Re: [PATCH 11/13] RISC-V: KVM: Use SBI sync SRET call when available

Atish Patra
2024-10-18T20:03:40Z

On Fri, Jul 19, 2024 at 9:10 AM Anup Patel <apatel@ventanamicro.com> wrote:
>
> Implement an optimized KVM world-switch using SBI sync SRET call
> when SBI nested acceleration extension is available. This improves
> KVM world-switch when KVM RISC-V is running as a Guest under some
> other hypervisor.
>
> Signed-off-by: Anup Patel <apatel@ventanamicro.com>
> ---
> arch/riscv/include/asm/kvm_nacl.h | 32 +++++++++++++++++++++
> arch/riscv/kvm/vcpu.c | 48 ++++++++++++++++++++++++++++---
> arch/riscv/kvm/vcpu_switch.S | 29 +++++++++++++++++++
> 3 files changed, 105 insertions(+), 4 deletions(-)
>
> diff --git a/arch/riscv/include/asm/kvm_nacl.h b/arch/riscv/include/asm/kvm_nacl.h
> index a704e8000a58..5e74238ea525 100644
> --- a/arch/riscv/include/asm/kvm_nacl.h
> +++ b/arch/riscv/include/asm/kvm_nacl.h
> @@ -12,6 +12,8 @@
> #include <asm/csr.h>
> #include <asm/sbi.h>
>
> +struct kvm_vcpu_arch;
> +
> DECLARE_STATIC_KEY_FALSE(kvm_riscv_nacl_available);
> #define kvm_riscv_nacl_available() \
> static_branch_unlikely(&kvm_riscv_nacl_available)
> @@ -43,6 +45,10 @@ void __kvm_riscv_nacl_hfence(void *shmem,
> unsigned long page_num,
> unsigned long page_count);
>
> +void __kvm_riscv_nacl_switch_to(struct kvm_vcpu_arch *vcpu_arch,
> + unsigned long sbi_ext_id,
> + unsigned long sbi_func_id);
> +
> int kvm_riscv_nacl_enable(void);
>
> void kvm_riscv_nacl_disable(void);
> @@ -64,6 +70,32 @@ int kvm_riscv_nacl_init(void);
> #define nacl_shmem_fast() \
> (kvm_riscv_nacl_available() ? nacl_shmem() : NULL)
>
> +#define nacl_scratch_read_long(__shmem, __offset) \
> +({ \
> + unsigned long *__p = (__shmem) + \
> + SBI_NACL_SHMEM_SCRATCH_OFFSET + \
> + (__offset); \
> + lelong_to_cpu(*__p); \
> +})
> +
> +#define nacl_scratch_write_long(__shmem, __offset, __val) \
> +do { \
> + unsigned long *__p = (__shmem) + \
> + SBI_NACL_SHMEM_SCRATCH_OFFSET + \
> + (__offset); \
> + *__p = cpu_to_lelong(__val); \
> +} while (0)
> +
> +#define nacl_scratch_write_longs(__shmem, __offset, __array, __count) \
> +do { \
> + unsigned int __i; \
> + unsigned long *__p = (__shmem) + \
> + SBI_NACL_SHMEM_SCRATCH_OFFSET + \
> + (__offset); \
> + for (__i = 0; __i < (__count); __i++) \
> + __p[__i] = cpu_to_lelong((__array)[__i]); \
> +} while (0)
> +

This should be in a separate patch along with other helpers ?

> #define nacl_sync_hfence(__e) \
> sbi_ecall(SBI_EXT_NACL, SBI_EXT_NACL_SYNC_HFENCE, \
> (__e), 0, 0, 0, 0, 0)
> diff --git a/arch/riscv/kvm/vcpu.c b/arch/riscv/kvm/vcpu.c
> index 00baaf1b0136..fe849fb1aaab 100644
> --- a/arch/riscv/kvm/vcpu.c
> +++ b/arch/riscv/kvm/vcpu.c
> @@ -759,19 +759,59 @@ static __always_inline void kvm_riscv_vcpu_swap_in_host_state(struct kvm_vcpu *v
> */
> static void noinstr kvm_riscv_vcpu_enter_exit(struct kvm_vcpu *vcpu)
> {
> + void *nsh;
> struct kvm_cpu_context *gcntx = &vcpu->arch.guest_context;
> struct kvm_cpu_context *hcntx = &vcpu->arch.host_context;
>
> kvm_riscv_vcpu_swap_in_guest_state(vcpu);
> guest_state_enter_irqoff();
>
> - hcntx->hstatus = ncsr_swap(CSR_HSTATUS, gcntx->hstatus);
> + if (kvm_riscv_nacl_sync_sret_available()) {
> + nsh = nacl_shmem();
>
> - nsync_csr(-1UL);
> + if (kvm_riscv_nacl_autoswap_csr_available()) {
> + hcntx->hstatus =
> + nacl_csr_read(nsh, CSR_HSTATUS);
> + nacl_scratch_write_long(nsh,
> + SBI_NACL_SHMEM_AUTOSWAP_OFFSET +
> + SBI_NACL_SHMEM_AUTOSWAP_HSTATUS,
> + gcntx->hstatus);
> + nacl_scratch_write_long(nsh,
> + SBI_NACL_SHMEM_AUTOSWAP_OFFSET,
> + SBI_NACL_SHMEM_AUTOSWAP_FLAG_HSTATUS);
> + } else if (kvm_riscv_nacl_sync_csr_available()) {
> + hcntx->hstatus = nacl_csr_swap(nsh,
> + CSR_HSTATUS, gcntx->hstatus);
> + } else {
> + hcntx->hstatus = csr_swap(CSR_HSTATUS, gcntx->hstatus);
> + }
>
> - __kvm_riscv_switch_to(&vcpu->arch);
> + nacl_scratch_write_longs(nsh,
> + SBI_NACL_SHMEM_SRET_OFFSET +
> + SBI_NACL_SHMEM_SRET_X(1),
> + &gcntx->ra,
> + SBI_NACL_SHMEM_SRET_X_LAST);
> +
> + __kvm_riscv_nacl_switch_to(&vcpu->arch, SBI_EXT_NACL,
> + SBI_EXT_NACL_SYNC_SRET);
> +
> + if (kvm_riscv_nacl_autoswap_csr_available()) {
> + nacl_scratch_write_long(nsh,
> + SBI_NACL_SHMEM_AUTOSWAP_OFFSET,
> + 0);
> + gcntx->hstatus = nacl_scratch_read_long(nsh,
> + SBI_NACL_SHMEM_AUTOSWAP_OFFSET +
> + SBI_NACL_SHMEM_AUTOSWAP_HSTATUS);
> + } else {
> + gcntx->hstatus = csr_swap(CSR_HSTATUS, hcntx->hstatus);
> + }
> + } else {
> + hcntx->hstatus = csr_swap(CSR_HSTATUS, gcntx->hstatus);
>
> - gcntx->hstatus = csr_swap(CSR_HSTATUS, hcntx->hstatus);
> + __kvm_riscv_switch_to(&vcpu->arch);
> +
> + gcntx->hstatus = csr_swap(CSR_HSTATUS, hcntx->hstatus);
> + }
>
> vcpu->arch.last_exit_cpu = vcpu->cpu;
> guest_state_exit_irqoff();
> diff --git a/arch/riscv/kvm/vcpu_switch.S b/arch/riscv/kvm/vcpu_switch.S
> index 9f13e5ce6a18..47686bcb21e0 100644
> --- a/arch/riscv/kvm/vcpu_switch.S
> +++ b/arch/riscv/kvm/vcpu_switch.S
> @@ -218,6 +218,35 @@ SYM_FUNC_START(__kvm_riscv_switch_to)
> ret
> SYM_FUNC_END(__kvm_riscv_switch_to)
>
> + /*
> + * Parameters:
> + * A0 <= Pointer to struct kvm_vcpu_arch
> + * A1 <= SBI extension ID
> + * A2 <= SBI function ID
> + */
> +SYM_FUNC_START(__kvm_riscv_nacl_switch_to)
> + SAVE_HOST_GPRS
> +
> + SAVE_HOST_AND_RESTORE_GUEST_CSRS .Lkvm_nacl_switch_return
> +
> + /* Resume Guest using SBI nested acceleration */
> + add a6, a2, zero
> + add a7, a1, zero
> + ecall
> +
> + /* Back to Host */
> + .align 2
> +.Lkvm_nacl_switch_return:
> + SAVE_GUEST_GPRS
> +
> + SAVE_GUEST_AND_RESTORE_HOST_CSRS
> +
> + RESTORE_HOST_GPRS
> +
> + /* Return to C code */
> + ret
> +SYM_FUNC_END(__kvm_riscv_nacl_switch_to)
> +
> SYM_CODE_START(__kvm_riscv_unpriv_trap)
> /*
> * We assume that faulting unpriv load/store instruction is
> --
> 2.34.1
>

Reviewed-by: Atish Patra <atishp@rivosinc.com>
--
Regards,
Atish