https://github.com/DavidSpickett created https://github.com/llvm/llvm-project/pull/165415
None >From f7c4f813060ee2abc8e13604d27d57555701997c Mon Sep 17 00:00:00 2001 From: David Spickett <[email protected]> Date: Tue, 28 Oct 2025 10:38:17 +0000 Subject: [PATCH] [lldb][docs] Add documentation for AArch64 Linux SME only support --- lldb/docs/use/aarch64-linux.md | 69 ++++++++++++++++++++++++++++++++++ 1 file changed, 69 insertions(+) diff --git a/lldb/docs/use/aarch64-linux.md b/lldb/docs/use/aarch64-linux.md index 164d25cef6721..1a301adce7a99 100644 --- a/lldb/docs/use/aarch64-linux.md +++ b/lldb/docs/use/aarch64-linux.md @@ -230,6 +230,75 @@ bytes. `zt0`'s value and whether it is active or not will be saved prior to expression evaluation and restored afterwards. +## SME Only Systems + +AArch64 systems may have both SVE and SME (that is, `FEAT_SVE` and `FEAT_SVE`), +or they can have only SME. If they only have SME, the system has the usual +SVE state, but that state and SVE instructions may only be used while in +streaming mode. + +The LLDB experience is very similar to SVE+SME systems, with a few notable +changes. + +### Registers + +When in streaming mode, registers act as they would on an SVE+SME system. +Outside of streaming mode LLDB will show the `Z` registers as zero extended copies +of the `V` registers. + +Writes to `Z` registers are allowed, but these are converted to `V` register writes +and so only the bottom 128-bits will be applied. Note that the size of the value +written to a `Z` register must match the current streaming vector length, even if +the process is in non-streaming mode. + +It is useful for LLDB developers to know that for an SME only system, +`lldb-server` describes the system as having `Z` registers with `V` registers +as subsets of those registers. We do not change the representation each time +the mode changes. + +A consequence of this is that if the user writes to a `V` register while in +non-streaming mode, it will be sent to `lldb-server` as a `Z` register write +of a zero extended value. Then `lldb-server` will convert that back into a `V` +register write with the value truncated to 128-bit. + +In streaming mode, it will be zero extended, sent as a `Z` write and written +to the real streaming SVE `Z` register without truncation. + +`P` registers will be shown as 0s outside of streaming mode. They cannot be written +to in this state. + +The `ffr` register will also be shown as 0s outside of streaming mode. In streaming +mode, use of `ffr` is forbidden so it will also show as 0s. It cannot be written +to in either state. + +(in the former, LLDB generates the fake value, in the latter, the kernel +generates it) + +The `ZA` and `ZT0` registers act as they would for an SVE+SME system. + +Since there is no non-streaming SVE, there is non non-streaming vector length. +Therefore even in non-streaming mode, the value shown in `vg` will be the +streaming vector length, and be equal to the value shown for `svg`. + +### Expression Evaluation + +Some instructions are illegal to use in streaming mode (unless `FEAT_SMEFA64` +is present). LLDB will not make any attempt to make expressions compatible +with the current mode. If part of the expression is not compatible, it will +result in a SIGILL that will be cleaned up as it normally would be. + +All register, ZA and mode state will be restored as normal after an expression. + +Note that to restore to a non-streaming state from a streaming state, LLDB uses +a special part of the Linux Kernel's +[SME ABI](https://docs.kernel.org/arch/arm64/sme.html). Where FPSIMD data is +written to the non-existent non-streaming SVE register set, with the vector +length set to 0 to cause the process to exit streaming mode. + +This is only used for this purpose. Otherwise, in non-streaming mode FP +registers are accessed using the FP register set, and in streaming mode using +the streaming SVE register set. + ## Guarded Control Stack Extension (GCS) GCS support includes the following new registers: _______________________________________________ lldb-commits mailing list [email protected] https://lists.llvm.org/cgi-bin/mailman/listinfo/lldb-commits
