linux-kernel - Re: [PATCH v9 09/17] x86/split_lock: Handle #AC exception for split lock

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Date:   Wed, 16 Oct 2019 16:50:36 +0200 (CEST)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Xiaoyao Li <xiaoyao.li@...el.com>
cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        Fenghua Yu <fenghua.yu@...el.com>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        H Peter Anvin <hpa@...or.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Dave Hansen <dave.hansen@...el.com>,
        Radim Krcmar <rkrcmar@...hat.com>,
        Ashok Raj <ashok.raj@...el.com>,
        Tony Luck <tony.luck@...el.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Sai Praneeth Prakhya <sai.praneeth.prakhya@...el.com>,
        Ravi V Shankar <ravi.v.shankar@...el.com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        x86 <x86@...nel.org>, kvm@...r.kernel.org
Subject: Re: [PATCH v9 09/17] x86/split_lock: Handle #AC exception for split
 lock

On Wed, 16 Oct 2019, Xiaoyao Li wrote:
> On 10/16/2019 7:58 PM, Paolo Bonzini wrote:
> > > With your proposal you render #AC useless even on hosts which have SMT
> > > disabled, which is just wrong. There are enough good reasons to disable
> > > SMT.
> > 
> > My lazy "solution" only applies to SMT enabled.  When SMT is either not
> > supported, or disabled as in "nosmt=force", we can virtualize it like
> > the posted patches have done so far.
> > 
> 
> Do we really need to divide it into two cases of SMT enabled and SMT disabled?

Yes. See the matrix I just sent.

> > > I agree that with SMT enabled the situation is truly bad, but we surely
> > > can
> > > be smarter than just disabling it globally unconditionally and forever.
> > > 
> > > Plus we want a knob which treats guests triggering #AC in the same way as
> > > we treat user space, i.e. kill them with SIGBUS.
> > 
> > Yes, that's a valid alternative.  But if SMT is possible, I think the
> > only sane possibilities are global disable and SIGBUS.  SIGBUS (or
> > better, a new KVM_RUN exit code) can be acceptable for debugging guests too.
> 
> If SIGBUS, why need to globally disable?

See the matrix I just sent.

> When there is an #AC due to split-lock in guest, KVM only has below two
> choices:
> 1) inject back into guest.
>    - If kvm advertise this feature to guest, and guest kernel is latest, and
> guest kernel must enable it too. It's the happy case that guest can handler it
> on its own purpose.
>    - Any other cases, guest get an unexpected #AC and crash.

That's just wrong for obvious reasons.

> 2) report to userspace (I think the same like a SIGBUS)

No. What guarantees that userspace qemu handles the SIGBUS sanely?

> So for simplicity, we can do what Paolo suggested that don't advertise this
> feature and report #AC to userspace when an #AC due to split-lock in guest
> *but* we never disable the host's split-lock detection due to guest's
> split-lock.

No, you can't.

Guess what happens when you just boot some existing guest on a #AC enabled
host without having updated qemu to handle the exit code/SIGBUS.

It simply will crash and burn in nonsensical ways. Same as reinjecting it
into the guest and letting it crash.

Thanks,

	tglx