Re: [PATCH v5 11/20] RISC-V: KVM: Handle WFI exits for VCPU

Anup Patel
2019-08-22T12:50:17Z

On Thu, Aug 22, 2019 at 5:49 PM Alexander Graf <graf@amazon.com> wrote:
>
> On 22.08.19 10:45, Anup Patel wrote:
> > We get illegal instruction trap whenever Guest/VM executes WFI
> > instruction.
> >
> > This patch handles WFI trap by blocking the trapped VCPU using
> > kvm_vcpu_block() API. The blocked VCPU will be automatically
> > resumed whenever a VCPU interrupt is injected from user-space
> > or from in-kernel IRQCHIP emulation.
> >
> > Signed-off-by: Anup Patel <anup.patel@wdc.com>
> > Acked-by: Paolo Bonzini <pbonzini@redhat.com>
> > Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
> > ---
> > arch/riscv/kvm/vcpu_exit.c | 88 ++++++++++++++++++++++++++++++++++++++
> > 1 file changed, 88 insertions(+)
> >
> > diff --git a/arch/riscv/kvm/vcpu_exit.c b/arch/riscv/kvm/vcpu_exit.c
> > index efc06198c259..fbc04fe335ad 100644
> > --- a/arch/riscv/kvm/vcpu_exit.c
> > +++ b/arch/riscv/kvm/vcpu_exit.c
> > @@ -12,6 +12,9 @@
> > #include <linux/kvm_host.h>
> > #include <asm/csr.h>
> >
> > +#define INSN_MASK_WFI 0xffffff00
> > +#define INSN_MATCH_WFI 0x10500000
> > +
> > #define INSN_MATCH_LB 0x3
> > #define INSN_MASK_LB 0x707f
> > #define INSN_MATCH_LH 0x1003
> > @@ -179,6 +182,87 @@ static ulong get_insn(struct kvm_vcpu *vcpu)
> > return val;
> > }
> >
> > +typedef int (*illegal_insn_func)(struct kvm_vcpu *vcpu,
> > + struct kvm_run *run,
> > + ulong insn);
> > +
> > +static int truly_illegal_insn(struct kvm_vcpu *vcpu,
> > + struct kvm_run *run,
> > + ulong insn)
> > +{
> > + /* TODO: Redirect trap to Guest VCPU */
> > + return -ENOTSUPP;
> > +}
> > +
> > +static int system_opcode_insn(struct kvm_vcpu *vcpu,
> > + struct kvm_run *run,
> > + ulong insn)
> > +{
> > + if ((insn & INSN_MASK_WFI) == INSN_MATCH_WFI) {
> > + vcpu->stat.wfi_exit_stat++;
> > + if (!kvm_arch_vcpu_runnable(vcpu)) {
> > + srcu_read_unlock(&vcpu->kvm->srcu, vcpu->arch.srcu_idx);
> > + kvm_vcpu_block(vcpu);
> > + vcpu->arch.srcu_idx = srcu_read_lock(&vcpu->kvm->srcu);
> > + kvm_clear_request(KVM_REQ_UNHALT, vcpu);
> > + }
> > + vcpu->arch.guest_context.sepc += INSN_LEN(insn);
> > + return 1;
> > + }
> > +
> > + return truly_illegal_insn(vcpu, run, insn);
> > +}
> > +
> > +static illegal_insn_func illegal_insn_table[32] = {
>
> Every time I did experiments on PowerPC with indirect tables like this
> over switch() in C, the switch() code won. CPUs are pretty good at
> predicting branches. Predicting indirect jumps however, they are
> terrible at.
>
> So unless you consider the jump table more readable / maintainable, I
> would suggest to use a simple switch() statement. It will be faster and
> smaller.

Yes, readability was the reason why we choose jump table but
I see your point. Most of the entries in jump table point to
truly_illegal_insn() so I guess switch case will be quite simple
here.

I will update this in next revision.

Regards,
Anup

>
>
> Alex
>
>
> > + truly_illegal_insn, /* 0 */
> > + truly_illegal_insn, /* 1 */
> > + truly_illegal_insn, /* 2 */
> > + truly_illegal_insn, /* 3 */
> > + truly_illegal_insn, /* 4 */
> > + truly_illegal_insn, /* 5 */
> > + truly_illegal_insn, /* 6 */
> > + truly_illegal_insn, /* 7 */
> > + truly_illegal_insn, /* 8 */
> > + truly_illegal_insn, /* 9 */
> > + truly_illegal_insn, /* 10 */
> > + truly_illegal_insn, /* 11 */
> > + truly_illegal_insn, /* 12 */
> > + truly_illegal_insn, /* 13 */
> > + truly_illegal_insn, /* 14 */
> > + truly_illegal_insn, /* 15 */
> > + truly_illegal_insn, /* 16 */
> > + truly_illegal_insn, /* 17 */
> > + truly_illegal_insn, /* 18 */
> > + truly_illegal_insn, /* 19 */
> > + truly_illegal_insn, /* 20 */
> > + truly_illegal_insn, /* 21 */
> > + truly_illegal_insn, /* 22 */
> > + truly_illegal_insn, /* 23 */
> > + truly_illegal_insn, /* 24 */
> > + truly_illegal_insn, /* 25 */
> > + truly_illegal_insn, /* 26 */
> > + truly_illegal_insn, /* 27 */
> > + system_opcode_insn, /* 28 */
> > + truly_illegal_insn, /* 29 */
> > + truly_illegal_insn, /* 30 */
> > + truly_illegal_insn /* 31 */
> > +};
> > +
> > +static int illegal_inst_fault(struct kvm_vcpu *vcpu, struct kvm_run *run,
> > + unsigned long stval)
> > +{
> > + ulong insn = stval;
> > +
> > + if (unlikely((insn & 3) != 3)) {
> > + if (insn == 0)
> > + insn = get_insn(vcpu);
> > + if ((insn & 3) != 3)
> > + return truly_illegal_insn(vcpu, run, insn);
> > + }
> > +
> > + return illegal_insn_table[(insn & 0x7c) >> 2](vcpu, run, insn);
> > +}
> > +
> > static int emulate_load(struct kvm_vcpu *vcpu, struct kvm_run *run,
> > unsigned long fault_addr)
> > {
> > @@ -439,6 +523,10 @@ int kvm_riscv_vcpu_exit(struct kvm_vcpu *vcpu, struct kvm_run *run,
> > ret = -EFAULT;
> > run->exit_reason = KVM_EXIT_UNKNOWN;
> > switch (scause) {
> > + case EXC_INST_ILLEGAL:
> > + if (vcpu->arch.guest_context.hstatus & HSTATUS_SPV)
> > + ret = illegal_inst_fault(vcpu, run, stval);
> > + break;
> > case EXC_INST_PAGE_FAULT:
> > case EXC_LOAD_PAGE_FAULT:
> > case EXC_STORE_PAGE_FAULT:
> >
>