[PATCH v3 0/4] Add Sstc extension support

Atish Patra atishp at rivosinc.com
Tue Apr 26 11:52:41 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)

To maintain backward compatibility, KVM directly updates the vstimecmp
if older kernel without sstc support is running in guest. Similary, the
M-mode firmware(OpenSBI) uses stimecmp for older kernel without sstc support. 

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
OpenSBI[2] and Qemu[3]. This series can also be found at [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/opensbi/tree/sstc_v2
[3] https://github.com/atishp04/qemu/tree/sstc_v2
[3] https://github.com/atishp04/linux/tree/sstc_v3

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            |  11 ++
arch/riscv/include/asm/hwcap.h          |   1 +
arch/riscv/include/asm/kvm_host.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             | 138 +++++++++++++++++++++++-
drivers/clocksource/timer-riscv.c       |  21 +++-
11 files changed, 193 insertions(+), 7 deletions(-)


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