Re: [PATCH 2/4] hw/cxl: Use available size parameter to index into register arrays.

[Jonathan Cameron via <qemu-devel@nongnu.org>]

On Thu, 14 Sep 2023 15:54:54 +0300
Michael Tokarev <mjt@tls.msk.ru> wrote:

> 13.09.2023 18:05, Jonathan Cameron via wrote:
> > Indexing has to be done into an array with the right size elements.
> > As such, the size parameter always matches the array element size
> > and can be used in place of the longer sizeof(*array)
> > 
> > Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
> > ---
> >   hw/cxl/cxl-component-utils.c | 8 ++++----
> >   1 file changed, 4 insertions(+), 4 deletions(-)
> > 
> > diff --git a/hw/cxl/cxl-component-utils.c b/hw/cxl/cxl-component-utils.c
> > index f3bbf0fd13..089e10b232 100644
> > --- a/hw/cxl/cxl-component-utils.c
> > +++ b/hw/cxl/cxl-component-utils.c
> > @@ -76,7 +76,7 @@ static uint64_t cxl_cache_mem_read_reg(void *opaque, hwaddr offset,
> >       if (cregs->special_ops && cregs->special_ops->read) {
> >           return cregs->special_ops->read(cxl_cstate, offset, size);
> >       } else {
> > -        return cregs->cache_mem_registers[offset / sizeof(*cregs->cache_mem_registers)];
> > +        return cregs->cache_mem_registers[offset / size];  
> 
> This is a though one, and smells wrong.
> 
> Though because it is not at all obvious where this "size" value comes from,
> have to find usage(s) of this function (cache_mem_ops) and think twice about
> the other parameters in there.  Also having in mind the previous comparison
> with 8.  In this part of the code, size should always be =4, but it takes
> hard time to figure this out.
> 
> Wrong - no, because of the above - the only 2 possible values are 4 and 8,
> but it's difficult to see what's going on, and you're making it worse.
> 
> Original code was at least clear you're getting a single register from
> an array of registers, with new code it is not clear at all.

Fair point.

> 
> What I'd probably use here is to add comment that size can be either 4 or 8,
> and use a switch similar to what you've used in first patch in this series.
> And have a static_assert(sizeof(register) == 4) or something like that
> here in this second branch.

Good idea.
> 
> So it is something like:
> 
> static uint64_t cxl_cache_mem_read_reg(void *opaque, hwaddr offset,
>                                         unsigned size)
> {
>      CXLComponentState *cxl_cstate = opaque;
>      ComponentRegisters *cregs = &cxl_cstate->crb;
> 
>      switch (size) {
>      case 8:
>          qemu_log_mask(LOG_UNIMP,
>                        "CXL 8 byte cache mem registers not implemented\n");
>          return 0;
> 
>      case 4:
>          if (cregs->special_ops && cregs->special_ops->read) {
>              return cregs->special_ops->read(cxl_cstate, offset, 4);
>          } else {
>              return cregs->cache_mem_registers[offset /
>                                                sizeof(*cregs->cache_mem_registers)];
>          }
> 
>      default:
>          /* this routine is called with size being either 4 or 8 only */
>          g_assert_not_reached();
>      }
> }
> 
> Note: I especially left the sizeof() here, instead of using a previously
> suggested static_assert() - because a register can be implemented using
> larger integers on the host, it does not need to be 4 bytes, - but only
> low 4 bytes can actually be used.

I don't follow.  Here cache_mem_registers is an array of uint32_t so
it is going to be 4 bytes on any host!  Obviously that's not true for
registers in general.

So I've added the assert as it is always valid and made it a  / 4

Note I ended up with just using size in the first place because I was
planning to add a local variable that was always the same size and that was silly.

> 
> This does not shorten the line (it does by wrapping it up), but it keep
> code correct and more understandable.  Adding size parameter there makes
> it much more cryptic.
> 
> Here and in other places.
> 
> This is just an example, not a suggestion.

It makes sense.  Sorry - thought I'd sent this last week!


Jonathan

> 
> /mjt
>