state enable bit for small priv-1.13-era extensions

[aswaterman]

Going forward, we'd like to allocate only a single stateen bit for small extensions defined between priv-spec releases, rather than allocating a bit for every small extension. This isn't an absolute rule; it's just an attempt to cut down on stateen bit count when practical.

The plan is that the qosid register will be controlled by the priv-1.13 stateen bit, and possibly other registers will, too.

[ved-rivos]

@aswaterman is there a position defined for this bit.

[aswaterman]

I'm pretty swamped right now. Can you make a proposal (and a PR once we an agree)?

[ved-rivos]

Will do.

Bits that are presently defined in xstateen0.

• Bit 0 - Custom State
• Bit 1 - fcsr for Zfinx and related extensions
• Bit 2 - jvt CSR for Zcmt
• Bit 56 - CD ?
• Bit 57 - hcontext CSR
• Bit 58 - all IMSIC state, including CSRs stopei and vstopei
• Bit 59 - all other state added by the AIA and not controlled by bits 60 and 58
• Bit 60 - CSRs siselect, sireg, vsiselect, and vsireg
• Bit 63 - controls access to sstateen0 and hstateen0

This issue proposes to define a single xstateen0 bit - bit 55 - to govern new state introduced by Priv-1.14-era extensions.

• Bit 55 - all state introduced by extensions included in Privileged ISA version 1.14

The state of the following extensions is expected to be governed by the bit 55:

• Ssqosid - CSR srmcfg (holds QoS identifiers)
• Ssclic - CSRs stvt, snxti, sintstatus, sintthresh, sscratchcsw, and sscratchcswl (S-mode CLIC extensions)
• Ssctr - indirect state accessed by siselect/vsiselect value of 0x200 through 0x2FF (Control transfer records extension)

[ved-rivos]

The bit 56 was initially intended for use by the counter delegation extension but was not pursued/approved. The updated definition is as follows:

• Bit 0 - Custom State
• Bit 1 - fcsr for Zfinx and related extensions
• Bit 2 - jvt CSR for Zcmt
• Bit 57 - hcontext CSR
• Bit 58 - all IMSIC state, including CSRs stopei and vstopei
• Bit 59 - all other state added by the AIA and not controlled by bits 60 and 58
• Bit 60 - CSRs siselect, sireg, vsiselect, and vsireg
• Bit 63 - controls access to sstateen0 and hstateen0

An architecture decisions shall be made, per 1.14-era extensions, to determine if the extension should be governed by the bit provided in xstateen0 for the 1.14-era extensions or the extension should be governed by an extension specific bit. This determination is pending for Ssclic and Ssctr.

Bit 56 in CSR xstateen0 is defined to govern new state introduced by Priv-1.14-era extensions. Access to the CSR srmcfg introduced by the Ssqosid extension shall be governed by the bit 56 of CSR mstateen0.

[aswaterman]

Agreed.

[ved-rivos]

Bit 56 in CSR mstateen0 controls access to CSR henvcfgh.

Bit 55 in CSR mstateen0 is defined to control access to new state introduced by the following small extensions of Priv-1.14-era:

• CSR srmcfg introduced by the forthcoming Ssqosid extension.