question about RISCV EFI stub support

[Atish Patra]

On Mon, Mar 1, 2021 at 6:43 AM Kefeng Wang <wangkefeng.wang at huawei.com> wrote:
>
> + Palmer, sorry forget add to cc list...
>
> On 2021/2/25 10:00, Atish Patra wrote:
> > On Wed, Feb 24, 2021 at 5:29 PM Kefeng Wang <wangkefeng.wang at huawei.com> wrote:
> >>
> >> On 2021/2/25 7:35, Atish Patra wrote:
> >>> On Wed, Feb 24, 2021 at 3:14 AM Kefeng Wang <wangkefeng.wang at huawei.com> wrote:
> >>>> Hi Atish and Ard,
> >>>>
> >>>> The commit "cb7d2dd5612a RISC-V: Add PE/COFF header for EFI stub" use
> >>>> c.li to
> >>>>
> >>>> add Magic "MZ" signature for PE/COFF header, which select RISCV_ISA_C,
> >>>> but "C"
> >>>>
> >>>> to the ISA is optional,  and if "C" is not support ,  EFI boot is not
> >>>> supported.
> >>>>
> >>>> Could we use ".word MZ_MAGIC" like x86 code or some other way to support
> >>>> "MZ" signature?
> >>>>
> >>> .word won't work but .short will. With .word, you end up 0000 in the 2
> >>> bytes which can be interpreted as illegal instruction.
> >> Yes, should use ".short" in RV.
> >>> I guess your larger objective is to make the C extension optional and
> >>> not enabled by default for the Linux kernel.
> >> "C" extension is optional, and the upstream kernel has the RISCV_ISA_C which
> >>
> >> could be configurable.  and I think the EFI_STUB should not depend or
> >> select it,
> >>
> >> and maybe we only want to support RV64G in some cases,  what's your option?
> >>
> > I think RISCV_ISA_C was enabled by default just to generate a smaller
> > binary size.
> > @palmer : was there any other reason ?
>
> Any comment about the too issue,
>
> 1)  directly use .shout MZ_MAGIC to decouple EFI_STUB and RISCV_ISA_C.
>
> 2)  don't set RISCV_ISA_C  to default y
>

The first one is fine but I am not sure if we shouldn't set
RISCV_ISA_C to default y.
RISC-V Linux kernel always maintained RV64GC as the minimum required
ISAs required and I don't see any
good reason to change it.

Is there any solid use case where you want to support only RV64G ?

> >>> Do you just want it for a custom kernel or the upstream kernel ?
> >>>
> >>>> Thanks,
> >>>>
> >>>> Kefeng
> >>>>
> >>>>
> >>>> _______________________________________________
> >>>> linux-riscv mailing list
> >>>> linux-riscv at lists.infradead.org
> >>>> http://lists.infradead.org/mailman/listinfo/linux-riscv
> >>>
> >
> >



-- 
Regards,
Atish