> -----Original Message----- > From: Anup Patel [mailto:[email protected]] > Sent: Wednesday, December 16, 2020 2:40 PM > To: Jiangyifei <[email protected]> > Cc: Anup Patel <[email protected]>; Atish Patra <[email protected]>; > Paul Walmsley <[email protected]>; Palmer Dabbelt > <[email protected]>; Albert Ou <[email protected]>; Paolo Bonzini > <[email protected]>; Zhanghailiang <[email protected]>; > KVM General <[email protected]>; yinyipeng <[email protected]>; > Zhangxiaofeng (F) <[email protected]>; > [email protected] List <[email protected]>; > [email protected]; linux-riscv <[email protected]>; > Wubin (H) <[email protected]>; dengkai (A) <[email protected]> > 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 <[email protected]> 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 > > [email protected] > > http://lists.infradead.org/mailman/listinfo/kvm-riscv
