All of lore.kernel.org
 help / color / mirror / Atom feed
From: Maxim Levitsky <mlevitsk@redhat.com>
To: Marcelo Tosatti <mtosatti@redhat.com>,
	Thomas Gleixner <tglx@linutronix.de>
Cc: kvm@vger.kernel.org, Paolo Bonzini <pbonzini@redhat.com>,
	Oliver Upton <oupton@google.com>, Ingo Molnar <mingo@redhat.com>,
	Sean Christopherson <sean.j.christopherson@intel.com>,
	open list <linux-kernel@vger.kernel.org>,
	Jonathan Corbet <corbet@lwn.net>,
	Wanpeng Li <wanpengli@tencent.com>,
	Borislav Petkov <bp@alien8.de>, Jim Mattson <jmattson@google.com>,
	"H. Peter Anvin" <hpa@zytor.com>,
	"open list:DOCUMENTATION" <linux-doc@vger.kernel.org>,
	Joerg Roedel <joro@8bytes.org>,
	"maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)"
	<x86@kernel.org>, Vitaly Kuznetsov <vkuznets@redhat.com>
Subject: Re: [PATCH 0/2] RFC: Precise TSC migration
Date: Thu, 03 Dec 2020 13:51:18 +0200	[thread overview]
Message-ID: <77e48fe09de55fa77a9e33a2c6212e42c83556be.camel@redhat.com> (raw)
In-Reply-To: <20201201150205.GA42117@fuller.cnet>

On Tue, 2020-12-01 at 12:02 -0300, Marcelo Tosatti wrote:
> On Tue, Dec 01, 2020 at 02:48:11PM +0100, Thomas Gleixner wrote:
> > On Mon, Nov 30 2020 at 16:16, Marcelo Tosatti wrote:
> > > > Besides, Linux guests don't sync the TSC via IA32_TSC write,
> > > > but rather use IA32_TSC_ADJUST which currently doesn't participate
> > > > in the tsc sync heruistics.
> > > 
> > > Linux should not try to sync the TSC with IA32_TSC_ADJUST. It expects
> > > the BIOS to boot with synced TSCs.
> > 
> > That's wishful thinking.
> > 
> > Reality is that BIOS tinkerers fail to get it right. TSC_ADJUST allows
> > us to undo the wreckage they create.
> > 
> > Thanks,
> > 
> >         tglx
> 
> Have not seen any multicore Dell/HP systems require that.
> 
> Anyway, for QEMU/KVM it should be synced (unless there is a bug
> in the sync logic in the first place).
> 

I agree with that, and that is why I suggested to make the guest
avoid TSC syncing when KVM is detected.
 
I don't mind how to implement this.
 
It can be either done with new CPUID bit, 
or always when KVM
is detected, 
(or even when *any* hypervisor is detected)
 
I also don't mind if we only disable tsc sync logic or
set X86_FEATURE_TSC_RELIABLE which will disable it
and the clocksource watchdog.


Best regards,
	Maxim Levitsky




  reply	other threads:[~2020-12-03 11:53 UTC|newest]

Thread overview: 32+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-11-30 13:35 [PATCH 0/2] RFC: Precise TSC migration Maxim Levitsky
2020-11-30 13:35 ` [PATCH 1/2] KVM: x86: implement KVM_SET_TSC_PRECISE/KVM_GET_TSC_PRECISE Maxim Levitsky
2020-11-30 14:33   ` Paolo Bonzini
2020-11-30 15:58     ` Maxim Levitsky
2020-11-30 17:01       ` Paolo Bonzini
2020-12-01 19:43   ` Thomas Gleixner
2020-12-03 11:11     ` Maxim Levitsky
2020-11-30 13:35 ` [PATCH 2/2] KVM: x86: introduce KVM_X86_QUIRK_TSC_HOST_ACCESS Maxim Levitsky
2020-11-30 13:54   ` Paolo Bonzini
2020-11-30 14:11     ` Maxim Levitsky
2020-11-30 14:15       ` Paolo Bonzini
2020-11-30 15:33         ` Maxim Levitsky
2020-11-30 13:38 ` [PATCH 0/2] RFC: Precise TSC migration (summary) Maxim Levitsky
2020-11-30 16:54 ` [PATCH 0/2] RFC: Precise TSC migration Andy Lutomirski
2020-11-30 16:59   ` Paolo Bonzini
2020-11-30 19:16 ` Marcelo Tosatti
2020-12-01 12:30   ` Maxim Levitsky
2020-12-01 19:48     ` Marcelo Tosatti
2020-12-03 11:39       ` Maxim Levitsky
2020-12-03 20:18         ` Marcelo Tosatti
2020-12-07 13:00           ` Maxim Levitsky
2020-12-01 13:48   ` Thomas Gleixner
2020-12-01 15:02     ` Marcelo Tosatti
2020-12-03 11:51       ` Maxim Levitsky [this message]
2020-12-01 14:01   ` Thomas Gleixner
2020-12-01 16:19     ` Andy Lutomirski
2020-12-03 11:57       ` Maxim Levitsky
2020-12-01 19:35 ` Thomas Gleixner
2020-12-03 11:41   ` Paolo Bonzini
2020-12-03 12:47   ` Maxim Levitsky
2020-11-30 13:38 Maxim Levitsky
2020-11-30 13:38 ` Maxim Levitsky

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=77e48fe09de55fa77a9e33a2c6212e42c83556be.camel@redhat.com \
    --to=mlevitsk@redhat.com \
    --cc=bp@alien8.de \
    --cc=corbet@lwn.net \
    --cc=hpa@zytor.com \
    --cc=jmattson@google.com \
    --cc=joro@8bytes.org \
    --cc=kvm@vger.kernel.org \
    --cc=linux-doc@vger.kernel.org \
    --cc=linux-kernel@vger.kernel.org \
    --cc=mingo@redhat.com \
    --cc=mtosatti@redhat.com \
    --cc=oupton@google.com \
    --cc=pbonzini@redhat.com \
    --cc=sean.j.christopherson@intel.com \
    --cc=tglx@linutronix.de \
    --cc=vkuznets@redhat.com \
    --cc=wanpengli@tencent.com \
    --cc=x86@kernel.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 an external index of several public inboxes,
see mirroring instructions on how to clone and mirror
all data and code used by this external index.