Date:   Mon, 13 Jun 2022 00:12:20 +0800
From:   Zhao Liu <zhao1.liu@...ux.intel.com>
To:     Anup Patel <apatel@...tanamicro.com>
Cc:     Anup Patel <anup@...infault.org>, kvm@...r.kernel.org,
        kvm-riscv@...ts.infradead.org, linux-riscv@...ts.infradead.org,
        linux-kernel@...r.kernel.org, Zhao Liu <zhao1.liu@...ux.intel.com>,
        Zhenyu Wang <zhenyuw@...ux.intel.com>
Subject: Re: [PATCH 3/3] RISC-V: KVM: Add extensible CSR emulation framework

kvm-riscv@...ts.infradead.org, linux-riscv@...ts.infradead.org,
linux-kernel@...r.kernel.org, Anup Patel <apatel@...tanamicro.com>,
Zhao Liu <zhao1.liu@...ux.intel.com>, Zhenyu Wang
<zhenyuw@...ux.intel.com>
Bcc: 
Subject: Re: [PATCH 3/3] RISC-V: KVM: Add extensible CSR emulation framework
Reply-To: 
In-Reply-To: <20220610050555.288251-4-apatel@...tanamicro.com>

On Fri, Jun 10, 2022 at 10:35:55AM +0530, Anup Patel wrote:
> Date: Fri, 10 Jun 2022 10:35:55 +0530
> From: Anup Patel <apatel@...tanamicro.com>
> Subject: [PATCH 3/3] RISC-V: KVM: Add extensible CSR emulation framework
> X-Mailer: git-send-email 2.34.1
> 
> We add an extensible CSR emulation framework which is based upon the
> existing system instruction emulation. This will be useful to upcoming
> AIA, PMU, Nested and other virtualization features.
> 
> The CSR emulation framework also has provision to emulate CSR in user
> space but this will be used only in very specific cases such as AIA
> IMSIC CSR emulation in user space or vendor specific CSR emulation
> in user space.
> 
> By default, all CSRs not handled by KVM RISC-V will be redirected back
> to Guest VCPU as illegal instruction trap.
> 
> Signed-off-by: Anup Patel <apatel@...tanamicro.com>
> ---
>  arch/riscv/include/asm/kvm_host.h      |   5 +
>  arch/riscv/include/asm/kvm_vcpu_insn.h |   6 +
>  arch/riscv/kvm/vcpu.c                  |  11 ++
>  arch/riscv/kvm/vcpu_insn.c             | 169 +++++++++++++++++++++++++
>  include/uapi/linux/kvm.h               |   8 ++
>  5 files changed, 199 insertions(+)
> 
> diff --git a/arch/riscv/include/asm/kvm_host.h b/arch/riscv/include/asm/kvm_host.h
> index 03103b86dd86..a54744d7e1ba 100644
> --- a/arch/riscv/include/asm/kvm_host.h
> +++ b/arch/riscv/include/asm/kvm_host.h
> @@ -64,6 +64,8 @@ struct kvm_vcpu_stat {
>  	u64 wfi_exit_stat;
>  	u64 mmio_exit_user;
>  	u64 mmio_exit_kernel;
> +	u64 csr_exit_user;
> +	u64 csr_exit_kernel;
>  	u64 exits;
>  };
>  
> @@ -209,6 +211,9 @@ struct kvm_vcpu_arch {
>  	/* MMIO instruction details */
>  	struct kvm_mmio_decode mmio_decode;
>  
> +	/* CSR instruction details */
> +	struct kvm_csr_decode csr_decode;
> +
>  	/* SBI context */
>  	struct kvm_sbi_context sbi_context;
>  
> diff --git a/arch/riscv/include/asm/kvm_vcpu_insn.h b/arch/riscv/include/asm/kvm_vcpu_insn.h
> index 3351eb61a251..350011c83581 100644
> --- a/arch/riscv/include/asm/kvm_vcpu_insn.h
> +++ b/arch/riscv/include/asm/kvm_vcpu_insn.h
> @@ -18,6 +18,11 @@ struct kvm_mmio_decode {
>  	int return_handled;
>  };
>  
> +struct kvm_csr_decode {
> +	unsigned long insn;
> +	int return_handled;
> +};
> +
>  /* Return values used by function emulating a particular instruction */
>  enum kvm_insn_return {
>  	KVM_INSN_EXIT_TO_USER_SPACE = 0,
> @@ -28,6 +33,7 @@ enum kvm_insn_return {
>  };
>  
>  void kvm_riscv_vcpu_wfi(struct kvm_vcpu *vcpu);
> +int kvm_riscv_vcpu_csr_return(struct kvm_vcpu *vcpu, struct kvm_run *run);
>  int kvm_riscv_vcpu_virtual_insn(struct kvm_vcpu *vcpu, struct kvm_run *run,
>  				struct kvm_cpu_trap *trap);
>  
> diff --git a/arch/riscv/kvm/vcpu.c b/arch/riscv/kvm/vcpu.c
> index 7f4ad5e4373a..cf9616da68f6 100644
> --- a/arch/riscv/kvm/vcpu.c
> +++ b/arch/riscv/kvm/vcpu.c
> @@ -26,6 +26,8 @@ const struct _kvm_stats_desc kvm_vcpu_stats_desc[] = {
>  	STATS_DESC_COUNTER(VCPU, wfi_exit_stat),
>  	STATS_DESC_COUNTER(VCPU, mmio_exit_user),
>  	STATS_DESC_COUNTER(VCPU, mmio_exit_kernel),
> +	STATS_DESC_COUNTER(VCPU, csr_exit_user),
> +	STATS_DESC_COUNTER(VCPU, csr_exit_kernel),
>  	STATS_DESC_COUNTER(VCPU, exits)
>  };
>  
> @@ -869,6 +871,15 @@ int kvm_arch_vcpu_ioctl_run(struct kvm_vcpu *vcpu)
>  		}
>  	}
>  
> +	/* Process CSR value returned from user-space */
> +	if (run->exit_reason == KVM_EXIT_RISCV_CSR) {
> +		ret = kvm_riscv_vcpu_csr_return(vcpu, vcpu->run);
> +		if (ret) {
> +			kvm_vcpu_srcu_read_unlock(vcpu);
> +			return ret;
> +		}
> +	}
> +


Hi Anup, what about a `switch` to handle exit_reason?
        switch(run->exit_reason) {
                case KVM_EXIT_MMIO:
                        ret = kvm_riscv_vcpu_mmio_return(vcpu, vcpu->run);
                        break;
                case KVM_EXIT_RISCV_SBI:
                        ret = kvm_riscv_vcpu_sbi_return(vcpu, vcpu->run);
                        break;
                case KVM_EXIT_RISCV_CSR:
                        ret = kvm_riscv_vcpu_csr_return(vcpu, vcpu->run);
                        break;
                case default:
                        break;
        }
        if (ret) {
                kvm_vcpu_srcu_read_unlock(vcpu);
                return ret;
        }

>  	if (run->immediate_exit) {
>  		kvm_vcpu_srcu_read_unlock(vcpu);
>  		return -EINTR;
> diff --git a/arch/riscv/kvm/vcpu_insn.c b/arch/riscv/kvm/vcpu_insn.c
> index 75ca62a7fba5..c9542ba98431 100644
> --- a/arch/riscv/kvm/vcpu_insn.c
> +++ b/arch/riscv/kvm/vcpu_insn.c
> @@ -14,6 +14,19 @@
>  #define INSN_MASK_WFI		0xffffffff
>  #define INSN_MATCH_WFI		0x10500073
>  
> +#define INSN_MATCH_CSRRW	0x1073
> +#define INSN_MASK_CSRRW		0x707f
> +#define INSN_MATCH_CSRRS	0x2073
> +#define INSN_MASK_CSRRS		0x707f
> +#define INSN_MATCH_CSRRC	0x3073
> +#define INSN_MASK_CSRRC		0x707f
> +#define INSN_MATCH_CSRRWI	0x5073
> +#define INSN_MASK_CSRRWI	0x707f
> +#define INSN_MATCH_CSRRSI	0x6073
> +#define INSN_MASK_CSRRSI	0x707f
> +#define INSN_MATCH_CSRRCI	0x7073
> +#define INSN_MASK_CSRRCI	0x707f
> +
>  #define INSN_MATCH_LB		0x3
>  #define INSN_MASK_LB		0x707f
>  #define INSN_MATCH_LH		0x1003
> @@ -71,6 +84,7 @@
>  #define SH_RS1			15
>  #define SH_RS2			20
>  #define SH_RS2C			2
> +#define MASK_RX			0x1f
>  
>  #define RV_X(x, s, n)		(((x) >> (s)) & ((1 << (n)) - 1))
>  #define RVC_LW_IMM(x)		((RV_X(x, 6, 1) << 2) | \
> @@ -189,7 +203,162 @@ static int wfi_insn(struct kvm_vcpu *vcpu, struct kvm_run *run, ulong insn)
>  	return KVM_INSN_CONTINUE_NEXT_SEPC;
>  }
>  
> +struct csr_func {
> +	unsigned int base;
> +	unsigned int count;
> +	/*
> +	 * Possible return values are as same as "func" callback in
> +	 * "struct insn_func".
> +	 */
> +	int (*func)(struct kvm_vcpu *vcpu, unsigned int csr_num,
> +		    unsigned long *val, unsigned long new_val,
> +		    unsigned long wr_mask);
> +};
> +
> +static const struct csr_func csr_funcs[] = { };
> +
> +/**
> + * kvm_riscv_vcpu_csr_return -- Handle CSR read/write after user space
> + *				emulation or in-kernel emulation
> + *
> + * @vcpu: The VCPU pointer
> + * @run:  The VCPU run struct containing the CSR data
> + *
> + * Returns > 0 upon failure and 0 upon success
> + */
> +int kvm_riscv_vcpu_csr_return(struct kvm_vcpu *vcpu, struct kvm_run *run)
> +{
> +	ulong insn;
> +
> +	if (vcpu->arch.csr_decode.return_handled)
> +		return 0;
> +	vcpu->arch.csr_decode.return_handled = 1;
> +
> +	/* Update destination register for CSR reads */
> +	insn = vcpu->arch.csr_decode.insn;
> +	if ((insn >> SH_RD) & MASK_RX)
> +		SET_RD(insn, &vcpu->arch.guest_context,
> +		       run->riscv_csr.ret_value);
> +
> +	/* Move to next instruction */
> +	vcpu->arch.guest_context.sepc += INSN_LEN(insn);
> +
> +	return 0;
> +}
> +
> +static int csr_insn(struct kvm_vcpu *vcpu, struct kvm_run *run, ulong insn)
> +{
> +	int i, rc = KVM_INSN_ILLEGAL_TRAP;
> +	unsigned int csr_num = insn >> SH_RS2;
> +	unsigned int rs1_num = (insn >> SH_RS1) & MASK_RX;
> +	ulong rs1_val = GET_RS1(insn, &vcpu->arch.guest_context);
> +	const struct csr_func *tcfn, *cfn = NULL;
> +	ulong val = 0, wr_mask = 0, new_val = 0;
> +
> +	/* Decode the CSR instruction */
> +	switch (GET_RM(insn)) {
> +	case 1:

It's better to define these rounding mode.
What about this name: #define INSN_RM_RTZ 1.

Thanks,
Zhao

> +		wr_mask = -1UL;
> +		new_val = rs1_val;
> +		break;
> +	case 2:
> +		wr_mask = rs1_val;
> +		new_val = -1UL;
> +		break;
> +	case 3:
> +		wr_mask = rs1_val;
> +		new_val = 0;
> +		break;
> +	case 5:
> +		wr_mask = -1UL;
> +		new_val = rs1_num;
> +		break;
> +	case 6:
> +		wr_mask = rs1_num;
> +		new_val = -1UL;
> +		break;
> +	case 7:
> +		wr_mask = rs1_num;
> +		new_val = 0;
> +		break;
> +	default:
> +		return rc;
> +	};
> +
> +	/* Save instruction decode info */
> +	vcpu->arch.csr_decode.insn = insn;
> +	vcpu->arch.csr_decode.return_handled = 0;
> +
> +	/* Update CSR details in kvm_run struct */
> +	run->riscv_csr.csr_num = csr_num;
> +	run->riscv_csr.new_value = new_val;
> +	run->riscv_csr.write_mask = wr_mask;
> +	run->riscv_csr.ret_value = 0;
> +
> +	/* Find in-kernel CSR function */
> +	for (i = 0; i < ARRAY_SIZE(csr_funcs); i++) {
> +		tcfn = &csr_funcs[i];
> +		if ((tcfn->base <= csr_num) &&
> +		    (csr_num < (tcfn->base + tcfn->count))) {
> +			cfn = tcfn;
> +			break;
> +		}
> +	}
> +
> +	/* First try in-kernel CSR emulation */
> +	if (cfn && cfn->func) {
> +		rc = cfn->func(vcpu, csr_num, &val, new_val, wr_mask);
> +		if (rc > KVM_INSN_EXIT_TO_USER_SPACE) {
> +			if (rc == KVM_INSN_CONTINUE_NEXT_SEPC) {
> +				run->riscv_csr.ret_value = val;
> +				vcpu->stat.csr_exit_kernel++;
> +				kvm_riscv_vcpu_csr_return(vcpu, run);
> +				rc = KVM_INSN_CONTINUE_SAME_SEPC;
> +			}
> +			return rc;
> +		}
> +	}
> +
> +	/* Exit to user-space for CSR emulation */
> +	if (rc <= KVM_INSN_EXIT_TO_USER_SPACE) {
> +		vcpu->stat.csr_exit_user++;
> +		run->exit_reason = KVM_EXIT_RISCV_CSR;
> +	}
> +
> +	return rc;
> +}
> +
>  static const struct insn_func system_opcode_funcs[] = {
> +	{
> +		.mask  = INSN_MASK_CSRRW,
> +		.match = INSN_MATCH_CSRRW,
> +		.func  = csr_insn,
> +	},
> +	{
> +		.mask  = INSN_MASK_CSRRS,
> +		.match = INSN_MATCH_CSRRS,
> +		.func  = csr_insn,
> +	},
> +	{
> +		.mask  = INSN_MASK_CSRRC,
> +		.match = INSN_MATCH_CSRRC,
> +		.func  = csr_insn,
> +	},
> +	{
> +		.mask  = INSN_MASK_CSRRWI,
> +		.match = INSN_MATCH_CSRRWI,
> +		.func  = csr_insn,
> +	},
> +	{
> +		.mask  = INSN_MASK_CSRRSI,
> +		.match = INSN_MATCH_CSRRSI,
> +		.func  = csr_insn,
> +	},
> +	{
> +		.mask  = INSN_MASK_CSRRCI,
> +		.match = INSN_MATCH_CSRRCI,
> +		.func  = csr_insn,
> +	},
>  	{
>  		.mask  = INSN_MASK_WFI,
>  		.match = INSN_MATCH_WFI,
> diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h
> index 5088bd9f1922..c48fd3d1c45b 100644
> --- a/include/uapi/linux/kvm.h
> +++ b/include/uapi/linux/kvm.h
> @@ -270,6 +270,7 @@ struct kvm_xen_exit {
>  #define KVM_EXIT_X86_BUS_LOCK     33
>  #define KVM_EXIT_XEN              34
>  #define KVM_EXIT_RISCV_SBI        35
> +#define KVM_EXIT_RISCV_CSR        36
>  
>  /* For KVM_EXIT_INTERNAL_ERROR */
>  /* Emulate instruction failed. */
> @@ -496,6 +497,13 @@ struct kvm_run {
>  			unsigned long args[6];
>  			unsigned long ret[2];
>  		} riscv_sbi;
> +		/* KVM_EXIT_RISCV_CSR */
> +		struct {
> +			unsigned long csr_num;
> +			unsigned long new_value;
> +			unsigned long write_mask;
> +			unsigned long ret_value;
> +		} riscv_csr;
>  		/* Fix the size of the union. */
>  		char padding[256];
>  	};
> -- 
> 2.34.1
> 
> 
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