Re: [PATCH 3/3] RISC-V: KVM: Add extensible CSR emulation framework

From: Anup Patel
Date: Mon Jun 13 2022 - 07:20:38 EST

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In reply to: Zhao Liu: "Re: [PATCH 3/3] RISC-V: KVM: Add extensible CSR emulation framework"
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On Sun, Jun 12, 2022 at 9:38 PM Zhao Liu <zhao1.liu@xxxxxxxxxxxxxxx> wrote:
>
> kvm-riscv@xxxxxxxxxxxxxxxxxxx, linux-riscv@xxxxxxxxxxxxxxxxxxx,
> linux-kernel@xxxxxxxxxxxxxxx, Anup Patel <apatel@xxxxxxxxxxxxxxxx>,
> Zhao Liu <zhao1.liu@xxxxxxxxxxxxxxx>, Zhenyu Wang
> <zhenyuw@xxxxxxxxxxxxxxx>
> Bcc:
> Subject: Re: [PATCH 3/3] RISC-V: KVM: Add extensible CSR emulation framework
> Reply-To:
> In-Reply-To: <20220610050555.288251-4-apatel@xxxxxxxxxxxxxxxx>
>
> 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@xxxxxxxxxxxxxxxx>
> > 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@xxxxxxxxxxxxxxxx>
> > ---
> > 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;
> }

I agree with your suggestion. I will use switch-case in v2.

>
> > 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.

Actually, there is no "Rm" field in CSR instruction encoding. Instead,
the BIT[14:12] of CSR instruction is "funct3" field. I will fix this in v2.

Also, instead of adding new defines for "funct3" field of CSR instruction,
we can simply use INSN_MATCH_xyz defines to avoid hard-coding.

Regards,
Anup

>
> 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
> >
> >
> > _______________________________________________
> > linux-riscv mailing list
> > linux-riscv@xxxxxxxxxxxxxxxxxxx
> > http://lists.infradead.org/mailman/listinfo/linux-riscv

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In reply to: Zhao Liu: "Re: [PATCH 3/3] RISC-V: KVM: Add extensible CSR emulation framework"
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