| Message ID | 20201203121839.308-1-jiangyifei@huawei.com (mailing list archive) |
|---|---|
| Headers | show |
| Series | Implement guest time scaling in RISC-V KVM | expand |
On Thu, Dec 3, 2020 at 5:51 PM Yifei Jiang <jiangyifei@huawei.com> wrote: > > This series implements guest time scaling based on RDTIME instruction > emulation so that we can allow migrating Guest/VM across Hosts with > different time frequency. > > Why not through para-virt. From arm's experience[1], para-virt implementation > doesn't really solve the problem for the following two main reasons: > - RDTIME not only be used in linux, but also in firmware and userspace. > - It is difficult to be compatible with nested virtualization. I think this approach is rather incomplete. Also, I don't see how para-virt time scaling will be difficult for nested virtualization. If trap-n-emulate TIME CSR for Guest Linux then it will have significant performance impact of systems where TIME CSR is implemented in HW. Best approach will be to have VDSO-style para-virt time-scale SBI calls (similar to what KVM x86 does). If the Guest software (Linux/Bootloader) does not enable para-virt time-scaling then we trap-n-emulate TIME CSR (this series). Please propose VDSO-style para-virt time-scale SBI call and expand this this series to provide both: 1. VDSO-style para-virt time-scaling 2. Trap-n-emulation of TIME CSR when #1 is disabled Regards, Anup > > [1] https://lore.kernel.org/patchwork/cover/1288153/ > > Yifei Jiang (3): > RISC-V: KVM: Change the method of calculating cycles to nanoseconds > RISC-V: KVM: Support dynamic time frequency from userspace > RISC-V: KVM: Implement guest time scaling > > arch/riscv/include/asm/csr.h | 3 ++ > arch/riscv/include/asm/kvm_vcpu_timer.h | 13 +++++-- > arch/riscv/kvm/vcpu_exit.c | 35 +++++++++++++++++ > arch/riscv/kvm/vcpu_timer.c | 51 ++++++++++++++++++++++--- > 4 files changed, 93 insertions(+), 9 deletions(-) > > -- > 2.19.1 > > > -- > kvm-riscv mailing list > kvm-riscv@lists.infradead.org > http://lists.infradead.org/mailman/listinfo/kvm-riscv
> -----Original Message----- > From: Anup Patel [mailto:anup@brainfault.org] > Sent: Wednesday, December 16, 2020 2:40 PM > To: Jiangyifei <jiangyifei@huawei.com> > Cc: Anup Patel <anup.patel@wdc.com>; Atish Patra <atish.patra@wdc.com>; > Paul Walmsley <paul.walmsley@sifive.com>; Palmer Dabbelt > <palmer@dabbelt.com>; Albert Ou <aou@eecs.berkeley.edu>; Paolo Bonzini > <pbonzini@redhat.com>; Zhanghailiang <zhang.zhanghailiang@huawei.com>; > KVM General <kvm@vger.kernel.org>; yinyipeng <yinyipeng1@huawei.com>; > Zhangxiaofeng (F) <victor.zhangxiaofeng@huawei.com>; > linux-kernel@vger.kernel.org List <linux-kernel@vger.kernel.org>; > kvm-riscv@lists.infradead.org; linux-riscv <linux-riscv@lists.infradead.org>; > Wubin (H) <wu.wubin@huawei.com>; dengkai (A) <dengkai1@huawei.com> > Subject: Re: [PATCH RFC 0/3] Implement guest time scaling in RISC-V KVM > > On Thu, Dec 3, 2020 at 5:51 PM Yifei Jiang <jiangyifei@huawei.com> wrote: > > > > This series implements guest time scaling based on RDTIME instruction > > emulation so that we can allow migrating Guest/VM across Hosts with > > different time frequency. > > > > Why not through para-virt. From arm's experience[1], para-virt > > implementation doesn't really solve the problem for the following two main > reasons: > > - RDTIME not only be used in linux, but also in firmware and userspace. > > - It is difficult to be compatible with nested virtualization. > > I think this approach is rather incomplete. Also, I don't see how para-virt time > scaling will be difficult for nested virtualization. > > If trap-n-emulate TIME CSR for Guest Linux then it will have significant > performance impact of systems where TIME CSR is implemented in HW. > > Best approach will be to have VDSO-style para-virt time-scale SBI calls (similar > to what KVM x86 does). If the Guest software (Linux/Bootloader) does not > enable para-virt time-scaling then we trap-n-emulate TIME CSR (this series). > > Please propose VDSO-style para-virt time-scale SBI call and expand this this > series to provide both: > 1. VDSO-style para-virt time-scaling > 2. Trap-n-emulation of TIME CSR when #1 is disabled > > Regards, > Anup > OK, it sounds good. We will look into the para-virt time-scaling for more details. Yifei > > > > [1] https://lore.kernel.org/patchwork/cover/1288153/ > > > > Yifei Jiang (3): > > RISC-V: KVM: Change the method of calculating cycles to nanoseconds > > RISC-V: KVM: Support dynamic time frequency from userspace > > RISC-V: KVM: Implement guest time scaling > > > > arch/riscv/include/asm/csr.h | 3 ++ > > arch/riscv/include/asm/kvm_vcpu_timer.h | 13 +++++-- > > arch/riscv/kvm/vcpu_exit.c | 35 +++++++++++++++++ > > arch/riscv/kvm/vcpu_timer.c | 51 > ++++++++++++++++++++++--- > > 4 files changed, 93 insertions(+), 9 deletions(-) > > > > -- > > 2.19.1 > > > > > > -- > > kvm-riscv mailing list > > kvm-riscv@lists.infradead.org > > http://lists.infradead.org/mailman/listinfo/kvm-riscv