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

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Date:   Thu, 11 Aug 2022 14:49:50 -0700 (PDT)
From:   Palmer Dabbelt <palmer@...belt.com>
To:     anup@...infault.org
CC:     Atish Patra <atishp@...osinc.com>, linux-kernel@...r.kernel.org,
        aou@...s.berkeley.edu, atishp@...shpatra.org,
        daniel.lezcano@...aro.org, guoren@...nel.org, heiko@...ech.de,
        kvm-riscv@...ts.infradead.org, kvm@...r.kernel.org,
        linux-riscv@...ts.infradead.org, pbonzini@...hat.com,
        Paul Walmsley <paul.walmsley@...ive.com>, robh@...nel.org,
        tglx@...utronix.de, research_trasio@....a4lg.com, wefu@...hat.com
Subject:     Re: [PATCH v7 0/4] Add Sstc extension support

On Fri, 22 Jul 2022 21:47:06 PDT (-0700), anup@...infault.org wrote:
> Hi Palmer,
>
> On Fri, Jul 22, 2022 at 10:20 PM Atish Patra <atishp@...osinc.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 ?

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
>>