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[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] Re: [PATCH v1 1/4] xen: introduce CONFIG_GENERIC_BUG_FRAME
On Fri, 2023-02-17 at 08:12 +0100, Jan Beulich wrote:
> On 16.02.2023 21:44, Oleksii wrote:
> > On Thu, 2023-02-16 at 12:19 +0000, Andrew Cooper wrote:
> > > On 16/02/2023 12:09 pm, Oleksii wrote:
> > > > On Thu, 2023-02-16 at 12:44 +0200, Oleksii wrote:
> > > > > On Thu, 2023-02-16 at 08:31 +0100, 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.
> > > > > Generally, the only thing that can't be abstracted ( as you
> > > > > mentioned
> > > > > is an instruction ).
> > > > >
> > > > > So we can make additional defines:
> > > > > 1. #define MODIFIER "" or "c" (depends on architecture) and
> > > > > use
> > > > > it
> > > > >
> > > > > the following way:
> > > > > ".pushsection .bug_frames.%"MODIFIER"[bf_type], \"a\",
> > > > > @progbits\n"
> > > > > ...
> > > > > 2. #define BUG_INSTR "ebreak" | "ud2" | "..." and use it in
> > > > > the
> > > > > following way:
> > > > > ".Lbug%=: "BUG_ISNTR"\n"
> > > > > ...
> > > > > Except for these defines which should be specified for each
> > > > > architecture
> > > > > all other code will be the same for all architectures.
> > > > >
> > > > > But as I understand with modifier 'c' can be issues that not
> > > > > all
> > > > > compilers support but for ARM and x86 before immediate is
> > > > > present
> > > > > punctuation # or $ which are removed by modifier 'c'. And I
> > > > > am
> > > > > wondering what other ways to remove punctuation before
> > > > > immediate
> > > > > exist.
> > > > >
> > > > > Do you think we should consider that as an issue?
> > > > >
> > > > > > > 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()?)
> > > > > Hmm. I don't know other ways to express an immediate operand
> > > > > except
> > > > > 'i'.
> > > > >
> > > > > It looks like type, line, msg are always 'constants'
> > > > > (
> > > > > they possibly can be passed without 'i' and used directly
> > > > > inside
> > > > > asm():
> > > > > ...
> > > > > ".pushsection .bug_frames." __stringify(type) ", \"a\",
> > > > > %progbits\n"\
> > > > > ...
> > > > > ) but the issue will be with 'ptr' for which we have to use
> > > > > 'i'.
> > > > >
> > > > > And I am not sure about all 'constants'. For example, I think
> > > > > that it
> > > > > can be an issue with 'line' = '((line & ((1 <<
> > > > > BUG_LINE_LO_WIDTH)
> > > > > -
> > > > > 1))
> > > > > << BUG_DISP_WIDTH)' if we will use that directly inside
> > > > > asm(...).
> > > > >
> > > > I think we can avoid 'i' by using 'r' + some kind of mov
> > > > instruction to
> > > > write a register value to memory. The code below is pseudo-
> > > > code:
> > > > #define _ASM_BUGFRAME_TEXT(second_frame)
> > > > ...
> > > > "loc_disp:\n"
> > > > " .long 0x0"
> > > > "mov [loc_disp], some_register"
> > > > ...
> > > > And the we have to define mov for each architecture.
> > >
> > > No, you really cannot. This is the asm equivalent of writing
> > > something
> > > like this:
> > >
> > > static struct bugframe __section(.bug_frames.1) foo = {
> > > .loc = insn - &foo + LINE_LO,
> > > .msg = "bug string" - &foo + LINE_HI,
> > > };
> > >
> > > It is a data structure, not executable code.
> > Thanks for the clarification.
> >
> > What about mainly using C for BUG_FRAME and only allocating
> > bug_frame
> > sections in assembly?
> >
> > Please look at POC below:
>
> That's still using statements (assignments), not initializers. We
> expect
> the table to be populated at build time, and we expect it to be read-
> only
> at runtime. Plus your approach once again increases overall size
> (just
> that this time you add code, not data).
If we have such requirements then the best option is to use as a
generic implementation the implementation provided by x86 ( as I
mentioned before, it mainly can be re-used for RISC-V ) and leave ARM
implementation mostly as is ( except for some minor changes ).
>
> Jan
>
> > #include <stdio.h>
> > #include <stdint.h>
> >
> > #define BUG_DISP_WIDTH 24
> > #define BUG_LINE_LO_WIDTH (31 - BUG_DISP_WIDTH)
> > #define BUG_LINE_HI_WIDTH (31 - BUG_DISP_WIDTH)
> >
> > struct bug_frame {
> > signed int loc_disp:BUG_DISP_WIDTH;
> > unsigned int line_hi:BUG_LINE_HI_WIDTH;
> > signed int ptr_disp:BUG_DISP_WIDTH;
> > unsigned int line_lo:BUG_LINE_LO_WIDTH;
> > signed int msg_disp[];
> > };
> >
> > #define bug_loc(b) ((const void *)(b) + (b)->loc_disp)
> > #define bug_ptr(b) ((const void *)(b) + (b)->ptr_disp)
> > #define bug_line(b) (((((b)->line_hi + ((b)->loc_disp < 0)) & \
> > ((1 << BUG_LINE_HI_WIDTH) - 1)) << \
> > BUG_LINE_LO_WIDTH) + \
> > (((b)->line_lo + ((b)->ptr_disp < 0))
> > &
> > \
> > ((1 << BUG_LINE_LO_WIDTH) - 1)))
> > #define bug_msg(b) ((const char *)(b) + (b)->msg_disp[1])
> >
> > #define ALLOCATE_BF_SECTION(has_msg) do
> > { \
> > asm (".pushsection bug_frames
> > \n"
> > \
> > ".long 0, 0
> > \n"
> > \
> > ".if " #has_msg
> > "\n"
> > \
> > "\t.long 0
> > \n"
> > \
> > "\t.long 0
> > \n"
> > \
> >
> > ".endif\n"
> > \
> > ".popsection"
> > );
> > \
> > } while (0)
> >
> > #define MERGE_(a,b) a##b
> > #define UNIQUE_BUG_INSTR_LABEL(a) MERGE_(unique_name_, a)
> >
> > #define BUG_FRAME(type, line, file, has_msg, msg) do
> > {
> > \
> > unsigned int line_lo = (((line) >> BUG_LINE_LO_WIDTH) <<
> > BUG_DISP_WIDTH);
> >
> > \
> > unsigned int line_hi = ((line & ((1 << BUG_LINE_LO_WIDTH) - 1))
> > <<
> > BUG_DISP_WIDTH);
> >
> > \
> >
> > ALLOCATE_BF_SECTION(1);
> > \
> > *(signed int *)(&bug_frames) = (unsigned long)
> > &&UNIQUE_BUG_INSTR_LABEL(line) - (unsigned long)&bug_frames +
> > line_lo;
> > \
> > *((signed int *)(&bug_frames) + 1) = (unsigned long)file -
> > (unsigned long)&bug_frames +
> > line_hi;
> > \
> > bug_var.msg_disp[1] = (signed int)((unsigned long)#msg -
> > (unsigned
> > long)&bug_frames);
> >
> > \
> >
> > UNIQUE_BUG_INSTR_LABEL(line):
> > \
> > asm volatile
> > ("nop");
> > } while (0)
> >
> > extern char bug_frames[];
> >
> > static struct bug_frame bug_var
> > __attribute__((section("bug_frames")));
> >
> > int main() {
> > BUG_FRAME(1, __LINE__, __FILE__, 1, "TEST");
> >
> > printf("bug_loc: %p\n", bug_loc(&bug_var));
> > printf("bug_ptr: 0x%x\n", bug_ptr(&bug_var));
> > printf("__FILE__: %s\n", (char *)bug_ptr(&bug_var));
> > printf("bug_line: %d\n", bug_line(&bug_var));
> > printf("msg: %s\n", bug_msg(&bug_var));
> >
> > BUG_FRAME(1, __LINE__, __FILE__, 1, "NEW TEST");
> >
> > printf("bug_loc: %p\n", bug_loc(&bug_var));
> > printf("bug_ptr: 0x%x\n", bug_ptr(&bug_var));
> > printf("__FILE__: %s\n", (char *)bug_ptr(&bug_var));
> > printf("bug_line: %d\n", bug_line(&bug_var));
> > printf("msg: %s\n", bug_msg(&bug_var));
> >
> > return 0;
> > }
> >
> > Do you think it makes any sense? In this case, all BUG_FRAME can be
> > re-
> > used among all architectures, and only bug instructions should be
> > changed.
> >
> > >
> > > ~Andrew
> > ~ Oleksii
>
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