[PATCH 0/5] Enable hstateen bits lazily for the KVM RISC-V Guests

[Radim Krčmář]

2025-05-05T14:39:25-07:00, Atish Patra <atishp at rivosinc.com>:
> This series adds support for enabling hstateen bits lazily at runtime
> instead of statically at bootime. The boot time enabling happens for
> all the guests if the required extensions are present in the host and/or
> guest. That may not be necessary if the guest never exercise that
> feature. We can enable the hstateen bits that controls the access lazily
> upon first access. This providers KVM more granular control of which
> feature is enabled in the guest at runtime.
>
> Currently, the following hstateen bits are supported to control the access
> from VS mode.
>
> 1. BIT(58): IMSIC     : STOPEI and IMSIC guest interrupt file
> 2. BIT(59): AIA       : SIPH/SIEH/STOPI
> 3. BIT(60): AIA_ISEL  : Indirect csr access via siselect/sireg
> 4. BIT(62): HSENVCFG  : SENVCFG access
> 5. BIT(63): SSTATEEN0 : SSTATEEN0 access
>
> KVM already support trap/enabling of BIT(58) and BIT(60) in order
> to support sw version of the guest interrupt file.

I don't think KVM toggles the hstateen bits at runtime, because that
would mean there is a bug even in current KVM.

>                                                    This series extends
> those to enable to correpsonding hstateen bits in PATCH1. The remaining
> patches adds lazy enabling support of the other bits.

The ISA has a peculiar design for hstateen/sstateen interaction:

  For every bit in an hstateen CSR that is zero (whether read-only zero
  or set to zero), the same bit appears as read-only zero in sstateen
  when accessed in VS-mode.

This means we must clear bit 63 in hstateen and trap on sstateen
accesses if any of the sstateen bits are not supposed to be read-only 0
to the guest while the hypervisor wants to have them as 0.

Thanks.