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From:   Anup Patel <anup@brainfault.org>
Date:   Sat, 23 Jul 2022 10:17:06 +0530
Message-ID: <CAAhSdy2UDwZYK+EaKHPXLxyKUMv-OX02EdaBDBgjNF0jdDJ7xQ@mail.gmail.com>
Subject: Re: [PATCH v7 0/4] Add Sstc extension support
To:     Atish Patra <atishp@rivosinc.com>
Cc:     linux-kernel@vger.kernel.org, Albert Ou <aou@eecs.berkeley.edu>,
        Atish Patra <atishp@atishpatra.org>,
        Daniel Lezcano <daniel.lezcano@linaro.org>,
        Guo Ren <guoren@kernel.org>, Heiko Stuebner <heiko@sntech.de>,
        kvm-riscv@lists.infradead.org, kvm@vger.kernel.org,
        linux-riscv@lists.infradead.org,
        Palmer Dabbelt <palmer@dabbelt.com>,
        Paolo Bonzini <pbonzini@redhat.com>,
        Paul Walmsley <paul.walmsley@sifive.com>,
        Rob Herring <robh@kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Tsukasa OI <research_trasio@irq.a4lg.com>,
        Wei Fu <wefu@redhat.com>
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Hi Palmer,

On Fri, Jul 22, 2022 at 10:20 PM Atish Patra <atishp@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 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 ?

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
>