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

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Date:   Wed, 29 Apr 2020 07:50:40 -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 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.

> As for the MTF, section 34.14.1 of the SDM, volume 3, clearly states:
> 
> The pseudocode above makes reference to the saving of VMX-critical
> state. This state consists of the following:
> (1) SS.DPL (the current privilege level); (2) RFLAGS.VM; (3) the state
> of blocking by STI and by MOV SS (see
> Table 24-3 in Section 24.4.2); (4) the state of virtual-NMI blocking
> (only if the processor is in VMX non-root oper-
> ation and the “virtual NMIs” VM-execution control is 1); and (5) an
> indication of whether an MTF VM exit is pending
> (see Section 25.5.2). These data may be saved internal to the
> processor or in the VMCS region of the current
> VMCS. Processors that do not support SMI recognition while there is
> blocking by STI or by MOV SS need not save
> the state of such blocking.
> 
> I haven't really looked at kvm's implementation of SMM (because Google
> doesn't support it), but it seems that the "MTF VM exit is pending"
> bit should be trivial to deal with. I assume we save the other
> VMX-critical state somewhere!

True, I spaced on the extistence of vmx_pre_{enter,leave}_smm().

I'll send a patch, the delta to what's in kvm/queue should actually be
quite small.