linux-kernel - Re: [PATCH RFC 3/7] KVM: timer: synchronize tsc-deadline timestamp for guest

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Message-ID: <196f0958-f5bf-0eb3-fbe8-71f1b6e3ff34@gmail.com>
Date:   Thu, 14 Dec 2017 09:54:55 +0800
From:   Quan Xu <quan.xu0@...il.com>
To:     Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
Cc:     pbonzini@...hat.com, rkrcmar@...hat.com, yang.zhang.wz@...il.com,
        kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        Ben Luo <bn0418@...il.com>
Subject: Re: [PATCH RFC 3/7] KVM: timer: synchronize tsc-deadline timestamp
 for guest



On 2017/12/08 23:06, Konrad Rzeszutek Wilk wrote:
> On Fri, Dec 08, 2017 at 04:39:46PM +0800, Quan Xu wrote:
>> From: Ben Luo <bn0418@...il.com>
>>
>> In general, KVM guest programs tsc-deadline timestamp to
>> MSR_IA32_TSC_DEADLINE MSR. This will cause a VM-exit, and
>> then KVM handles this timer for guest.
>>
>> The tsc-deadline timestamp is mostly recorded in share page
>> with less VM-exit. We Introduce a periodically working kthread
>> to scan share page and synchronize timer setting for guest
>> on a dedicated CPU.
> That sounds like a race. Meaning the guest may put too small window
> and this 'working thread to scan' may not get to it fast enough?
yes, you are right. So ..
> .
> Meaning we miss the deadline to inject the timer in the guest.
>
> Or is this part of this PV MSR semantics - that it will only work
> for certain amount of values and anything less than say 1ms
> should not use the PV MSR?

..
for these timers, We have to program these tsc-deadline timestamps
to MSR_IA32_TSC_DEADLINE as normal, which will cause VM-exit and KVM will
signal the working thread through IPI to program timer, instead of
registering on current CPU (patch 0004).

more detail in patch 0007.

Quan
Alibaba Cloud