Re: [PATCH v1 1/4] xen: introduce CONFIG_GENERIC_BUG_FRAME

[Oleksii <oleksii.kurochko@xxxxxxxxx>]

On Thu, 2023-02-16 at 10:48 +0000, Andrew Cooper wrote:
> On 16/02/2023 7:31 am, Jan Beulich wrote:
> > On 15.02.2023 18:59, Oleksii wrote:
> > > Hello Jan and community,
> > > 
> > > I experimented and switched RISC-V to x86 implementation. All
> > > that I
> > > changed in x86 implementation for RISC-V was _ASM_BUGFRAME_TEXT.
> > > Other
> > > things are the same as for x86.
> > > 
> > > For RISC-V it is fine to skip '%c' modifier so _ASM_BUGFRAME_TEXT
> > > will
> > > look like:
> > > 
> > > #define _ASM_BUGFRAME_TEXT(second_frame) \
> > >     ".Lbug%=: ebreak\n"   
> > >     ".pushsection .bug_frames.%[bf_type], \"a\", @progbits\n"
> > >     ".p2align 2\n"
> > >     ".Lfrm%=:\n"
> > >     ".long (.Lbug%= - .Lfrm%=) + %[bf_line_hi]\n"
> > >     ".long (%[bf_ptr] - .Lfrm%=) + %[bf_line_lo]\n"
> > >     ".if " #second_frame "\n"
> > >     ".long 0, %[bf_msg] - .Lfrm%=\n"
> > >     ".endif\n"
> > >     ".popsection\n"
> > I expect this could be further abstracted such that only the actual
> > instruction is arch-specific.
> > 
> > > The only thing I am worried about is:
> > > 
> > > #define _ASM_BUGFRAME_INFO(type, line, ptr, msg) \
> > >   [bf_type] "i" (type), ...
> > > because as I understand it can be an issue with 'i' modifier in
> > > case of
> > > PIE. I am not sure that Xen enables PIE somewhere but still...
> > > If it is not an issue then we can use x86 implementation as a
> > > generic
> > > one.
> > "i" is not generally an issue with PIE, it only is when the value
> > is the
> > address of a symbol. Here "type" is a constant in all cases. (Or
> > else
> > how would you express an immediate operand of an instruction in an
> > asm()?)
> 
> At a guess, the problem isn't type.  It's the line number, which ends
> up
> in a relocation.
Sure. I missed that.
> 
> That said, I'm not sure an architecture could reasonably function
> without a generic 4-byte PC-relative relocation...
> 
> ~Andrew