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Subject: Re: [PATCH v6 5/5] x86/kvm: Avoid dynamic allocation of pvclock
 data when SEV is active
From:   Sean Christopherson <sean.j.christopherson@intel.com>
To:     Borislav Petkov <bp@suse.de>, Brijesh Singh <brijesh.singh@amd.com>
Cc:     x86@kernel.org, linux-kernel@vger.kernel.org, kvm@vger.kernel.org,
        Tom Lendacky <thomas.lendacky@amd.com>,
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Date:   Mon, 10 Sep 2018 09:13:23 -0700
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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...