Re: [PATCH] x86: refine guest_mode()

[Jan Beulich <jbeulich@xxxxxxxx>]

On 27.04.2020 11:59, Roger Pau Monné wrote:
> On Mon, Apr 27, 2020 at 10:03:05AM +0200, Jan Beulich wrote:
>> --- a/xen/include/asm-x86/regs.h
>> +++ b/xen/include/asm-x86/regs.h
>> @@ -10,9 +10,10 @@
>>      /* Frame pointer must point into current CPU stack. */                  
>>   \
>>      ASSERT(diff < STACK_SIZE);                                              
>>   \
>>      /* If not a guest frame, it must be a hypervisor frame. */              
>>   \
>> -    ASSERT((diff == 0) || (r->cs == __HYPERVISOR_CS));                      
>>   \
>> +    if ( diff < PRIMARY_STACK_SIZE )                                        
>>   \
>> +        ASSERT(!diff || ((r)->cs == __HYPERVISOR_CS));                      
>>   \
> 
> Why not use:
> 
> ASSERT(diff >= PRIMARY_STACK_SIZE || !diff || ((r)->cs == __HYPERVISOR_CS));

Except for the longer (without being helpful imo) string reported if
the assertion triggers, I see not difference.

> I'm not sure I fully understand this layout, is it possible that you
> also need to account for the size of cpu_info?

Depends on how paranoid we want the checking here to be, but in going
further I wouldn't want this to become sub-page fine-grained if we
aren't first doing e.g. what I'm mentioning in the post-commit-message
remark.

Jan