Re: [PATCH 1 of 2 V3] x86/IST: Create set_ist() helper function

[Andrew Cooper <andrew.cooper3@citrix.com>]

On 07/12/2012 11:45, Jan Beulich wrote:
>>>> On 06.12.12 at 22:42, Andrew Cooper <andrew.cooper3@citrix.com> wrote:
>> ... to save using open-coded bitwise operations, and update all IST
>> manipulation sites to use the function.
>>
>> Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
>>
>> --
>>
>> I am not overly happy with the name set_ist(), and certainly not tied to
>> it.  However, I am unable to think of a better name. set_idt_ist() is
>> wrong, as is set_irq_ist(), while set_idt_entry_ist() just seems to
>> cludgy.  The comment and parameter types do explicitly state what is
>> expected t be passed, but suggestions welcome for a better name.
>>
>> diff -r bc624b00d6d6 -r 43f86afe90be xen/arch/x86/hvm/svm/svm.c
>> --- a/xen/arch/x86/hvm/svm/svm.c
>> +++ b/xen/arch/x86/hvm/svm/svm.c
>> @@ -869,9 +869,9 @@ static void svm_ctxt_switch_from(struct 
>>      svm_vmload(per_cpu(root_vmcb, cpu));
>>  
>>      /* Resume use of ISTs now that the host TR is reinstated. */
>> -    idt_tables[cpu][TRAP_double_fault].a  |= IST_DF << 32;
>> -    idt_tables[cpu][TRAP_nmi].a           |= IST_NMI << 32;
>> -    idt_tables[cpu][TRAP_machine_check].a |= IST_MCE << 32;
>> +    set_ist(&idt_tables[cpu][TRAP_double_fault],  IST_DF);
>> +    set_ist(&idt_tables[cpu][TRAP_nmi],           IST_NMI);
>> +    set_ist(&idt_tables[cpu][TRAP_machine_check], IST_MCE);
>>  }
>>  
>>  static void svm_ctxt_switch_to(struct vcpu *v)
>> @@ -893,9 +893,9 @@ static void svm_ctxt_switch_to(struct vc
>>       * Cannot use ISTs for NMI/#MC/#DF while we are running with the guest TR.
>>       * But this doesn't matter: the IST is only req'd to handle SYSCALL/SYSRET.
>>       */
>> -    idt_tables[cpu][TRAP_double_fault].a  &= ~(7UL << 32);
>> -    idt_tables[cpu][TRAP_nmi].a           &= ~(7UL << 32);
>> -    idt_tables[cpu][TRAP_machine_check].a &= ~(7UL << 32);
>> +    set_ist(&idt_tables[cpu][TRAP_double_fault],  IST_NONE);
>> +    set_ist(&idt_tables[cpu][TRAP_nmi],           IST_NONE);
>> +    set_ist(&idt_tables[cpu][TRAP_machine_check], IST_NONE);
>>  
>>      svm_restore_dr(v);
>>  
>> diff -r bc624b00d6d6 -r 43f86afe90be xen/arch/x86/x86_64/traps.c
>> --- a/xen/arch/x86/x86_64/traps.c
>> +++ b/xen/arch/x86/x86_64/traps.c
>> @@ -370,9 +370,9 @@ void __devinit subarch_percpu_traps_init
>>      {
>>          /* Specify dedicated interrupt stacks for NMI, #DF, and #MC. */
>>          set_intr_gate(TRAP_double_fault, &double_fault);
>> -        idt_table[TRAP_double_fault].a  |= IST_DF << 32;
>> -        idt_table[TRAP_nmi].a           |= IST_NMI << 32;
>> -        idt_table[TRAP_machine_check].a |= IST_MCE << 32;
>> +        set_ist(&idt_table[TRAP_double_fault],  IST_DF);
>> +        set_ist(&idt_table[TRAP_nmi],           IST_NMI);
>> +        set_ist(&idt_table[TRAP_machine_check], IST_MCE);
>>  
>>          /*
>>           * The 32-on-64 hypercall entry vector is only accessible from ring 
>> 1.
>> diff -r bc624b00d6d6 -r 43f86afe90be xen/include/asm-x86/processor.h
>> --- a/xen/include/asm-x86/processor.h
>> +++ b/xen/include/asm-x86/processor.h
>> @@ -425,10 +425,20 @@ struct tss_struct {
>>      u8 __cacheline_filler[24];
>>  } __cacheline_aligned __attribute__((packed));
>>  
>> -#define IST_DF  1UL
>> -#define IST_NMI 2UL
>> -#define IST_MCE 3UL
>> -#define IST_MAX 3UL
>> +#define IST_NONE 0UL
>> +#define IST_DF   1UL
>> +#define IST_NMI  2UL
>> +#define IST_MCE  3UL
>> +#define IST_MAX  3UL
>> +
>> +/* Set the interrupt stack table used by a particular interrupt
>> + * descriptor table entry. */
>> +static always_inline void set_ist(idt_entry_t * idt, unsigned long ist)
>> +{
>> +    /* ist is a 3 bit field, 32 bits into the idt entry. */
>> +    ASSERT( ist < 8 );
> This ought to check against IST_MAX.

Ok

>
>> +    idt->a = ( idt->a & ~(7UL << 32) ) | ( (ist & 7UL) << 32 );
> And with the check above, the right most & is pretty pointless.
>
> Jan

I was trying to protect against overflowing into the rest of the bits. 
I would hope that all actual instantiations use the IST_ macros, and the
compiler will optimise it away, but in the case that someone calls
set_ist(<entry>, 100), the compiler will do the right thing.

Thinking about it though, anyone calling set_ist() with a value greater
than 7 probably has larger bugs in their code.

I will remove it.

~Andrew

>
>> +}
>>  
>>  #define IDT_ENTRIES 256
>>  extern idt_entry_t idt_table[];
>>
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>
>