Re: [PATCH v5 08/20] RISC-V: KVM: Implement KVM_GET_ONE_REG/KVM_SET_ONE_REG ioctls

Anup Patel
2019-08-23T11:20:17Z

On Thu, Aug 22, 2019 at 7:42 PM Alexander Graf <graf@amazon.com> wrote:
>
>
>
> On 22.08.19 16:00, Anup Patel wrote:
> > On Thu, Aug 22, 2019 at 5:31 PM Alexander Graf <graf@amazon.com> wrote:
> >>
> >> On 22.08.19 10:44, Anup Patel wrote:
> >>> For KVM RISC-V, we use KVM_GET_ONE_REG/KVM_SET_ONE_REG ioctls to access
> >>> VCPU config and registers from user-space.
> >>>
> >>> We have three types of VCPU registers:
> >>> 1. CONFIG - these are VCPU config and capabilities
> >>> 2. CORE - these are VCPU general purpose registers
> >>> 3. CSR - these are VCPU control and status registers
> >>>
> >>> The CONFIG registers available to user-space are ISA and TIMEBASE. Out
> >>> of these, TIMEBASE is a read-only register which inform user-space about
> >>> VCPU timer base frequency. The ISA register is a read and write register
> >>> where user-space can only write the desired VCPU ISA capabilities before
> >>> running the VCPU.
> >>>
> >>> The CORE registers available to user-space are PC, RA, SP, GP, TP, A0-A7,
> >>> T0-T6, S0-S11 and MODE. Most of these are RISC-V general registers except
> >>> PC and MODE. The PC register represents program counter whereas the MODE
> >>> register represent VCPU privilege mode (i.e. S/U-mode).
> >>>
> >>> The CSRs available to user-space are SSTATUS, SIE, STVEC, SSCRATCH, SEPC,
> >>> SCAUSE, STVAL, SIP, and SATP. All of these are read/write registers.
> >>>
> >>> In future, more VCPU register types will be added (such as FP) for the
> >>> KVM_GET_ONE_REG/KVM_SET_ONE_REG ioctls.
> >>>
> >>> 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/include/uapi/asm/kvm.h | 40 ++++-
> >>> arch/riscv/kvm/vcpu.c | 235 +++++++++++++++++++++++++++++-
> >>> 2 files changed, 272 insertions(+), 3 deletions(-)
> >>>
> >>> diff --git a/arch/riscv/include/uapi/asm/kvm.h b/arch/riscv/include/uapi/asm/kvm.h
> >>> index 6dbc056d58ba..024f220eb17e 100644
> >>> --- a/arch/riscv/include/uapi/asm/kvm.h
> >>> +++ b/arch/riscv/include/uapi/asm/kvm.h
> >>> @@ -23,8 +23,15 @@
> >>>
> >>> /* for KVM_GET_REGS and KVM_SET_REGS */
> >>> struct kvm_regs {
> >>> + /* out (KVM_GET_REGS) / in (KVM_SET_REGS) */
> >>> + struct user_regs_struct regs;
> >>> + unsigned long mode;
> >>
> >> Is there any particular reason you're reusing kvm_regs and don't invent
> >> your own struct? kvm_regs is explicitly meant for the get_regs and
> >> set_regs ioctls.
> >
> > We are implementing only ONE_REG interface so most of these
> > structs are unused hence we tried to reuse these struct instead
> > of introducing new structs. (Similar to KVM ARM64)
> >
> >>
> >>> };
> >>>
> >>> +/* Possible privilege modes for kvm_regs */
> >>> +#define KVM_RISCV_MODE_S 1
> >>> +#define KVM_RISCV_MODE_U 0
> >>> +
> >>> /* for KVM_GET_FPU and KVM_SET_FPU */
> >>> struct kvm_fpu {
> >>> };
> >>> @@ -41,10 +48,41 @@ struct kvm_guest_debug_arch {
> >>> struct kvm_sync_regs {
> >>> };
> >>>
> >>> -/* dummy definition */
> >>> +/* for KVM_GET_SREGS and KVM_SET_SREGS */
> >>> struct kvm_sregs {
> >>> + unsigned long sstatus;
> >>> + unsigned long sie;
> >>> + unsigned long stvec;
> >>> + unsigned long sscratch;
> >>> + unsigned long sepc;
> >>> + unsigned long scause;
> >>> + unsigned long stval;
> >>> + unsigned long sip;
> >>> + unsigned long satp;
> >>
> >> Same comment here.
> >
> > Same as above, we are trying to use unused struct.
> >
> >>
> >>> };
> >>>
> >>> +#define KVM_REG_SIZE(id) \
> >>> + (1U << (((id) & KVM_REG_SIZE_MASK) >> KVM_REG_SIZE_SHIFT))
> >>> +
> >>> +/* If you need to interpret the index values, here is the key: */
> >>> +#define KVM_REG_RISCV_TYPE_MASK 0x00000000FF000000
> >>> +#define KVM_REG_RISCV_TYPE_SHIFT 24
> >>> +
> >>> +/* Config registers are mapped as type 1 */
> >>> +#define KVM_REG_RISCV_CONFIG (0x01 << KVM_REG_RISCV_TYPE_SHIFT)
> >>> +#define KVM_REG_RISCV_CONFIG_ISA 0x0
> >>> +#define KVM_REG_RISCV_CONFIG_TIMEBASE 0x1
> >>> +
> >>> +/* Core registers are mapped as type 2 */
> >>> +#define KVM_REG_RISCV_CORE (0x02 << KVM_REG_RISCV_TYPE_SHIFT)
> >>> +#define KVM_REG_RISCV_CORE_REG(name) \
> >>> + (offsetof(struct kvm_regs, name) / sizeof(unsigned long))
> >>
> >> I see, you're trying to implicitly use the struct offsets as index.
> >>
> >> I'm not a really big fan of it, but I can't pinpoint exactly why just
> >> yet. It just seems too magical (read: potentially breaking down the
> >> road) for me.
> >>
> >>> +
> >>> +/* Control and status registers are mapped as type 3 */
> >>> +#define KVM_REG_RISCV_CSR (0x03 << KVM_REG_RISCV_TYPE_SHIFT)
> >>> +#define KVM_REG_RISCV_CSR_REG(name) \
> >>> + (offsetof(struct kvm_sregs, name) / sizeof(unsigned long))
> >>> +
> >>> #endif
> >>>
> >>> #endif /* __LINUX_KVM_RISCV_H */
> >>> diff --git a/arch/riscv/kvm/vcpu.c b/arch/riscv/kvm/vcpu.c
> >>> index 7f59e85c6af8..9396a83c0611 100644
> >>> --- a/arch/riscv/kvm/vcpu.c
> >>> +++ b/arch/riscv/kvm/vcpu.c
> >>> @@ -164,6 +164,215 @@ vm_fault_t kvm_arch_vcpu_fault(struct kvm_vcpu *vcpu, struct vm_fault *vmf)
> >>> return VM_FAULT_SIGBUS;
> >>> }
> >>>
> >>> +static int kvm_riscv_vcpu_get_reg_config(struct kvm_vcpu *vcpu,
> >>> + const struct kvm_one_reg *reg)
> >>> +{
> >>> + unsigned long __user *uaddr =
> >>> + (unsigned long __user *)(unsigned long)reg->addr;
> >>> + unsigned long reg_num = reg->id & ~(KVM_REG_ARCH_MASK |
> >>> + KVM_REG_SIZE_MASK |
> >>> + KVM_REG_RISCV_CONFIG);
> >>> + unsigned long reg_val;
> >>> +
> >>> + if (KVM_REG_SIZE(reg->id) != sizeof(unsigned long))
> >>> + return -EINVAL;
> >>> +
> >>> + switch (reg_num) {
> >>> + case KVM_REG_RISCV_CONFIG_ISA:
> >>> + reg_val = vcpu->arch.isa;
> >>> + break;
> >>> + case KVM_REG_RISCV_CONFIG_TIMEBASE:
> >>> + reg_val = riscv_timebase;
> >>
> >> What does this reflect? The current guest time hopefully not? An offset?
> >> Related to what?
> >
> > riscv_timebase is the frequency in HZ of the system timer.
> >
> > The name "timebase" is not appropriate but we have been
> > carrying it since quite some time now.
>
> What do you mean by "some time"? So far I only see a kernel internal
> variable named after it. That's dramatically different from something
> exposed via uapi.
>
> Just name it tbfreq.

Sure, I will use TBFREQ name.

>
> So if this is the frequency, where is the offset? You will need it on
> save/restore. If you're saying that's out of scope for now, that's fine
> with me too :).

tbfreq is read-only and fixed.

The Guest tbfreq has to be same as Host tbfreq. This means we
can only migrate Guest from Host A to Host B only if:
1. They have matching ISA capabilities
2. They have matching tbfreq

Regards,
Anup

>
>
> Alex