Re: [RESEND][PATCH 1/6] x86/bugs: Add asm helpers for executing VERW

[Pawan Gupta]

On Sat, Oct 21, 2023 at 12:55:45AM +0100, Andrew Cooper wrote:
> On 20/10/2023 9:44 pm, Pawan Gupta wrote:
> > +#define EXEC_VERW				\
> > +	__EXEC_VERW(551f);			\
> > +	/* nopl __KERNEL_DS(%rax) */		\
> > +	.byte 0x0f, 0x1f, 0x80, 0x00, 0x00;	\
> > +551:	.word __KERNEL_DS;			\
> 
> Is this actually wise from a perf point of view?
> 
> You're causing a data access to the instruction stream, and not only
> that, the immediate next instruction.  Some parts don't take kindly to
> snoops hitting L1I.
> 
> A better option would be to simply have
> 
> .section .text.entry
> .align CACHELINE
> mds_verw_sel:
>     .word __KERNEL_DS
>     int3
> .align CACHELINE
> 
> 
> And then just have EXEC_VERW be
> 
>     verw mds_verw_sel(%rip)
> 
> in the fastpaths.  That keeps the memory operand in .text.entry it works
> on Meltdown-vulnerable CPUs, but creates effectively a data cacheline
> that isn't mixed into anywhere in the frontend, which also gets far
> better locality of reference.

With .text.entry section I am getting getting below warnings and an
error:

-----------------------------------------------------------------
    LD      vmlinux.o
  vmlinux.o: warning: objtool: .text.entry+0x0: unreachable instruction
  vmlinux.o: warning: objtool: .text.entry+0x40: unreachable instruction
  vmlinux.o: warning: objtool: .text.entry+0x80: unreachable instruction
  vmlinux.o: warning: objtool: .text.entry+0xc0: unreachable instruction
  vmlinux.o: warning: objtool: .text.entry+0x100: unreachable instruction
  vmlinux.o: warning: objtool: .text.entry+0x140: unreachable instruction
  vmlinux.o: warning: objtool: .text.entry+0x180: unreachable instruction
  vmlinux.o: warning: objtool: .text.entry+0x1c0: unreachable instruction
  vmlinux.o: warning: objtool: .text.entry+0x200: unreachable instruction
  vmlinux.o: warning: objtool: .text.entry+0x240: unreachable instruction
  vmlinux.o: warning: objtool: .text.entry+0x280: unreachable instruction
  vmlinux.o: warning: objtool: .text.entry+0x2c0: unreachable instruction
  vmlinux.o: warning: objtool: .text.entry+0x300: unreachable instruction
  vmlinux.o: warning: objtool: .altinstr_replacement+0x2c: relocation to !ENDBR: .text.entry+0x0
  vmlinux.o: warning: objtool: .altinstr_replacement+0x1c4: relocation to !ENDBR: .text.entry+0x0
  vmlinux.o: warning: objtool: .altinstr_replacement+0x1d0: relocation to !ENDBR: .text.entry+0x0
  vmlinux.o: warning: objtool: .altinstr_replacement+0x2d2: relocation to !ENDBR: .text.entry+0x80
  vmlinux.o: warning: objtool: .altinstr_replacement+0x5d5: relocation to !ENDBR: .text.entry+0xc0
    OBJCOPY modules.builtin.modinfo
    GEN     modules.builtin
    MODPOST vmlinux.symvers
    UPD     include/generated/utsversion.h
    CC      init/version-timestamp.o
    LD      .tmp_vmlinux.kallsyms1
  ld: error: unplaced orphan section `.text.entry' from `vmlinux.o'
  make[2]: *** [scripts/Makefile.vmlinux:36: vmlinux] Error 1
-----------------------------------------------------------------

... because my config has CONFIG_LD_ORPHAN_WARN_LEVEL="error" and
objtool needs to be told about this entry.

Do you think its worth fighting these warnings and error, or simply use
.rodata section for verw memory operand?