RFC: Dedicated Clock Source and Clock Event Source for HS-mode and VS-mode
The QEMU “virt” machine emulates both TIME CSR and HTIMEDELTA CSR so no trapping happens when accessing TIME and HTIMEDELTA CSRs from VS/VU mode on QEMU. Although, QEMU “sifive_u” machine does not emulate these CSRs because on real SiFive Unleashed TIME CSR is trap-n-emulated by M-mode.
Also, (like Andrew already mentioned) the MCOUNTEREN.TM and HCOUNTER.TM bits handles the delegation of TIME CSR properly.
From: tech-privileged@... <tech-privileged@...> On Behalf Of zhaosiqi (A) via lists.riscv.org
Sent: 04 August 2020 13:59
To: Andrew Waterman <andrew@...>
Subject: 答复: [RISC-V] [tech-privileged] RFC: Dedicated Clock Source and Clock Event Source for HS-mode and VS-mode
I see. This is new information for me though. The ‘that is’ in the current spec makes the sentence look like an reiteration of the functionality of the htimedelta itself. I totally didn’t expect this added bit of information about the changing of the trapping rule of the time CSR in there. (Judging from the current implementation in QEMU and KVM, reading time CSR in a virtual machine does trap.) Perhaps add a note to clarify this point?
However, there is still one new bit. Our proposal says that the no-trapping behavior can be turned off, so that the hypervisor can choose to emulate the time CSR for a virtual machine. This can provide the flexibility for implementing simpler systems where the aliases or the htimedelta does not exist.
Andrew Waterman [mailto:andrew@...]
On Tue, Aug 4, 2020 at 12:46 AM zhaosiqi (A) <zhaosiqi3@...> wrote:
Yeah, my point is that the current definition of htimedelta already says what you want it to say about the time CSR. Reading the time CSR in VS-mode or VU-mode isn't required to trap, and it returns the value you want it to return.
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