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Message-ID: <CAAhV-H5g7KXK08vqKOR5HTsPKZ7X3CBa9fgfSTavnN7m9D_9AA@mail.gmail.com>
Date: Wed, 17 Dec 2025 12:55:19 +0800
From: Huacai Chen <chenhuacai@...nel.org>
To: Xianglai Li <lixianglai@...ngson.cn>
Cc: loongarch@...ts.linux.dev, linux-kernel@...r.kernel.org,
kvm@...r.kernel.org, stable@...r.kernel.org, WANG Xuerui <kernel@...0n.name>,
Tianrui Zhao <zhaotianrui@...ngson.cn>, Bibo Mao <maobibo@...ngson.cn>,
Charlie Jenkins <charlie@...osinc.com>, Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH 1/2] LoongArch: KVM: Compile the switch.S file directly
into the kernel
Hi, Xianglai,
On Wed, Dec 17, 2025 at 11:49 AM Xianglai Li <lixianglai@...ngson.cn> 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.
>
> Cc: stable@...r.kernel.org
> Signed-off-by: Xianglai Li <lixianglai@...ngson.cn>
> ---
> Cc: Huacai Chen <chenhuacai@...nel.org>
> Cc: WANG Xuerui <kernel@...0n.name>
> Cc: Tianrui Zhao <zhaotianrui@...ngson.cn>
> Cc: Bibo Mao <maobibo@...ngson.cn>
> Cc: Charlie Jenkins <charlie@...osinc.com>
> Cc: Xianglai Li <lixianglai@...ngson.cn>
> Cc: Thomas Gleixner <tglx@...utronix.de>
>
> arch/loongarch/Kbuild | 2 +-
> arch/loongarch/include/asm/asm-prototypes.h | 16 ++++++++++
> arch/loongarch/include/asm/kvm_host.h | 5 +--
> arch/loongarch/include/asm/kvm_vcpu.h | 20 ++++++------
> arch/loongarch/kvm/Makefile | 2 +-
> arch/loongarch/kvm/main.c | 35 ++-------------------
> arch/loongarch/kvm/switch.S | 22 ++++++++++---
> 7 files changed, 49 insertions(+), 53 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..eb591276d191 100644
> --- a/arch/loongarch/include/asm/asm-prototypes.h
> +++ b/arch/loongarch/include/asm/asm-prototypes.h
> @@ -20,3 +20,19 @@ asmlinkage void noinstr __no_stack_protector ret_from_kernel_thread(struct task_
> struct pt_regs *regs,
> int (*fn)(void *),
> void *fn_arg);
> +
> +void kvm_exc_entry(void);
> +int kvm_enter_guest(void *run, void *vcpu);
> +
> +#ifdef CONFIG_CPU_HAS_LSX
> +void kvm_save_lsx(void *fpu);
> +void kvm_restore_lsx(void *fpu);
> +#endif
> +
> +#ifdef CONFIG_CPU_HAS_LASX
> +void kvm_save_lasx(void *fpu);
> +void kvm_restore_lasx(void *fpu);
> +#endif
> +
> +void kvm_save_fpu(void *fpu);
> +void kvm_restore_fpu(void *fpu);
> diff --git a/arch/loongarch/include/asm/kvm_host.h b/arch/loongarch/include/asm/kvm_host.h
> index e4fe5b8e8149..0aa7679536cc 100644
> --- a/arch/loongarch/include/asm/kvm_host.h
> +++ b/arch/loongarch/include/asm/kvm_host.h
> @@ -85,7 +85,6 @@ 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
> @@ -344,11 +343,9 @@ enum hrtimer_restart kvm_swtimer_wakeup(struct hrtimer *timer);
> void kvm_arch_flush_remote_tlbs_memslot(struct kvm *kvm, const struct kvm_memory_slot *memslot);
> void kvm_init_vmcs(struct kvm *kvm);
> void kvm_exc_entry(void);
> -int kvm_enter_guest(struct kvm_run *run, struct kvm_vcpu *vcpu);
> +int kvm_enter_guest(void *run, void *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/include/asm/kvm_vcpu.h b/arch/loongarch/include/asm/kvm_vcpu.h
> index 3784ab4ccdb5..8af98a3d7b0c 100644
> --- a/arch/loongarch/include/asm/kvm_vcpu.h
> +++ b/arch/loongarch/include/asm/kvm_vcpu.h
> @@ -53,28 +53,28 @@ void kvm_deliver_exception(struct kvm_vcpu *vcpu);
>
> void kvm_own_fpu(struct kvm_vcpu *vcpu);
> void kvm_lose_fpu(struct kvm_vcpu *vcpu);
> -void kvm_save_fpu(struct loongarch_fpu *fpu);
> -void kvm_restore_fpu(struct loongarch_fpu *fpu);
> +void kvm_save_fpu(void *fpu);
> +void kvm_restore_fpu(void *fpu);
Why are these modifications needed?
Huacai
> void kvm_restore_fcsr(struct loongarch_fpu *fpu);
>
> #ifdef CONFIG_CPU_HAS_LSX
> int kvm_own_lsx(struct kvm_vcpu *vcpu);
> -void kvm_save_lsx(struct loongarch_fpu *fpu);
> -void kvm_restore_lsx(struct loongarch_fpu *fpu);
> +void kvm_save_lsx(void *fpu);
> +void kvm_restore_lsx(void *fpu);
> #else
> static inline int kvm_own_lsx(struct kvm_vcpu *vcpu) { return -EINVAL; }
> -static inline void kvm_save_lsx(struct loongarch_fpu *fpu) { }
> -static inline void kvm_restore_lsx(struct loongarch_fpu *fpu) { }
> +static inline void kvm_save_lsx(void *fpu) { }
> +static inline void kvm_restore_lsx(void *fpu) { }
> #endif
>
> #ifdef CONFIG_CPU_HAS_LASX
> int kvm_own_lasx(struct kvm_vcpu *vcpu);
> -void kvm_save_lasx(struct loongarch_fpu *fpu);
> -void kvm_restore_lasx(struct loongarch_fpu *fpu);
> +void kvm_save_lasx(void *fpu);
> +void kvm_restore_lasx(void *fpu);
> #else
> static inline int kvm_own_lasx(struct kvm_vcpu *vcpu) { return -EINVAL; }
> -static inline void kvm_save_lasx(struct loongarch_fpu *fpu) { }
> -static inline void kvm_restore_lasx(struct loongarch_fpu *fpu) { }
> +static inline void kvm_save_lasx(void *fpu) { }
> +static inline void kvm_restore_lasx(void *fpu) { }
> #endif
>
> #ifdef CONFIG_CPU_HAS_LBT
> 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 80ea63d465b8..67d234540ed4 100644
> --- a/arch/loongarch/kvm/main.c
> +++ b/arch/loongarch/kvm/main.c
> @@ -340,8 +340,7 @@ 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);
> @@ -357,30 +356,8 @@ 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;
> @@ -414,16 +391,10 @@ 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..93845ce53651 100644
> --- a/arch/loongarch/kvm/switch.S
> +++ b/arch/loongarch/kvm/switch.S
> @@ -5,6 +5,7 @@
>
> #include <linux/linkage.h>
> #include <asm/asm.h>
> +#include <asm/page.h>
> #include <asm/asmmacro.h>
> #include <asm/loongarch.h>
> #include <asm/regdef.h>
> @@ -100,10 +101,18 @@
> * - 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
> .cfi_sections .debug_frame
> SYM_CODE_START(kvm_exc_entry)
> + .p2align PAGE_SHIFT
> UNWIND_HINT_UNDEFINED
> csrwr a2, KVM_TEMP_KS
> csrrd a2, KVM_VCPU_KS
> @@ -190,8 +199,8 @@ 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(kvm_exc_entry)
>
> /*
> * int kvm_enter_guest(struct kvm_run *run, struct kvm_vcpu *vcpu)
> @@ -215,8 +224,8 @@ 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)
> +EXPORT_SYMBOL(kvm_enter_guest)
>
> SYM_FUNC_START(kvm_save_fpu)
> fpu_save_csr a0 t1
> @@ -224,6 +233,7 @@ SYM_FUNC_START(kvm_save_fpu)
> fpu_save_cc a0 t1 t2
> jr ra
> SYM_FUNC_END(kvm_save_fpu)
> +EXPORT_SYMBOL(kvm_save_fpu)
>
> SYM_FUNC_START(kvm_restore_fpu)
> fpu_restore_double a0 t1
> @@ -231,6 +241,7 @@ SYM_FUNC_START(kvm_restore_fpu)
> fpu_restore_cc a0 t1 t2
> jr ra
> SYM_FUNC_END(kvm_restore_fpu)
> +EXPORT_SYMBOL(kvm_restore_fpu)
>
> #ifdef CONFIG_CPU_HAS_LSX
> SYM_FUNC_START(kvm_save_lsx)
> @@ -239,6 +250,7 @@ SYM_FUNC_START(kvm_save_lsx)
> lsx_save_data a0 t1
> jr ra
> SYM_FUNC_END(kvm_save_lsx)
> +EXPORT_SYMBOL(kvm_save_lsx)
>
> SYM_FUNC_START(kvm_restore_lsx)
> lsx_restore_data a0 t1
> @@ -246,6 +258,7 @@ SYM_FUNC_START(kvm_restore_lsx)
> fpu_restore_csr a0 t1 t2
> jr ra
> SYM_FUNC_END(kvm_restore_lsx)
> +EXPORT_SYMBOL(kvm_restore_lsx)
> #endif
>
> #ifdef CONFIG_CPU_HAS_LASX
> @@ -255,6 +268,7 @@ SYM_FUNC_START(kvm_save_lasx)
> lasx_save_data a0 t1
> jr ra
> SYM_FUNC_END(kvm_save_lasx)
> +EXPORT_SYMBOL(kvm_save_lasx)
>
> SYM_FUNC_START(kvm_restore_lasx)
> lasx_restore_data a0 t1
> @@ -262,10 +276,8 @@ SYM_FUNC_START(kvm_restore_lasx)
> fpu_restore_csr a0 t1 t2
> jr ra
> SYM_FUNC_END(kvm_restore_lasx)
> +EXPORT_SYMBOL(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
>
>
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