mirror of
https://github.com/xen-project/xen.git
synced 2026-10-28 02:01:24 +01:00
Baptiste Le Duc
861e44a3ba
Zihintpause is in required_extensions[] so Xen panics when it is missing, while it does not strictly require hardware support for it. The PAUSE hint (Zihintpause extension) is encoded as FENCE W, 0 (0x0100000F). In accordance with the RISC-V Unprivileged ISA, HINTs are encoded in the space of valid standard instructions. On platforms without Zihintpause support, executing 0x0100000F is treated as a standard FENCE with an empty successor set, which acts as a NOP and never generates an illegal instruction trap. Zihintpause is therefore purely an optimization hint. Drop it from required_extensions[] so Xen can still boot on systems that don't advertise it, and print a warning in that case. Assisted-by: Claude:claude-opus-5 Signed-off-by: Baptiste Le Duc <baptiste.le-duc@vates.tech> Reviewed-by: Oleksii Kurochko <oleksii.kurochko@gmail.com>
38 lines
1.5 KiB
Plaintext
38 lines
1.5 KiB
Plaintext
System requirements
|
|
===================
|
|
|
|
The following extensions are required by Xen. Xen panics at boot if any of
|
|
them is missing from the riscv,isa property of the CPU nodes:
|
|
- I, M, A, Zicsr, Zifencei:
|
|
Base ISA Xen is compiled for.
|
|
- C:
|
|
Required only when Xen is built with CONFIG_RISCV_ISA_C=y.
|
|
- H:
|
|
Provides additional instructions and CSRs that control the new stage of
|
|
address translation and support hosting a guest OS in virtual S-mode
|
|
(VS-mode).
|
|
- Zbb:
|
|
RISC-V doesn't have a CLZ instruction in the base ISA.
|
|
As a consequence, __builtin_ffs() emits a library call to ffs() on GCC,
|
|
or a de Bruijn sequence on Clang.
|
|
Zbb extension adds a CLZ instruction, after which __builtin_ffs() emits
|
|
a very simple sequence.
|
|
The similar issue occurs with other __builtin_<bitop>, so it is needed to
|
|
provide a generic version of bitops in RISC-V bitops.h
|
|
- Svpbmt:
|
|
Mandatory to enable changing the memory attributes of a page.
|
|
For platforms that do not support Svpbmt, it is necessary to introduce a
|
|
similar mechanism as described in:
|
|
https://lore.kernel.org/all/20241102000843.1301099-1-samuel.holland@sifive.com/
|
|
|
|
Note: this list must be kept in sync with required_extensions[] in
|
|
xen/arch/riscv/cpufeature.c.
|
|
|
|
Optional extensions
|
|
===================
|
|
|
|
Other extensions recognised by Xen are listed in riscv_isa_ext[] in
|
|
xen/arch/riscv/cpufeature.c. Xen uses them when present, but they are not
|
|
needed to boot. The second column of that table says whether the extension
|
|
is also exposed to guests.
|