[PATCH v5 0/4] Add Sstc extension support

Atish Patra atishp at rivosinc.com
Wed Jul 20 12:23:38 PDT 2022

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)

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 v4->v5:
1. Added RB tag.
2. Changed the pr-format.
3. Rebased on 5.19-rc7 and kvm-queue.
4. Moved the henvcfg modification from hardware enable to vcpu_load.

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_v4
[3] https://github.com/atishp04/linux/tree/sstc_v5

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

arch/riscv/include/asm/csr.h            |   5 +
arch/riscv/include/asm/hwcap.h          |   1 +
arch/riscv/include/asm/kvm_vcpu_timer.h |   7 ++
arch/riscv/include/uapi/asm/kvm.h       |   1 +
arch/riscv/kernel/cpu.c                 |   1 +
arch/riscv/kernel/cpufeature.c          |   1 +
arch/riscv/kvm/vcpu.c                   |   7 +-
arch/riscv/kvm/vcpu_timer.c             | 144 +++++++++++++++++++++++-
drivers/clocksource/timer-riscv.c       |  24 +++-
9 files changed, 183 insertions(+), 8 deletions(-)


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