Re: [Xen-devel] [PATCH 3/6] x86/shadow: Use ERR_PTR infrastructure for sh_emulate_map_dest()

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 22.06.17 at 10:21, <andrew.cooper3@xxxxxxxxxx> wrote:
> On 22/06/2017 09:14, Jan Beulich wrote:
>>>>> On 21.06.17 at 17:12, <andrew.cooper3@xxxxxxxxxx> wrote:
>>> sh_emulate_map_dest() predates the introduction of the generic ERR_PTR()
>>> infrasturcture, but take the opportunity to avoid opencoding it.
>>>
>>> The chosen error constants require need to be negative to work with 
> IS_ERR(),
>>> but no other changes.
>> Drop one of "require" or "need"?
>>
>>> --- a/xen/arch/x86/mm/shadow/private.h
>>> +++ b/xen/arch/x86/mm/shadow/private.h
>>> @@ -395,10 +395,9 @@ void shadow_unhook_mappings(struct domain *d, mfn_t 
> smfn, int user_only);
>>>  
>>>  /* Returns a mapped pointer to write to, or one of the following error
>>>   * indicators. */
>>> -#define MAPPING_UNHANDLEABLE ((void *)(unsigned long)X86EMUL_UNHANDLEABLE)
>>> -#define MAPPING_EXCEPTION    ((void *)(unsigned long)X86EMUL_EXCEPTION)
>>> -#define MAPPING_SILENT_FAIL  ((void *)(unsigned long)X86EMUL_OKAY)
>>> -#define sh_emulate_map_dest_failed(rc) ((unsigned long)(rc) <= 3)
>>> +#define MAPPING_UNHANDLEABLE ERR_PTR(~(long)X86EMUL_UNHANDLEABLE)
>>> +#define MAPPING_EXCEPTION    ERR_PTR(~(long)X86EMUL_EXCEPTION)
>>> +#define MAPPING_SILENT_FAIL  ERR_PTR(~(long)X86EMUL_OKAY)
>> While this doesn't change with your patch, this last definition has a
>> string dependency on X86EMUL_OKAY to be zero, yet there's no
>> respective BUILD_BUG_ON() anywhere. Would you mind adding
>> one at once? In any event
>> Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
> 
> There is no longer any dependence on X86EMUL_OKAY being 0 (so far as I
> can see).  All this logic would work fine if OKAY had the value 10 for
> example.

The mapping failure is "silent" only if X86EMUL_OKAY is zero, afaict.

> The only requirement (now) is that the X86EMUL constants are all within
> MAX_ERRNO so they count as errors as far as the ERR_PTR logic is
> concerned, and I suppose those should gain BUILD_BUG_ON()s

That would be helpful too.

Jan


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