[PATCH v4 0/4] Add Sstc extension support

[Anup Patel]

On Fri, May 27, 2022 at 9:59 AM Atish Patra <atishp at rivosinc.com> wrote:
>
> This series implements Sstc extension support which was ratified recently.
> Before the Sstc extension, an SBI call is necessary to generate timer
> interrupts as only M-mode have access to the timecompare registers. Thus,
> there is significant latency to generate timer interrupts at kernel.
> For virtualized enviornments, its even worse as the KVM handles the SBI call
> and uses a software timer to emulate the timecomapre register.
>
> Sstc extension solves both these problems by defining a stimecmp/vstimecmp
> at supervisor (host/guest) level. It allows kernel to program a timer and
> recieve interrupt without supervisor execution enviornment (M-mode/HS mode)
> intervention.
>
> KVM directly updates the vstimecmp as well if the guest kernel invokes the SBI
> call instead of updating stimecmp directly. This is required because KVM will
> enable sstc extension if the hardware supports it unless the VMM explicitly
> disables it for that guest. The hardware is expected to compare the
> vstimecmp at every cycle if sstc is enabled and any stale value in vstimecmp
> will lead to spurious timer interrupts. This also helps maintaining the
> backward compatibility with older kernels.
>
> Similary, the M-mode firmware(OpenSBI) uses stimecmp for older kernel
> without sstc support as STIP bit in mip is read only for hardware with sstc.
>
> The PATCH 1 & 2 enables the basic infrastructure around Sstc extension while
> PATCH 3 lets kernel use the Sstc extension if it is available in hardware.
> PATCH 4 implements the Sstc extension in KVM.
>
> This series has been tested on Qemu(RV32 & RV64) with additional patches in
> Qemu[2]. This series can also be found at [3].
>
> Changes from v3->v4:
> 1. Rebased on 5.18-rc6
> 2. Unified vstimemp & next_cycles.
> 3. Addressed comments in PATCH 3 & 4.
>
> Changes from v2->v3:
> 1. Dropped unrelated KVM fixes from this series.
> 2. Rebased on 5.18-rc3.
>
> Changes from v1->v2:
> 1. Separate the static key from kvm usage
> 2. Makde the sstc specific static key local to the driver/clocksource
> 3. Moved the vstimecmp update code to the vcpu_timer
> 4. Used function pointers instead of static key to invoke vstimecmp vs
>    hrtimer at the run time. This will help in future for migration of vms
>    from/to sstc enabled hardware to non-sstc enabled hardware.
> 5. Unified the vstimer & timer to 1 timer as only one of them will be used
>    at runtime.
>
> [1] https://drive.google.com/file/d/1m84Re2yK8m_vbW7TspvevCDR82MOBaSX/view
> [2] https://github.com/atishp04/qemu/tree/sstc_v3
> [3] https://github.com/atishp04/linux/tree/sstc_v4
>
> Atish Patra (4):
> RISC-V: Add SSTC extension CSR details
> RISC-V: Enable sstc extension parsing from DT
> RISC-V: Prefer sstc extension if available
> RISC-V: KVM: Support sstc extension

Please don't forget to CC kvm-riscv mailing list for KVM RISC-V patches.

We have a patchwork setup for KVM RISC-V will also miss a series if
patches are not CCed.

Regards,
Anup

>
> arch/riscv/include/asm/csr.h            |  11 ++
> arch/riscv/include/asm/hwcap.h          |   1 +
> arch/riscv/include/asm/kvm_vcpu_timer.h |   8 +-
> arch/riscv/include/uapi/asm/kvm.h       |   1 +
> arch/riscv/kernel/cpu.c                 |   1 +
> arch/riscv/kernel/cpufeature.c          |   1 +
> arch/riscv/kvm/main.c                   |  12 +-
> arch/riscv/kvm/vcpu.c                   |   5 +-
> arch/riscv/kvm/vcpu_timer.c             | 144 +++++++++++++++++++++++-
> drivers/clocksource/timer-riscv.c       |  24 +++-
> 10 files changed, 198 insertions(+), 10 deletions(-)
>
> --
> 2.25.1
>