From: David Woodhouse <dwmw2@infradead.org>
To: Sean Christopherson <seanjc@google.com>,
Vitaly Kuznetsov <vkuznets@redhat.com>
Cc: jgross@suse.com, linux-hyperv@vger.kernel.org,
dave.hansen@linux.intel.com, hpa@zytor.com, kvm@vger.kernel.org,
joe.jin@oracle.com, pv-drivers@vmware.com, x86@kernel.org,
wanpengli@tencent.com, pbonzini@redhat.com,
virtualization@lists.linux-foundation.org, peterz@infradead.org,
akaher@vmware.com, mingo@redhat.com, bp@alien8.de,
amakhalov@vmware.com, xen-devel@lists.xenproject.org,
tglx@linutronix.de, boris.ostrovsky@oracle.com,
linux-kernel@vger.kernel.org
Subject: Re: [PATCH RFC 1/1] x86/paravirt: introduce param to disable pv sched_clock
Date: Thu, 19 Oct 2023 16:47:05 +0100 [thread overview]
Message-ID: <2f1459d7c3e3e81cdca931e104c3ade71dfcfee5.camel@infradead.org> (raw)
In-Reply-To: <ZTFOCqMCuSiH8VEt@google.com>
[-- Attachment #1.1: Type: text/plain, Size: 1815 bytes --]
On Thu, 2023-10-19 at 08:40 -0700, Sean Christopherson wrote:
>
> > Normally, it should be up to the hypervisor to tell the guest which
> > clock to use, i.e. if TSC is reliable or not. Let me put my question
> > this way: if TSC on the particular host is good for everything, why
> > does the hypervisor advertises 'kvmclock' to its guests?
>
> I suspect there are two reasons.
>
> 1. As is likely the case in our fleet, no one revisited the set of advertised
> PV features when defining the VM shapes for a new generation of hardware, or
> whoever did the reviews wasn't aware that advertising kvmclock is actually
> suboptimal. All the PV clock stuff in KVM is quite labyrinthian, so it's
> not hard to imagine it getting overlooked.
>
> 2. Legacy VMs. If VMs have been running with a PV clock for years, forcing
> them to switch to a new clocksource is high-risk, low-reward.
Doubly true for Xen guests (given that the Xen clocksource is identical
to the KVM clocksource).
> > If for some 'historical reasons' we can't revoke features we can always
> > introduce a new PV feature bit saying that TSC is preferred.
Don't we already have one? It's the PVCLOCK_TSC_STABLE_BIT. Why would a
guest ever use kvmclock if the PVCLOCK_TSC_STABLE_BIT is set?
The *point* in the kvmclock is that the hypervisor can mess with the
epoch/scaling to try to compensate for TSC brokenness as the host
scales/sleeps/etc.
And the *problem* with the kvmclock is that it does just that, even
when the host TSC hasn't done anything wrong and the kvmclock shouldn't
have changed at all.
If the PVCLOCK_TSC_STABLE_BIT is set, a guest should just use the guest
TSC directly without looking to the kvmclock for adjusting it.
No?
[-- Attachment #1.2: smime.p7s --]
[-- Type: application/pkcs7-signature, Size: 5965 bytes --]
[-- Attachment #2: Type: text/plain, Size: 183 bytes --]
_______________________________________________
Virtualization mailing list
Virtualization@lists.linux-foundation.org
https://lists.linuxfoundation.org/mailman/listinfo/virtualization
next prev parent reply other threads:[~2023-10-19 15:47 UTC|newest]
Thread overview: 4+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-10-18 22:11 [PATCH RFC 1/1] x86/paravirt: introduce param to disable pv sched_clock Dongli Zhang
2023-10-19 11:55 ` Vitaly Kuznetsov
[not found] ` <ZTFOCqMCuSiH8VEt@google.com>
2023-10-19 15:47 ` David Woodhouse [this message]
2023-10-19 23:11 ` Dongli Zhang
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=2f1459d7c3e3e81cdca931e104c3ade71dfcfee5.camel@infradead.org \
--to=dwmw2@infradead.org \
--cc=akaher@vmware.com \
--cc=amakhalov@vmware.com \
--cc=boris.ostrovsky@oracle.com \
--cc=bp@alien8.de \
--cc=dave.hansen@linux.intel.com \
--cc=hpa@zytor.com \
--cc=jgross@suse.com \
--cc=joe.jin@oracle.com \
--cc=kvm@vger.kernel.org \
--cc=linux-hyperv@vger.kernel.org \
--cc=linux-kernel@vger.kernel.org \
--cc=mingo@redhat.com \
--cc=pbonzini@redhat.com \
--cc=peterz@infradead.org \
--cc=pv-drivers@vmware.com \
--cc=seanjc@google.com \
--cc=tglx@linutronix.de \
--cc=virtualization@lists.linux-foundation.org \
--cc=vkuznets@redhat.com \
--cc=wanpengli@tencent.com \
--cc=x86@kernel.org \
--cc=xen-devel@lists.xenproject.org \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).