Messages in this thread | | | Subject | Re: [RFC PATCH 13/16] RISC-V: KVM: Add timer functionality | From | Paolo Bonzini <> | Date | Wed, 31 Jul 2019 08:58:09 +0200 |
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On 31/07/19 03:55, Atish Patra wrote: > On Tue, 2019-07-30 at 13:26 +0200, Paolo Bonzini wrote: >> On 29/07/19 13:57, Anup Patel wrote: >>> + if (delta_ns > VCPU_TIMER_PROGRAM_THRESHOLD_NS) { >>> + hrtimer_start(&t->hrt, ktime_add_ns(ktime_get(), >>> delta_ns), >> >> I think the guest would prefer if you saved the time before enabling >> interrupts on the host, and use that here instead of ktime_get(). >> Otherwise the timer could be delayed arbitrarily by host interrupts. >> >> (Because the RISC-V SBI timer is relative only---which is >> unfortunate--- > > Just to clarify: RISC-V SBI timer call passes absolute time. > > https://elixir.bootlin.com/linux/v5.3-rc2/source/drivers/clocksource/timer-riscv.c#L32 > > That's why we compute a delta between absolute time passed via SBI and > current time. hrtimer is programmed to trigger only after the delta > time from now.
Nevermind, I got lost in all the conversions.
One important issue is the lack of ability to program a delta between HS/HU-mode cycles and VS/VU-mode cycles. Without this, it's impossible to do virtual machine migration (except with hcounteren trap-and-emulate, which I think we agree is not acceptable). I found the open issue at https://github.com/riscv/riscv-isa-manual/issues/298 and commented on it.
Paolo
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