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Date:   Wed, 16 Oct 2019 16:43:26 +0200 (CEST)
From:   Thomas Gleixner <tglx@linutronix.de>
To:     Xiaoyao Li <xiaoyao.li@intel.com>
cc:     Paolo Bonzini <pbonzini@redhat.com>,
        Sean Christopherson <sean.j.christopherson@intel.com>,
        Fenghua Yu <fenghua.yu@intel.com>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        H Peter Anvin <hpa@zytor.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Dave Hansen <dave.hansen@intel.com>,
        Radim Krcmar <rkrcmar@redhat.com>,
        Ashok Raj <ashok.raj@intel.com>,
        Tony Luck <tony.luck@intel.com>,
        Dan Williams <dan.j.williams@intel.com>,
        Sai Praneeth Prakhya <sai.praneeth.prakhya@intel.com>,
        Ravi V Shankar <ravi.v.shankar@intel.com>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        x86 <x86@kernel.org>, kvm@vger.kernel.org
Subject: Re: [PATCH v9 09/17] x86/split_lock: Handle #AC exception for split
 lock
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On Wed, 16 Oct 2019, Xiaoyao Li wrote:
> On 10/16/2019 7:26 PM, Paolo Bonzini wrote:
> > Old guests are prevalent enough that enabling split-lock detection by
> > default would be a big usability issue.  And even ignoring that, you
> > would get the issue you describe below:
> 
> Right, whether enabling split-lock detection is made by the administrator. The
> administrator is supposed to know the consequence of enabling it. Enabling it
> means don't want any split-lock happens in userspace, of course VMM softwares
> are under control.

I have no idea what you are talking about, but the whole thing is trivial
enough to describe in a decision matrix:

N | #AC       | #AC enabled | SMT | Ctrl    | Guest | Action
R | available | on host     |     | exposed | #AC   |
--|-----------|-------------|-----|---------|-------|---------------------
  |           |             |     |         |       |
0 | N         |     x       |  x  |   N     |   x   | None
  |           |             |     |         |       |
1 | Y         |     N       |  x  |   N     |   x   | None
  |           |             |     |         |       |
2 | Y         |     Y       |  x  |   Y     |   Y   | Forward to guest
  |           |             |     |         |       |
3 | Y         |     Y       |  N  |   Y     |   N   | A) Store in vCPU and
  |           |             |     |         |       |    toggle on VMENTER/EXIT
  |           |             |     |         |       |
  |           |             |     |         |       | B) SIGBUS or KVM exit code
  |           |             |     |         |       |
4 | Y         |     Y       |  Y  |   Y     |   N   | A) Disable globally on
  |           |             |     |         |       |    host. Store in vCPU/guest
  |           |             |     |         |       |    state and evtl. reenable
  |           |             |     |         |       |    when guest goes away.
  |           |             |     |         |       | 
  |           |             |     |         |       | B) SIGBUS or KVM exit code

  [234] need proper accounting and tracepoints in KVM

  [34]  need a policy decision in KVM

Now there are a two possible state transitions:

 #AC enabled on host during runtime

   Existing guests are not notified. Nothing changes.


 #AC disabled on host during runtime

   That only affects state #2 from the above table and there are two
   possible solutions:

     1) Do nothing.

     2) Issue a notification to the guest. This would be doable at least
     	for Linux guests because any guest kernel which handles #AC is
	at least the same generation as the host which added #AC.

   	Whether it's worth it, I don't know, but it makes sense at least
	for consistency reasons.

     For a first step I'd go for 'Do nothing'

SMT state transitions could be handled in a similar way, but I don't think
it's worth the trouble. The above should cover everything at least on a
best effort basis.

Thanks,

	tglx