On Wed, Sep 23, 2026 at 12:36 PM Shreshth Srivastava <[email protected]> wrote: > > On 11.09.26 10:41, Juergen Gross wrote: > > When building a kernel with CONFIG_PARAVIRT_XXL the paravirt > > infrastructure will always use functions for reading or writing MSRs, > > even when running on bare metal. > > Hi Juergen, > > This doesn't build with CONFIG_PARAVIRT_XXL=y. 16/17 and 17/17 are where > it breaks, but the cause is the .byte fallbacks: ASM_WRMSRNS_IMM from > 08/17 and ASM_RDMSR_IMM from 10/17 don't end in a separator, unlike the > .insn variants above them. > > #define ASM_RDMSR_IMM \ > " .byte 0xc4,0xe7,0x7b,0xf6,0xc0; .long %c[msr]" > > That worked while they were only ever the last argument of an > ALTERNATIVE(), which appends its own newline. 16/17 concatenates them > with ASM_CLRERR: > > ASM_RDMSR_IMM ASM_CLRERR, X86_FEATURE_MSR_IMM, \ > > so the .long operand runs into the xor. From > make arch/x86/kernel/cpu/common.s: > > .byte 0xc4,0xe7,0x7b,0xf6,0xc0; .long 266xor %rdx,%rdx > > paravirt-msr.h:165: Error: junk at end of line, first unrecognized > character is `x' > > clang reports "error: unexpected token" in the same place. 71 objects > fail, the same 71 either way, no vmlinux. > > msr.h chooses between the .insn form and the .byte fallback with: > > #if defined(CONFIG_AS_IS_GNU) && CONFIG_AS_VERSION >= 24100 > > Two kinds of toolchain end up on the .byte side of that test: > > - GNU as older than 2.41. RHEL 9 and CentOS Stream 9 ship 2.35, and > Documentation/process/changes.rst sets the minimum at 2.30, so this > is a supported configuration rather than an old outlier. > - clang, any version. CONFIG_AS_IS_GNU is never set for clang, so the > && short-circuits and the version comparison is never reached. Your > 08/17 comment already notes that clang has no .insn support.
Indeed, looks like we're missing support for .insn for x86. Filed https://github.com/llvm/llvm-project/issues/225916. (Please do file bugs against the toolchain when you encounter issues like this, and cc someone from kernel development). > > gcc with binutils 2.41 or newer takes the .insn path, where both macros > do end in a separator, and is unaffected. > > Reproduced on v7.3-rc2 with your v3 00/13, v2 0/5 and v5 00/17 applied in > that order, x86_64 defconfig plus HYPERVISOR_GUEST, PARAVIRT, XEN and > XEN_PV. gcc 11.5.0 with GNU as 2.35.2, and clang 21.1.7. > > Terminating both fallbacks fixes it, and both toolchains then build > vmlinux with no errors or warnings: > > diff --git a/arch/x86/include/asm/msr.h b/arch/x86/include/asm/msr.h > index eba325ecfe4c..529c13553c63 100644 > --- a/arch/x86/include/asm/msr.h > +++ b/arch/x86/include/asm/msr.h > @@ -78,9 +78,9 @@ static inline void do_trace_rdpmc(u32 msr, u64 val, int > failed) {} > * form MSR access instructions reference %rax as the register operand. > */ > #define ASM_RDMSR_IMM \ > - " .byte 0xc4,0xe7,0x7b,0xf6,0xc0; .long %c[msr]" > + " .byte 0xc4,0xe7,0x7b,0xf6,0xc0; .long %c[msr]\n\t" > #define ASM_WRMSRNS_IMM \ > - " .byte 0xc4,0xe7,0x7a,0xf6,0xc0; .long %c[msr]" > + " .byte 0xc4,0xe7,0x7a,0xf6,0xc0; .long %c[msr]\n\t" > #endif > > #define RDMSR_AND_SAVE_RESULT \ > > ASM_WRMSRNS needs no change, _ASM_BYTES() already emits a semicolon. > > The WRMSRNS line belongs in 08/17 and the RDMSR line in 10/17. > > Thanks, > Shreshth -- Thanks, ~Nick Desaulniers
