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1. "Svinval" Extension for Fine-Grained Address-Translation Cache Invalidation, Version 1.0

The Svinval extension splits SFENCE.VMA, HFENCE.VVMA, and HFENCE.GVMA instructions into finer-grained invalidation and ordering operations that can be more efficiently batched or pipelined on certain classes of high-performance implementation.

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The SINVAL.VMA instruction invalidates any address-translation cache entries that an SFENCE.VMA instruction with the same values of rs1 and rs2 would invalidate. However, unlike SFENCE.VMA, SINVAL.VMA instructions are only ordered with respect to SFENCE.VMA, SFENCE.W.INVAL, and SFENCE.INVAL.IR instructions as defined below.

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The SFENCE.W.INVAL instruction guarantees that any previous stores already visible to the current RISC-V hart are ordered before subsequent SINVAL.VMA instructions executed by the same hart. The SFENCE.INVAL.IR instruction guarantees that any previous SINVAL.VMA instructions executed by the current hart are ordered before subsequent implicit references by that hart to the memory-management data structures.

When executed in order (but not necessarily consecutively) by a single hart, the sequence SFENCE.W.INVAL, SINVAL.VMA, and SFENCE.INVAL.IR has the same effect as a hypothetical SFENCE.VMA instruction in which:

  • the values of rs1 and rs2 for the SFENCE.VMA are the same as those used in the SINVAL.VMA,

  • reads and writes prior to the SFENCE.W.INVAL are considered to be those prior to the SFENCE.VMA, and

  • reads and writes following the SFENCE.INVAL.IR are considered to be those subsequent to the SFENCE.VMA.

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If the hypervisor extension is implemented, the Svinval extension also provides two additional instructions: HINVAL.VVMA and HINVAL.GVMA. These have the same semantics as SINVAL.VMA, except that they combine with SFENCE.W.INVAL and SFENCE.INVAL.IR to replace HFENCE.VVMA and HFENCE.GVMA, respectively, instead of SFENCE.VMA. In addition, HINVAL.GVMA uses VMIDs instead of ASIDs.

SINVAL.VMA, HINVAL.VVMA, and HINVAL.GVMA require the same permissions and raise the same exceptions as SFENCE.VMA, HFENCE.VVMA, and HFENCE.GVMA, respectively. In particular, an attempt to execute any of these instructions in U-mode always raises an illegal-instruction exception, and an attempt to execute SINVAL.VMA or HINVAL.GVMA in S-mode or HS-mode when mstatus.TVM=1 also raises an illegal-instruction exception. An attempt to execute HINVAL.VVMA or HINVAL.GVMA in VS-mode or VU-mode, or to execute SINVAL.VMA in VU-mode, raises a virtual-instruction exception. When hstatus.VTVM=1, an attempt to execute SINVAL.VMA in VS-mode also raises a virtual instruction exception.

Attempting to execute SFENCE.W.INVAL or SFENCE.INVAL.IR in U-mode raises an illegal-instruction exception. Doing so in VU-mode raises a virtual-instruction exception. SFENCE.W.INVAL and SFENCE.INVAL.IR are unaffected by the mstatus.TVM and hstatus.VTVM fields and hence are always permitted in S-mode and VS-mode.

SFENCE.W.INVAL and SFENCE.INVAL.IR instructions do not need to be trapped when mstatus.TVM=1 or when hstatus.VTVM=1, as they only have ordering effects but no visible side effects. Trapping of the SINVAL.VMA instruction is sufficient to enable emulation of the intended overall TLB maintenance functionality.

In typical usage, software will invalidate a range of virtual addresses in the address-translation caches by executing an SFENCE.W.INVAL instruction, executing a series of SINVAL.VMA, HINVAL.VVMA, or HINVAL.GVMA instructions to the addresses (and optionally ASIDs or VMIDs) in question, and then executing an SFENCE.INVAL.IR instruction.

High-performance implementations will be able to pipeline the address-translation cache invalidation operations, and will defer any pipeline stalls or other memory ordering enforcement until an SFENCE.W.INVAL, SFENCE.INVAL.IR, SFENCE.VMA, HFENCE.GVMA, or HFENCE.VVMA instruction is executed.

Simpler implementations may implement SINVAL.VMA, HINVAL.VVMA, and HINVAL.GVMA identically to SFENCE.VMA, HFENCE.VVMA, and HFENCE.GVMA, respectively, while implementing SFENCE.W.INVAL and SFENCE.INVAL.IR instructions as no-ops.