RISC-V vector registers not visible

[lorcan-oneill]

According to the OpenOCD 0.12.0 User's Guide (extract below) the RISC-V vector registers should be accessible. However, we can't see them when using VSCode and OpenOCD/GDB. Do some flags need to be set for that somewhere? We're using xpack-openocd version 0.12.0-3.

This User’s Guide documents release 0.12.0, dated 20 December 2022, of the Open On-Chip Debugger (OpenOCD).
16.10.2
Vector Registers
For harts that implement the vector extension, OpenOCD provides access to the relevant CSRs, as well as the vector registers (v0-v31).

[ilg-ul]

Unfortunately I don't have experience with the vector extension. :-(

[lorcan-oneill]

Thanks for your quick reply! It's my first time messaging on this channel. Is there an audience here that could have experience of this, or am I on my own with this question?

…

________________________________ From: Liviu Ionescu Sent: Thursday, April 17, 2025 5:18 PM To: xpack-dev-tools/riscv-none-elf-gcc-xpack Cc: Lorcan Oneill - M72430; Author Subject: Re: [xpack-dev-tools/riscv-none-elf-gcc-xpack] RISC-V vector registers not visible (Discussion #41) EXTERNAL EMAIL: Do not click links or open attachments unless you know the content is safe Unfortunately I don't have experience with the vector extension. :-( — Reply to this email directly, view it on GitHub<#41 (comment)>, or unsubscribe<https://github.com/notifications/unsubscribe-auth/A6TWBUG4TYQFEUNNFEG2KSL2Z7H3XAVCNFSM6AAAAAB3LIWCJKVHI2DSMVQWIX3LMV43URDJONRXK43TNFXW4Q3PNVWWK3TUHMYTEOBWHE3DKMA>. You are receiving this because you authored the thread.

[TommyMurphyTM1234]

Anything useful here?

• https://openocd.org/doc-release/html/Architecture-and-Core-Commands.html#Vector-Registers

What version of the RISC-V vector extension does your target support? If it's pre 1.0 then I think that you are out of luck:

• Update to version 1.0 of the vector spec. riscv-collab/riscv-openocd#505

Not sure if there's anything of relevance here?

• https://github.com/riscv-collab/riscv-openocd/pulls?q=is%3Apr+vector+registers

But the OpenOCD documentation seems to suggest that the vector (spec 1.0) registers should be visible by default:

• https://openocd.org/doc-release/html/Architecture-and-Core-Commands.html#Vector-Registers

[lorcan-oneill]

Great! Lots of useful info there. Yes we're using 1.0 so we should be ok but I just haven't been able to see the vector registers in VSCode yet (can see the using the OpenOCD RISC-V fork but that hasn't been upstreamed yet). I'll have a good look at that tomorrow.
Thanks a lot!

[lorcan-oneill]

Hmm. Some interesting stuff there. We can see the vector registers if we use the RISC-V fork of OpenOCD so it's unlikely to be a VSCode or GDB problem. We can also define the vector control registers (but not the vector registers V0-V31) using custom register definitions with the upstream version.
There are some comments about the vslide1down instruction in the links. I wonder if it's something to do with that, although from the comments and documentation it doesn't look like it.

[TommyMurphyTM1234]

We can see the vector registers if we use the RISC-V fork of OpenOCD

In that case it's quite possible that the relevant changes have simply not yet been upstreamed from the RISC-V OpenOCD fork repo to the master OpenOCD project repo. Synchronisation of changes in the two repos (in both directions) only happens sporadically and whenever someone has the time and inclination to do the necessary work.

What OpenOCD were you using previously? The xPack version or a build from the master branch of the upstream OpenOCD repo. The latter will almost certainly be more up to date than the former so if you haven't tried the upstream version it might be worth trying that too?

We can also define the vector control registers (but not the vector registers V0-V31) using custom register definitions with the upstream version. There are some comments about the vslide1down instruction in the links. I wonder if it's something to do with that, although from the comments and documentation it doesn't look like it.

Might be worth opening an issue on the RISC-V OpenOCD fork repo to ask for clarification on this?

[lorcan-oneill]

From the comments in the RISC-V fork, it has not been upstreamed for a long time and there's not much hope of it happening anytime soon. According to the OpenOCD 0.12.0 spec the vector registers should be there and the xPack version we're using is 0.12.0-3 so it should be ok.
Yes it's probably worth our effort to build the upstream version and use that just in case it's missing from xPack for some reason.
It looks like not many people are using the vector extension with open source tools so far!

[ilg-ul]

just in case it's missing from xPack for some reason

Usually there are 2-3 xPack OpenOCD releases per year, so it is more or less up to date. But it does not merge the commits from the RISC-V fork, if you need them I'm afraid you have to compile your own binaries.

[lorcan-oneill]

Yes I realise that and have seen the discussion about upstreaming the RISC-V fork. Everyone is really busy, which is totally understandable.
The documentation for 0.12.0 clearly states that the vector registers are there but we can't see them. I presume that documentation relates to the official OpenOCD release, not the RISC-V fork. So unless there is a difference between the official 0.12.0 and the xPack 0.12.0, the vector registers should be visible when using the xPack version.