[PATCH] lib: sbi: Ensure SBI extension is available

[Andrew Jones]

On Thu, Apr 27, 2023 at 03:26:20PM +0800, Xiang W wrote:
> 在 2023-04-27星期四的 09:00 +0200，Andrew Jones写道：
> > On Thu, Apr 27, 2023 at 09:02:14AM +0800, Xiang W wrote:
> > > 在 2023-04-26星期三的 19:27 +0200，Andrew Jones写道：
> > > > Ensure attempts to invoke SBI extension functions fail with
> > > > SBI_ERR_NOT_SUPPORTED when the extension's probe function has
> > > > reported that the extension is not available. By adding a new
> > > > status member to the extension which has three states
> > > > (uninitialized, available, unavailable), we ensure that the
> > > > probe function is only invoked once, lazily, upon first use.
> > > 
> > > Adding this state may cause errors. For example: the id numberof an extension is from extid_start to extid_end, which has an
> > > unused ID. Searching for an ID in this extension will return NULL
> > > if an unused ID is searched first.
> > 
> > Hi Xiang,
> > 
> > You're right. Either sbi_ecall_extension::status needs to be a function of
> > extid, like probe is, or we should just always call probe (when it's
> > available) and leave any optimizations to the individual probe functions.
> > How about this?
> > 
> > diff --git a/lib/sbi/sbi_ecall.c b/lib/sbi/sbi_ecall.c
> > index 76a1ae9ab733..c86e4e8c9134 100644
> > --- a/lib/sbi/sbi_ecall.c
> > +++ b/lib/sbi/sbi_ecall.c
> > @@ -42,16 +42,18 @@ static SBI_LIST_HEAD(ecall_exts_list);
> >  
> >  struct sbi_ecall_extension *sbi_ecall_find_extension(unsigned long extid)
> >  {
> > -       struct sbi_ecall_extension *t, *ret = NULL;
> > +       struct sbi_ecall_extension *t;
> > +       unsigned long out_val;
> >  
> >         sbi_list_for_each_entry(t, &ecall_exts_list, head) {
> >                 if (t->extid_start <= extid && extid <= t->extid_end) {
> > -                       ret = t;
> > -                       break;
> > +                       if (t->probe && (t->probe(extid, &out_val) || !out_val))
> > +                               return NULL;
> > +                       return t;
> >                 }
> >         }
> >  
> > -       return ret;
> > +       return NULL;
> >  }
> >  
> >  int sbi_ecall_register_extension(struct sbi_ecall_extension *ext)
> > 
> > Thanks,
> > drew
> Good. If so modified we can modify sbi_ecall_base_probe as well.
> As follows:
> 
> diff --git a/lib/sbi/sbi_ecall_base.c b/lib/sbi/sbi_ecall_base.c
> index 786d2ac..e3aa02b 100644
> --- a/lib/sbi/sbi_ecall_base.c
> +++ b/lib/sbi/sbi_ecall_base.c
> @@ -20,15 +20,7 @@ static int sbi_ecall_base_probe(unsigned long extid, unsigned long *out_val)
>         struct sbi_ecall_extension *ext;
>  
>         ext = sbi_ecall_find_extension(extid);
> -       if (!ext) {
> -               *out_val = 0;
> -               return 0;
> -       }
> -
> -       if (ext->probe)
> -               return ext->probe(extid, out_val);
> -
> -       *out_val = 1;
> +       *out_val = ext ? 1 : 0;
>         return 0;
>  }
>

If we do this, then we lose the return value of the extension's probe
function, which is currently getting propagated all the way back to
the SBI caller. sbi_ecall_base_probe() isn't in a fast path, so even
though probe will get invoked twice, I guess we can afford it.

Thanks,
drew