Re: [PATCH V4 1/2] LoongArch: KVM: Compile the switch.S file directly into the kernel

[Huacai Chen]

On Mon, Apr 27, 2026 at 4:43 PM lixianglai <lixianglai@xxxxxxxxxxx> wrote:
>
> Hi Huacai Chen:
> > Hi, Xianglai,
> >
> > After this patch I get:
> > arch/loongarch/kvm/switch.o: warning: objtool: kvm_enter_guest+0x0:
> > unreachable instruction
> >
> > But if I restore SYM_FUNC_START/SYM_FUNC_END for kvm_enter_guest, the
> > warning disappear. So please make sure which one is correct.
> Oh~ Here, we should use SYM_FUNC_START/SYM_FUNC_END. I also reproduced
> this issue locally.
> After making the modification, the compilation did not notice this
> compilation warning.
Jinyang,

What's your opinion? I remember you suggest to use
SYM_CODE_START/SYM_CODE_END here.

Huacai

>
>
> Thanks,
> Xianglai.
>
> > Huacai
> >
> > On Tue, Apr 14, 2026 at 7:57 PM Xianglai Li <lixianglai@xxxxxxxxxxx> wrote:
> >> If we directly compile the switch.S file into the kernel, the address of
> >> the kvm_exc_entry function will definitely be within the DMW memory area.
> >> Therefore, we will no longer need to perform a copy relocation of
> >> kvm_exc_entry.
> >>
> >> Based on the above description, compile switch.S directly into the kernel,
> >> and then remove the copy relocation execution logic for the kvm_exc_entry
> >> function.
> >>
> >> Signed-off-by: Xianglai Li <lixianglai@xxxxxxxxxxx>
> >> ---
> >> Cc: Huacai Chen <chenhuacai@xxxxxxxxxx>
> >> Cc: Tianrui Zhao <zhaotianrui@xxxxxxxxxxx>
> >> Cc: Bibo Mao <maobibo@xxxxxxxxxxx>
> >>
> >>   arch/loongarch/Kbuild                       |  2 +-
> >>   arch/loongarch/include/asm/asm-prototypes.h | 21 +++++++++++++
> >>   arch/loongarch/include/asm/kvm_host.h       |  3 --
> >>   arch/loongarch/kvm/Makefile                 |  2 +-
> >>   arch/loongarch/kvm/main.c                   | 35 ++-------------------
> >>   arch/loongarch/kvm/switch.S                 | 27 +++++++++++-----
> >>   6 files changed, 46 insertions(+), 44 deletions(-)
> >>
> >> diff --git a/arch/loongarch/Kbuild b/arch/loongarch/Kbuild
> >> index beb8499dd8ed..1c7a0dbe5e72 100644
> >> --- a/arch/loongarch/Kbuild
> >> +++ b/arch/loongarch/Kbuild
> >> @@ -3,7 +3,7 @@ obj-y += mm/
> >>   obj-y += net/
> >>   obj-y += vdso/
> >>
> >> -obj-$(CONFIG_KVM) += kvm/
> >> +obj-$(subst m,y,$(CONFIG_KVM)) += kvm/
> >>
> >>   # for cleaning
> >>   subdir- += boot
> >> diff --git a/arch/loongarch/include/asm/asm-prototypes.h b/arch/loongarch/include/asm/asm-prototypes.h
> >> index 704066b4f736..e8ce153691e5 100644
> >> --- a/arch/loongarch/include/asm/asm-prototypes.h
> >> +++ b/arch/loongarch/include/asm/asm-prototypes.h
> >> @@ -20,3 +20,24 @@ asmlinkage void noinstr __no_stack_protector ret_from_kernel_thread(struct task_
> >>                                                                      struct pt_regs *regs,
> >>                                                                      int (*fn)(void *),
> >>                                                                      void *fn_arg);
> >> +
> >> +struct kvm_run;
> >> +struct kvm_vcpu;
> >> +
> >> +void kvm_exc_entry(void);
> >> +int  kvm_enter_guest(struct kvm_run *run, struct kvm_vcpu *vcpu);
> >> +
> >> +struct loongarch_fpu;
> >> +
> >> +#ifdef CONFIG_CPU_HAS_LSX
> >> +void kvm_save_lsx(struct loongarch_fpu *fpu);
> >> +void kvm_restore_lsx(struct loongarch_fpu *fpu);
> >> +#endif
> >> +
> >> +#ifdef CONFIG_CPU_HAS_LASX
> >> +void kvm_save_lasx(struct loongarch_fpu *fpu);
> >> +void kvm_restore_lasx(struct loongarch_fpu *fpu);
> >> +#endif
> >> +
> >> +void kvm_save_fpu(struct loongarch_fpu *fpu);
> >> +void kvm_restore_fpu(struct loongarch_fpu *fpu);
> >> diff --git a/arch/loongarch/include/asm/kvm_host.h b/arch/loongarch/include/asm/kvm_host.h
> >> index 19eb5e5c3984..0bcdffc14c5f 100644
> >> --- a/arch/loongarch/include/asm/kvm_host.h
> >> +++ b/arch/loongarch/include/asm/kvm_host.h
> >> @@ -86,7 +86,6 @@ struct kvm_context {
> >>   struct kvm_world_switch {
> >>          int (*exc_entry)(void);
> >>          int (*enter_guest)(struct kvm_run *run, struct kvm_vcpu *vcpu);
> >> -       unsigned long page_order;
> >>   };
> >>
> >>   #define MAX_PGTABLE_LEVELS     4
> >> @@ -356,8 +355,6 @@ void kvm_exc_entry(void);
> >>   int  kvm_enter_guest(struct kvm_run *run, struct kvm_vcpu *vcpu);
> >>
> >>   extern unsigned long vpid_mask;
> >> -extern const unsigned long kvm_exception_size;
> >> -extern const unsigned long kvm_enter_guest_size;
> >>   extern struct kvm_world_switch *kvm_loongarch_ops;
> >>
> >>   #define SW_GCSR                (1 << 0)
> >> diff --git a/arch/loongarch/kvm/Makefile b/arch/loongarch/kvm/Makefile
> >> index cb41d9265662..fe665054f824 100644
> >> --- a/arch/loongarch/kvm/Makefile
> >> +++ b/arch/loongarch/kvm/Makefile
> >> @@ -11,7 +11,7 @@ kvm-y += exit.o
> >>   kvm-y += interrupt.o
> >>   kvm-y += main.o
> >>   kvm-y += mmu.o
> >> -kvm-y += switch.o
> >> +obj-y += switch.o
> >>   kvm-y += timer.o
> >>   kvm-y += tlb.o
> >>   kvm-y += vcpu.o
> >> diff --git a/arch/loongarch/kvm/main.c b/arch/loongarch/kvm/main.c
> >> index 2c593ac7892f..18800a38b150 100644
> >> --- a/arch/loongarch/kvm/main.c
> >> +++ b/arch/loongarch/kvm/main.c
> >> @@ -348,8 +348,7 @@ void kvm_arch_disable_virtualization_cpu(void)
> >>
> >>   static int kvm_loongarch_env_init(void)
> >>   {
> >> -       int cpu, order, ret;
> >> -       void *addr;
> >> +       int cpu, ret;
> >>          struct kvm_context *context;
> >>
> >>          vmcs = alloc_percpu(struct kvm_context);
> >> @@ -365,30 +364,8 @@ static int kvm_loongarch_env_init(void)
> >>                  return -ENOMEM;
> >>          }
> >>
> >> -       /*
> >> -        * PGD register is shared between root kernel and kvm hypervisor.
> >> -        * So world switch entry should be in DMW area rather than TLB area
> >> -        * to avoid page fault reenter.
> >> -        *
> >> -        * In future if hardware pagetable walking is supported, we won't
> >> -        * need to copy world switch code to DMW area.
> >> -        */
> >> -       order = get_order(kvm_exception_size + kvm_enter_guest_size);
> >> -       addr = (void *)__get_free_pages(GFP_KERNEL, order);
> >> -       if (!addr) {
> >> -               free_percpu(vmcs);
> >> -               vmcs = NULL;
> >> -               kfree(kvm_loongarch_ops);
> >> -               kvm_loongarch_ops = NULL;
> >> -               return -ENOMEM;
> >> -       }
> >> -
> >> -       memcpy(addr, kvm_exc_entry, kvm_exception_size);
> >> -       memcpy(addr + kvm_exception_size, kvm_enter_guest, kvm_enter_guest_size);
> >> -       flush_icache_range((unsigned long)addr, (unsigned long)addr + kvm_exception_size + kvm_enter_guest_size);
> >> -       kvm_loongarch_ops->exc_entry = addr;
> >> -       kvm_loongarch_ops->enter_guest = addr + kvm_exception_size;
> >> -       kvm_loongarch_ops->page_order = order;
> >> +       kvm_loongarch_ops->exc_entry = (void *)kvm_exc_entry;
> >> +       kvm_loongarch_ops->enter_guest = (void *)kvm_enter_guest;
> >>
> >>          vpid_mask = read_csr_gstat();
> >>          vpid_mask = (vpid_mask & CSR_GSTAT_GIDBIT) >> CSR_GSTAT_GIDBIT_SHIFT;
> >> @@ -422,16 +399,10 @@ static int kvm_loongarch_env_init(void)
> >>
> >>   static void kvm_loongarch_env_exit(void)
> >>   {
> >> -       unsigned long addr;
> >> -
> >>          if (vmcs)
> >>                  free_percpu(vmcs);
> >>
> >>          if (kvm_loongarch_ops) {
> >> -               if (kvm_loongarch_ops->exc_entry) {
> >> -                       addr = (unsigned long)kvm_loongarch_ops->exc_entry;
> >> -                       free_pages(addr, kvm_loongarch_ops->page_order);
> >> -               }
> >>                  kfree(kvm_loongarch_ops);
> >>          }
> >>
> >> diff --git a/arch/loongarch/kvm/switch.S b/arch/loongarch/kvm/switch.S
> >> index f1768b7a6194..1a5636790ef9 100644
> >> --- a/arch/loongarch/kvm/switch.S
> >> +++ b/arch/loongarch/kvm/switch.S
> >> @@ -5,10 +5,12 @@
> >>
> >>   #include <linux/linkage.h>
> >>   #include <asm/asm.h>
> >> +#include <asm/page.h>
> >>   #include <asm/asmmacro.h>
> >>   #include <asm/loongarch.h>
> >>   #include <asm/regdef.h>
> >>   #include <asm/unwind_hints.h>
> >> +#include <linux/kvm_types.h>
> >>
> >>   #define HGPR_OFFSET(x)         (PT_R0 + 8*x)
> >>   #define GGPR_OFFSET(x)         (KVM_ARCH_GGPR + 8*x)
> >> @@ -100,8 +102,16 @@
> >>           *  -        is still in guest mode, such as pgd table/vmid registers etc,
> >>           *  -        will fix with hw page walk enabled in future
> >>           * load kvm_vcpu from reserved CSR KVM_VCPU_KS, and save a2 to KVM_TEMP_KS
> >> +        *
> >> +        * PGD register is shared between root kernel and kvm hypervisor.
> >> +        * So world switch entry should be in DMW area rather than TLB area
> >> +        * to avoid page fault reenter.
> >> +        *
> >> +        * In future if hardware pagetable walking is supported, we won't
> >> +        * need to copy world switch code to DMW area.
> >>           */
> >>          .text
> >> +       .p2align PAGE_SHIFT
> >>          .cfi_sections   .debug_frame
> >>   SYM_CODE_START(kvm_exc_entry)
> >>          UNWIND_HINT_UNDEFINED
> >> @@ -190,8 +200,8 @@ ret_to_host:
> >>          kvm_restore_host_gpr    a2
> >>          jr      ra
> >>
> >> -SYM_INNER_LABEL(kvm_exc_entry_end, SYM_L_LOCAL)
> >>   SYM_CODE_END(kvm_exc_entry)
> >> +EXPORT_SYMBOL_FOR_KVM(kvm_exc_entry)
> >>
> >>   /*
> >>    * int kvm_enter_guest(struct kvm_run *run, struct kvm_vcpu *vcpu)
> >> @@ -200,7 +210,7 @@ SYM_CODE_END(kvm_exc_entry)
> >>    *  a0: kvm_run* run
> >>    *  a1: kvm_vcpu* vcpu
> >>    */
> >> -SYM_FUNC_START(kvm_enter_guest)
> >> +SYM_CODE_START(kvm_enter_guest)
> >>          /* Allocate space in stack bottom */
> >>          addi.d  a2, sp, -PT_SIZE
> >>          /* Save host GPRs */
> >> @@ -215,8 +225,8 @@ SYM_FUNC_START(kvm_enter_guest)
> >>          /* Save kvm_vcpu to kscratch */
> >>          csrwr   a1, KVM_VCPU_KS
> >>          kvm_switch_to_guest
> >> -SYM_INNER_LABEL(kvm_enter_guest_end, SYM_L_LOCAL)
> >> -SYM_FUNC_END(kvm_enter_guest)
> >> +SYM_CODE_END(kvm_enter_guest)
> >> +EXPORT_SYMBOL_FOR_KVM(kvm_enter_guest)
> >>
> >>   SYM_FUNC_START(kvm_save_fpu)
> >>          fpu_save_csr    a0 t1
> >> @@ -224,6 +234,7 @@ SYM_FUNC_START(kvm_save_fpu)
> >>          fpu_save_cc     a0 t1 t2
> >>          jr              ra
> >>   SYM_FUNC_END(kvm_save_fpu)
> >> +EXPORT_SYMBOL_FOR_KVM(kvm_save_fpu)
> >>
> >>   SYM_FUNC_START(kvm_restore_fpu)
> >>          fpu_restore_double a0 t1
> >> @@ -231,6 +242,7 @@ SYM_FUNC_START(kvm_restore_fpu)
> >>          fpu_restore_cc     a0 t1 t2
> >>          jr                 ra
> >>   SYM_FUNC_END(kvm_restore_fpu)
> >> +EXPORT_SYMBOL_FOR_KVM(kvm_restore_fpu)
> >>
> >>   #ifdef CONFIG_CPU_HAS_LSX
> >>   SYM_FUNC_START(kvm_save_lsx)
> >> @@ -239,6 +251,7 @@ SYM_FUNC_START(kvm_save_lsx)
> >>          lsx_save_data   a0 t1
> >>          jr              ra
> >>   SYM_FUNC_END(kvm_save_lsx)
> >> +EXPORT_SYMBOL_FOR_KVM(kvm_save_lsx)
> >>
> >>   SYM_FUNC_START(kvm_restore_lsx)
> >>          lsx_restore_data a0 t1
> >> @@ -246,6 +259,7 @@ SYM_FUNC_START(kvm_restore_lsx)
> >>          fpu_restore_csr  a0 t1 t2
> >>          jr               ra
> >>   SYM_FUNC_END(kvm_restore_lsx)
> >> +EXPORT_SYMBOL_FOR_KVM(kvm_restore_lsx)
> >>   #endif
> >>
> >>   #ifdef CONFIG_CPU_HAS_LASX
> >> @@ -255,6 +269,7 @@ SYM_FUNC_START(kvm_save_lasx)
> >>          lasx_save_data  a0 t1
> >>          jr              ra
> >>   SYM_FUNC_END(kvm_save_lasx)
> >> +EXPORT_SYMBOL_FOR_KVM(kvm_save_lasx)
> >>
> >>   SYM_FUNC_START(kvm_restore_lasx)
> >>          lasx_restore_data a0 t1
> >> @@ -262,10 +277,8 @@ SYM_FUNC_START(kvm_restore_lasx)
> >>          fpu_restore_csr   a0 t1 t2
> >>          jr                ra
> >>   SYM_FUNC_END(kvm_restore_lasx)
> >> +EXPORT_SYMBOL_FOR_KVM(kvm_restore_lasx)
> >>   #endif
> >> -       .section ".rodata"
> >> -SYM_DATA(kvm_exception_size, .quad kvm_exc_entry_end - kvm_exc_entry)
> >> -SYM_DATA(kvm_enter_guest_size, .quad kvm_enter_guest_end - kvm_enter_guest)
> >>
> >>   #ifdef CONFIG_CPU_HAS_LBT
> >>   STACK_FRAME_NON_STANDARD kvm_restore_fpu
> >> --
> >> 2.39.1
> >>
>
>