Re: [PATCH v7 0/4] Add Sstc extension support

[Palmer Dabbelt]

On Fri, 22 Jul 2022 21:47:06 PDT (-0700), anup@xxxxxxxxxxxxxx wrote:
> Hi Palmer,
>
> On Fri, Jul 22, 2022 at 10:20 PM Atish Patra <atishp@xxxxxxxxxxxx> 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 v6->v7:
> > 1. Fixed a compilation error reported by 0-day bot.
> >
> > 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_v6
> > [3] https://github.com/atishp04/linux/tree/sstc_v7
> >
> > 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
>
> The PATCH4 is dependent on the KVM patches in queue for 5.20.
>
> I suggest you take PATCH1, PATCH2 and PATCH3. I will send
> PATCH4 in second batch/PR for 5.20 assuming you will send the
> first three patches in your first PR for 5.20
>
> Does this sound okay to you ?

Sorry for being slow here, I just merged the non-KVM ones onto 
riscv/for-next.  LMK if you want me to try and sort out the KVM bits, 
the branch base is at palmer/riscv-sstc assuming that's easier for you 
to just merge in locally.

> Regards,
> Anup
>
> > 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       |  25 +++-
> > 9 files changed, 185 insertions(+), 8 deletions(-)
> >
> > --
> > 2.25.1