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 v5->v6: 1. Moved SSTC extension enum below SVPBMT. 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_v5 [3] https://github.com/atishp04/linux/tree/sstc_v6 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 | 8 +- arch/riscv/kvm/vcpu_timer.c | 144 +++++++++++++++++++++++- drivers/clocksource/timer-riscv.c | 24 +++- 9 files changed, 184 insertions(+), 8 deletions(-) -- 2.25.1
