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Date: Wed, 10 Apr 2024 13:09:45 +0100
From: David Woodhouse <dwmw2@infradead.org>
To: paul@xen.org, Paul Durrant <xadimgnik@gmail.com>,
 Jack Allister <jalliste@amazon.com>
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On 10 April 2024 11:29:13 BST, Paul Durrant <xadimgnik@gmail=2Ecom> wrote:
>On 10/04/2024 10:52, Jack Allister wrote:
>> +	 * It's possible that this vCPU doesn't have a HVCLOCK configured
>> +	 * but the other vCPUs may=2E If this is the case calculate based
>> +	 * upon the time gathered in the seqcount but do not update the
>> +	 * vCPU specific PVTI=2E If we have one, then use that=2E
>
>Given this is a per-vCPU ioctl, why not fail in the case the vCPU doesn't=
 have HVCLOCK configured? Or is your intention that a GET/SET should always=
 work if TSC is stable?

It definitely needs to work for SET even when the vCPU hasn't been run yet=
 (and doesn't have a hvclock in vcpu->arch=2Ehv_clock)=2E

I think it should ideally work for GET too=2E I did try arguing that if th=
e vCPU hasn't set up its pvclock then why would it care if it's inaccurate?=
 But there's a pathological case of AMP where one vCPU is dedicated to an R=
TOS or something, and only the *other* vCPUs bring up their pvclock=2E

This of course brings you to the question of why we have it as a per-vCPU =
ioctl at all? It only needs to be done *once* to get/set the KVM-wide clock
 And a function of *this* vCPU's TSC=2E And the point is that if we're in =
use_master_clock mode, that's consistent across *all* vCPUs=2E There would =
be a bunch of additional complexity in making it a VM ioctl though, especia=
lly around the question of what to do if userspace tries to restore it when=
 there *aren't* any vCPUs yet=2E So we didn't do that=2E