linux-kernel - Re: [PATCH 09/13] KVM: nVMX: Prioritize SMI over nested IRQ/NMI

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Date:   Wed, 29 Apr 2020 13:06:29 -0700
From:   Sean Christopherson <sean.j.christopherson@...el.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 list <kvm@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Oliver Upton <oupton@...gle.com>,
        Peter Shier <pshier@...gle.com>
Subject: Re: [PATCH 09/13] KVM: nVMX: Prioritize SMI over nested IRQ/NMI

On Wed, Apr 29, 2020 at 07:50:40AM -0700, Sean Christopherson wrote:
> On Tue, Apr 28, 2020 at 04:16:16PM -0700, Jim Mattson wrote:
> > On Tue, Apr 28, 2020 at 3:59 PM Sean Christopherson
> > <sean.j.christopherson@...el.com> wrote:
> > >
> > > On Tue, Apr 28, 2020 at 03:04:02PM -0700, Jim Mattson wrote:
> > > > From the SDM, volume 3:
> > > >
> > > > • System-management interrupts (SMIs), INIT signals, and higher
> > > > priority events take priority over MTF VM exits.
> > > >
> > > > I think this block needs to be moved up.
> > >
> > > Hrm.  It definitely needs to be moved above the preemption timer, though I
> > > can't find any public documentation about the preemption timer's priority.
> > > Preemption timer is lower priority than MTF, ergo it's not in the same
> > > class as SMI.
> > >
> > > Regarding SMI vs. MTF and #DB trap, to actually prioritize SMIs above MTF
> > > and #DBs, we'd need to save/restore MTF and pending #DBs via SMRAM.  I
> > > think it makes sense to take the easy road and keep SMI after the traps,
> > > with a comment to say it's technically wrong but not worth fixing.
> > 
> > Pending debug exceptions should just go in the pending debug
> > exceptions field. End of story and end of complications. I don't
> > understand why kvm is so averse to using this field the way it was
> > intended.
> 
> Ah, it took my brain a bit to catch on.  I assume you're suggesting calling
> nested_vmx_updated_pending_dbg() so that the pending #DB naturally gets
> propagated to/from vmcs12 on SMI/RSM?  I think that should work.

This works for L2, but not L1 :-(  And L2 can't be fixed without first
fixing L1 because inject_pending_event() also incorrectly prioritizes #DB
over SMI.  For L1, utilizing SMRAM to save/restore the pending #DB is
likely the easiest solution as it avoids having to add new state for
migration.

I have everything coded up but it'll probably take a few weeks to test and
get it sent out, need to focus on other stuff for a while.