {
>          /* This fault must be due to <INT n> instruction. */
>          const struct trap_info *ti;
> @@ -3299,9 +3304,9 @@ void do_general_protection(struct cpu_us
>          return;
>      }
>  
> + hardware_gp:
>      DEBUGGER_trap_fatal(TRAP_gp_fault, regs);
>  
> - hardware_gp:
>      show_execution_state(regs);
>      panic("GENERAL PROTECTION FAULT\n[error_code=%04x]", regs->error_code);
>  }
> --- a/xen/arch/x86/x86_64/entry.S
> +++ b/xen/arch/x86/x86_64/entry.S
> @@ -338,7 +338,7 @@ int80_slow_path:
>           * Setup entry vector and error code as if this was a GPF caused by 
> an
>           * IDT entry with DPL==0.
>           */
> -        movl  $((0x80 << 3) | 0x2),UREGS_error_code(%rsp)
> +        movl  $((0x80 << 3) | X86_XEC_IDT),UREGS_error_code(%rsp)
>          SAVE_PRESERVED
>          movl  $TRAP_gp_fault,UREGS_entry_vector(%rsp)
>          /* A GPF wouldn't have incremented the instruction pointer. */
> --- a/xen/include/asm-x86/processor.h
> +++ b/xen/include/asm-x86/processor.h
> @@ -143,6 +143,11 @@
>  #define PFEC_page_paged     (1U<<5)
>  #define PFEC_page_shared    (1U<<6)
>  
> +/* Other exception error code values. */
> +#define X86_XEC_EXT         (_AC(1,U) << 0)
> +#define X86_XEC_IDT         (_AC(1,U) << 1)
> +#define X86_XEC_TI          (_AC(1,U) << 2)
> +
>  #define XEN_MINIMAL_CR4 (X86_CR4_PGE | X86_CR4_PAE)
>  
>  #define XEN_SYSCALL_MASK (X86_EFLAGS_AC|X86_EFLAGS_VM|X86_EFLAGS_RF|    \
>
>


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