On Thu, Dec 3, 2020 at 5:51 PM Yifei Jiang <jiangyifei@xxxxxxxxxx> 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@xxxxxxxxxxxxxxxxxxx > http://lists.infradead.org/mailman/listinfo/kvm-riscv
