Re: [PATCH v2 01/13] hw/intc/arm_gicv3_its: Fix event ID bounds checks

[Peter Maydell]

On Fri, 28 Jan 2022 at 01:33, Richard Henderson
<richard.henderson@linaro.org> wrote:
>
> On 1/12/22 04:10, Peter Maydell wrote:
> > In process_its_cmd() and process_mapti() we must check the
> > event ID against a limit defined by the size field in the DTE,
> > which specifies the number of ID bits minus one. Convert
> > this code to our num_foo convention:
> >   * change the variable names
> >   * use uint64_t and 1ULL when calculating the number
> >     of valid event IDs, because DTE.SIZE is 5 bits and
> >     so num_eventids may be up to 2^32
> >   * fix the off-by-one error in the comparison
> >
> > Signed-off-by: Peter Maydell<peter.maydell@linaro.org>
> > ---
> >   hw/intc/arm_gicv3_its.c | 18 ++++++++++--------
> >   1 file changed, 10 insertions(+), 8 deletions(-)
>
> Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
>
>
> > +        num_eventids = 1ULL << (FIELD_EX64(dte, DTE, SIZE) + 1);
>
> Could be written 2 << N, instead of 1 << (N + 1).

It could, but the spec defines the field as containing
"number of supported bits, minus 1", so I think that
using an expression that matches that is clearer.

Aside: clang optimizes both of these expressions to
the same thing; gcc does not:

https://godbolt.org/z/nsz4Mdxhq

(not that it will make a perf difference we care about here).

-- PMM