linux-kernel - Re: [PATCH v2 05/21] KVM: nVMX: Prioritize TSS T-flag #DBs over Monitor Trap Flag

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Date:   Thu, 7 Jul 2022 17:14:40 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Jim Mattson <jmattson@...gle.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, Oliver Upton <oupton@...gle.com>,
        Peter Shier <pshier@...gle.com>
Subject: Re: [PATCH v2 05/21] KVM: nVMX: Prioritize TSS T-flag #DBs over
 Monitor Trap Flag

On Wed, Jul 06, 2022, Jim Mattson wrote:
> On Tue, Jun 14, 2022 at 1:47 PM Sean Christopherson <seanjc@...gle.com> wrote:
> >
> > Service TSS T-flag #DBs prior to pending MTFs, as such #DBs are higher
> > priority than MTF.  KVM itself doesn't emulate TSS #DBs, and any such
> 
> Is there a KVM erratum for that?

Nope, just this hilarious TODO:

	/*
	 * TODO: What about debug traps on tss switch?
	 *       Are we supposed to inject them and update dr6?
	 */

> > exceptions injected from L1 will be handled by hardware (or morphed to
> > a fault-like exception if injection fails), but theoretically userspace
> > could pend a TSS T-flag #DB in conjunction with a pending MTF.
> >
> > Note, there's no known use case this fixes, it's purely to be technically
> > correct with respect to Intel's SDM.
> 
> A test would be nice. :-)

LOL, yeah, but ensuring userspace-injected TSS T-bit #DBs work isn't exactly on
my list of top 100 things to look at.