From mboxrd@z Thu Jan  1 00:00:00 1970
From: Sean Christopherson <sean.j.christopherson@intel.com>
Subject: Re: [PATCH v6 5/5] x86/kvm: Avoid dynamic allocation of pvclock
 data when SEV is active
Date: Mon, 10 Sep 2018 09:13:23 -0700
Message-ID: <1536596003.11460.82.camel@intel.com>
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         <20180910122727.GE21815@zn.tnic>
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         <20180910155329.GB20286@zn.tnic>
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Cc: x86@kernel.org, linux-kernel@vger.kernel.org, kvm@vger.kernel.org,
        Tom Lendacky <thomas.lendacky@amd.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        "H. Peter Anvin" <hpa@zytor.com>,
        Paolo Bonzini <pbonzini@redhat.com>,
        Radim =?UTF-8?Q?Kr=C4=8Dm=C3=A1=C5=99?= <rkrcmar@redhat.com>
To: Borislav Petkov <bp@suse.de>, Brijesh Singh <brijesh.singh@amd.com>
Return-path: <linux-kernel-owner@vger.kernel.org>
In-Reply-To: <20180910155329.GB20286@zn.tnic>
Sender: linux-kernel-owner@vger.kernel.org
List-Id: kvm.vger.kernel.org

On Mon, 2018-09-10 at 17:53 +0200, Borislav Petkov wrote:
> On Mon, Sep 10, 2018 at 08:15:38AM -0500, Brijesh Singh wrote:
> > 
> > > 
> > > Now, the real question from all this SNAFU is, why can't all those point
> > > to a single struct pvclock_vsyscall_time_info and all CPUs read a single
> > > thing? Why do they have to be per-CPU and thus waste so much memory?
> You forgot to answer to the real question - why do we need those things
> to be perCPU and why can't we use a single instance to share with *all*
> CPUs?

I can't speak to the actual TSC stuff, but...

The pvclock ABI includes a per-vCPU bit, PVCLOCK_GUEST_STOPPED, to indicate
that the VM has been paused by the host.  The guest uses this information to
update its watchdogs to avoid false positives.  I have no idea if there are
use cases for setting STOPPED on a subset of vCPUs, but the ABI allows it so
here we are...