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Message-ID: <ad734262-f9f5-6fd4-41cb-f3ff4e21c151@loongson.cn>
Date: Tue, 23 Dec 2025 10:41:12 +0800
From: lixianglai <lixianglai@...ngson.cn>
To: Huacai Chen <chenhuacai@...nel.org>
Cc: Bibo Mao <maobibo@...ngson.cn>, 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>,
Charlie Jenkins <charlie@...osinc.com>, Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH V2 1/2] LoongArch: KVM: Compile the switch.S file directly
into the kernel
Hi Huacai Chen:
> Hi, Xianglai,
>
> On Tue, Dec 23, 2025 at 10:38 AM lixianglai <lixianglai@...ngson.cn> wrote:
>> Hi Bibo Mao:
>>>
>>> On 2025/12/22 下午7:34, Xianglai Li 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 | 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 | 22 ++++++++++---
>>>> 6 files changed, 43 insertions(+), 42 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 e4fe5b8e8149..1a1be10e3803 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
>>>> @@ -347,8 +346,6 @@ 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 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
> I'm not sure, but if this line can be moved after .text, it seems better.
I will try and test it as suggested, and if it works, I will fix it
Thanks!
Xianglai.
> Huacai
>
>>>> 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)
>>> one small nit, could EXPORT_SYMBOL_FOR_KVM() be used here compared
>>> with EXPORT_SYMBOL()?
>>>
>> Ok! will fix it in next version
>> Thanks!
>> Xianglai.
>>
>>> Regards
>>> Bibo Mao
>>>> 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
>>>>
>>
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