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

Andrew Jones
2023-10-19T07:58:04Z

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@ventanamicro.com>
> ---
> 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.

> +
> /* 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;
}

>
> 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