linux-kernel - Re: [PATCH v7 6/6] x86/split_lock: Fix the split lock #AC handling when running as guest

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Date:   Thu, 14 Oct 2021 15:04:25 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Xiaoyao Li <xiaoyao.li@...el.com>
Cc:     Sathyanarayanan Kuppuswamy 
        <sathyanarayanan.kuppuwamy@...ux.intel.com>,
        Kuppuswamy Sathyanarayanan 
        <sathyanarayanan.kuppuswamy@...ux.intel.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        x86@...nel.org, Paolo Bonzini <pbonzini@...hat.com>,
        David Hildenbrand <david@...hat.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        "H . Peter Anvin" <hpa@...or.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Tony Luck <tony.luck@...el.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Andi Kleen <ak@...ux.intel.com>,
        Kirill Shutemov <kirill.shutemov@...ux.intel.com>,
        Kuppuswamy Sathyanarayanan <knsathya@...nel.org>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v7 6/6] x86/split_lock: Fix the split lock #AC handling
 when running as guest

On Thu, Oct 14, 2021, Xiaoyao Li wrote:
> On 10/14/2021 5:32 AM, Sathyanarayanan Kuppuswamy wrote:
> > + Xiaoyao
> > 
> > On 10/13/21 1:30 PM, Sean Christopherson wrote:
> > > On Tue, Oct 05, 2021, Kuppuswamy Sathyanarayanan wrote:
> > > > From: Xiaoyao Li <xiaoyao.li@...el.com>
> > > > 
> > > > If running as guest and hypervisor enables
> > > > MSR_TEST_CTRL.SPLIT_LOCK_DETECT during its running, it can get split
> > > > lock #AC even though sld_state is sld_off.
> > > That's a hypervisor bug, no?  The hypervisor should never inject a fault
> > > that the guest cannot reasonably expect.
> 
> What if hypervisor doesn't intercept #AC and host enables SPLIT_LOCK_DETECT
> during guest running? That's exactly the case TDX is facing.

That's a hypervisor bug.  Since it sounds like the TDX Module buries its head in
the sand for split-lock #AC, KVM should refuse to run TDX guests if split-lock #AC
is enabled.  Ideally the TDX Module would provide support for conditionally
intercepting #AC, e.g. intercept and re-inject "normal" #AC, and exit to the VMM
for split-lock #AC.  That would give VMMs the option of enabling split-lock
detection in fatal mode for guests.