Re: [PATCH v2 1/2] gnttab: adjust pin count overflow checks

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 15/01/2021 15:14, Jan Beulich wrote:
> It's at least odd to check counters which aren't going to be
> incremented, resulting in failure just because prior operations may
> have left an entry in an unusual state.

I wouldn't say it's an unusual state.  It can happen legally when you
map the same gref 128 times

Why a guest would do this in normal operation is a different question.

Perhaps "prior operations may have reached the refcount limit" ?

>  And it's also not helpful to
> use open-coded literal numbers in these checks.
>
> Calculate the increment values first and derive from them the mask to
> use in the checks.
>
> Also move the pin count checks ahead of the calculation of the status
> (and for copy also sha2) pointers: They're not needed in the failure
> cases, and this way the compiler may also have an easier time keeping
> the variables at least transiently in registers for the subsequent uses.
>
> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

Reviewed-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>