Re: [PATCH v2 3/8] RISC-V: KVM: Allow some SBI extensions to be disabled by default

[Anup Patel]

On Thu, Oct 19, 2023 at 1:27 PM Andrew Jones <ajones@xxxxxxxxxxxxxxxx> wrote:
>
> On Thu, Oct 12, 2023 at 10:45:04AM +0530, Anup Patel wrote:
> > Currently, all SBI extensions are enabled by default which is
> > problematic for SBI extensions (such as DBCN) which are forwarded
> > to the KVM user-space because we might have an older KVM user-space
> > which is not aware/ready to handle newer SBI extensions. Ideally,
> > the SBI extensions forwarded to the KVM user-space must be
> > disabled by default.
> >
> > To address above, we allow certain SBI extensions to be disabled
> > by default so that KVM user-space must explicitly enable such
> > SBI extensions to receive forwarded calls from Guest VCPU.
> >
> > Signed-off-by: Anup Patel <apatel@xxxxxxxxxxxxxxxx>
> > ---
> >  arch/riscv/include/asm/kvm_vcpu_sbi.h |  4 +++
> >  arch/riscv/kvm/vcpu.c                 |  6 ++++
> >  arch/riscv/kvm/vcpu_sbi.c             | 45 ++++++++++++++++-----------
> >  3 files changed, 36 insertions(+), 19 deletions(-)
> >
> > diff --git a/arch/riscv/include/asm/kvm_vcpu_sbi.h b/arch/riscv/include/asm/kvm_vcpu_sbi.h
> > index 8d6d4dce8a5e..c02bda5559d7 100644
> > --- a/arch/riscv/include/asm/kvm_vcpu_sbi.h
> > +++ b/arch/riscv/include/asm/kvm_vcpu_sbi.h
> > @@ -35,6 +35,9 @@ struct kvm_vcpu_sbi_return {
> >  struct kvm_vcpu_sbi_extension {
> >       unsigned long extid_start;
> >       unsigned long extid_end;
> > +
> > +     bool default_unavail;
> > +
> >       /**
> >        * SBI extension handler. It can be defined for a given extension or group of
> >        * extension. But it should always return linux error codes rather than SBI
> > @@ -59,6 +62,7 @@ int kvm_riscv_vcpu_get_reg_sbi_ext(struct kvm_vcpu *vcpu,
> >  const struct kvm_vcpu_sbi_extension *kvm_vcpu_sbi_find_ext(
> >                               struct kvm_vcpu *vcpu, unsigned long extid);
> >  int kvm_riscv_vcpu_sbi_ecall(struct kvm_vcpu *vcpu, struct kvm_run *run);
> > +void kvm_riscv_vcpu_sbi_init(struct kvm_vcpu *vcpu);
> >
> >  #ifdef CONFIG_RISCV_SBI_V01
> >  extern const struct kvm_vcpu_sbi_extension vcpu_sbi_ext_v01;
> > diff --git a/arch/riscv/kvm/vcpu.c b/arch/riscv/kvm/vcpu.c
> > index c061a1c5fe98..e087c809073c 100644
> > --- a/arch/riscv/kvm/vcpu.c
> > +++ b/arch/riscv/kvm/vcpu.c
> > @@ -141,6 +141,12 @@ int kvm_arch_vcpu_create(struct kvm_vcpu *vcpu)
> >       if (rc)
> >               return rc;
> >
> > +     /*
> > +      * Setup SBI extensions
> > +      * NOTE: This must be the last thing to be initialized.
> > +      */
> > +     kvm_riscv_vcpu_sbi_init(vcpu);
>
> With this, we no longer defer probing to the first access (whether that's
> by the guest or KVM userspace). With our current small set of SBI
> extensions where only a single one has a probe function, then this
> simpler approach is good enough. We can always go back to the lazy
> approach later if needed.

I agree. We can fallback to lazy probing in the future if required.

>
> > +
> >       /* Reset VCPU */
> >       kvm_riscv_reset_vcpu(vcpu);
> >
> > diff --git a/arch/riscv/kvm/vcpu_sbi.c b/arch/riscv/kvm/vcpu_sbi.c
> > index 9cd97091c723..1b1cee86efda 100644
> > --- a/arch/riscv/kvm/vcpu_sbi.c
> > +++ b/arch/riscv/kvm/vcpu_sbi.c
> > @@ -155,14 +155,8 @@ static int riscv_vcpu_set_sbi_ext_single(struct kvm_vcpu *vcpu,
> >       if (!sext)
> >               return -ENOENT;
> >
> > -     /*
> > -      * We can't set the extension status to available here, since it may
> > -      * have a probe() function which needs to confirm availability first,
> > -      * but it may be too early to call that here. We can set the status to
> > -      * unavailable, though.
> > -      */
> > -     if (!reg_val)
> > -             scontext->ext_status[sext->ext_idx] =
> > +     scontext->ext_status[sext->ext_idx] = (reg_val) ?
> > +                     KVM_RISCV_SBI_EXT_AVAILABLE :
> >                       KVM_RISCV_SBI_EXT_UNAVAILABLE;
>
> We're missing the change to riscv_vcpu_get_sbi_ext_single() which should
> also drop the comment block explaining the limits to status knowledge
> without initial probing (which we now do) and then just check for
> available, i.e.
>
> diff --git a/arch/riscv/kvm/vcpu_sbi.c b/arch/riscv/kvm/vcpu_sbi.c
> index bb76c3cf633f..92c42d9aba1c 100644
> --- a/arch/riscv/kvm/vcpu_sbi.c
> +++ b/arch/riscv/kvm/vcpu_sbi.c
> @@ -186,15 +186,8 @@ static int riscv_vcpu_get_sbi_ext_single(struct kvm_vcpu *vcpu,
>         if (!sext)
>                 return -ENOENT;
>
> -       /*
> -        * If the extension status is still uninitialized, then we should probe
> -        * to determine if it's available, but it may be too early to do that
> -        * here. The best we can do is report that the extension has not been
> -        * disabled, i.e. we return 1 when the extension is available and also
> -        * when it only may be available.
> -        */
> -       *reg_val = scontext->ext_status[sext->ext_idx] !=
> -                               KVM_RISCV_SBI_EXT_UNAVAILABLE;
> +       *reg_val = scontext->ext_status[sext->ext_idx] ==
> +                               KVM_RISCV_SBI_EXT_AVAILABLE;
>
>         return 0;
>  }

Thanks, I will include this change in the next revision.

>
> >
> >       return 0;
> > @@ -337,18 +331,8 @@ const struct kvm_vcpu_sbi_extension *kvm_vcpu_sbi_find_ext(
> >                           scontext->ext_status[entry->ext_idx] ==
> >                                               KVM_RISCV_SBI_EXT_AVAILABLE)
> >                               return ext;
> > -                     if (scontext->ext_status[entry->ext_idx] ==
> > -                                             KVM_RISCV_SBI_EXT_UNAVAILABLE)
> > -                             return NULL;
> > -                     if (ext->probe && !ext->probe(vcpu)) {
> > -                             scontext->ext_status[entry->ext_idx] =
> > -                                     KVM_RISCV_SBI_EXT_UNAVAILABLE;
> > -                             return NULL;
> > -                     }
> >
> > -                     scontext->ext_status[entry->ext_idx] =
> > -                             KVM_RISCV_SBI_EXT_AVAILABLE;
> > -                     return ext;
> > +                     return NULL;
> >               }
> >       }
> >
> > @@ -419,3 +403,26 @@ int kvm_riscv_vcpu_sbi_ecall(struct kvm_vcpu *vcpu, struct kvm_run *run)
> >
> >       return ret;
> >  }
> > +
> > +void kvm_riscv_vcpu_sbi_init(struct kvm_vcpu *vcpu)
> > +{
> > +     struct kvm_vcpu_sbi_context *scontext = &vcpu->arch.sbi_context;
> > +     const struct kvm_riscv_sbi_extension_entry *entry;
> > +     const struct kvm_vcpu_sbi_extension *ext;
> > +     int i;
> > +
> > +     for (i = 0; i < ARRAY_SIZE(sbi_ext); i++) {
> > +             entry = &sbi_ext[i];
> > +             ext = entry->ext_ptr;
> > +
> > +             if (ext->probe && !ext->probe(vcpu)) {
> > +                     scontext->ext_status[entry->ext_idx] =
> > +                             KVM_RISCV_SBI_EXT_UNAVAILABLE;
> > +                     continue;
> > +             }
> > +
> > +             scontext->ext_status[entry->ext_idx] = ext->default_unavail ?
> > +                                     KVM_RISCV_SBI_EXT_UNAVAILABLE :
> > +                                     KVM_RISCV_SBI_EXT_AVAILABLE;
> > +     }
> > +}
> > --
> > 2.34.1
> >
>
> Thanks,
> drew

Regards,
Anup