Re: [Xen-devel] [PATCH] xen/grant: Fix signed/unsigned comparisons issues

[Jan Beulich <jbeulich@xxxxxxxx>]

On 28.02.2020 10:09, Julien Grall wrote:
> Hi Jan,
> 
> On 28/02/2020 08:41, Jan Beulich wrote:
>> On 27.02.2020 19:46, Andrew Cooper wrote:
>>> Each function takes an unsigned count, and loops based on a signed i.  This
>>> causes problems when between 2 and 4 billion operations are requested.
>>
>> I can see problems, but not (as the title says) with comparison issues
>> (the signed i gets converted to unsigned for the purpose of the
>> comparison).
>>
>>> In practice, signed-ness issues aren't possible because count exceeding
>>> INT_MAX is excluded earlier in do_grant_op(), but the code reads as if it is
>>> buggy, and GCC obviously can't spot this either.
>>>
>>> Bloat-o-meter reports:
>>>    add/remove: 0/0 grow/shrink: 0/4 up/down: 0/-95 (-95)
>>>    Function                                     old     new   delta
>>>    do_grant_table_op                           7155    7140     -15
>>>    gnttab_transfer                             2732    2716     -16
>>>    gnttab_unmap_grant_ref                       771     739     -32
>>>    gnttab_unmap_and_replace                     771     739     -32
>>>    Total: Before=2996364, After=2996269, chg -0.00%
>>>
>>> and inspection of gnttab_unmap_grant_ref() at least reveals one fewer local
>>> variables on the stack.
>>
>> This is a good thing for x86. However, having started to familiarize
>> myself a tiny bit with RISC-V, I'm no longer certain our present way
>> of preferring unsigned int for array indexing variable and alike is
>> actually a good thing without further abstraction. Nevertheless, this
>> isn't an immediate issue, so ...
> 
> Would you mind expanding a bit more about this comment here? How 
> unsigned int is going to make things worse on RISC-V?

Other than x86-64 and Arm64, 64-bit (and wider) RISC-V sign-extend
the result of 32-bit operations by default. Hence for an unsigned
32-bit type to be used as array index, an additional zero-extending
insn is going to be needed (just like for our existing ports a
sign-extending insn is needed when array indexing variables are
calculated as 32-bit signed quantities, which is one of the reasons
why right now we prefer unsigned int in such cases).

Jan

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