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Date:   Mon, 26 Aug 2019 07:06:51 -0700
From:   Sean Christopherson <sean.j.christopherson@intel.com>
To:     Andy Lutomirski <luto@kernel.org>
Cc:     Paolo Bonzini <pbonzini@redhat.com>,
        Radim =?utf-8?B?S3LEjW3DocWZ?= <rkrcmar@redhat.com>,
        Vitaly Kuznetsov <vkuznets@redhat.com>,
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        Jim Mattson <jmattson@google.com>,
        Joerg Roedel <joro@8bytes.org>, kvm list <kvm@vger.kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Nadav Amit <nadav.amit@gmail.com>
Subject: Re: [PATCH] KVM: x86: Don't update RIP or do single-step on faulting
 emulation
Message-ID: <20190826140651.GA19381@linux.intel.com>
References: <20190823205544.24052-1-sean.j.christopherson@intel.com>
 <CALCETrXFRAmgqyxtsknpnQaMtVU-hqMRPYR=4Q5JtBgNGxuSGQ@mail.gmail.com>
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On Fri, Aug 23, 2019 at 03:46:20PM -0700, Andy Lutomirski wrote:
> On Fri, Aug 23, 2019 at 1:55 PM Sean Christopherson
> <sean.j.christopherson@intel.com> wrote:
> >
> > Don't advance RIP or inject a single-step #DB if emulation signals a
> > fault.  This logic applies to all state updates that are conditional on
> > clean retirement of the emulation instruction, e.g. updating RFLAGS was
> > previously handled by commit 38827dbd3fb85 ("KVM: x86: Do not update
> > EFLAGS on faulting emulation").
> >
> > Not advancing RIP is likely a nop, i.e. ctxt->eip isn't updated with
> > ctxt->_eip until emulation "retires" anyways.  Skipping #DB injection
> > fixes a bug reported by Andy Lutomirski where a #UD on SYSCALL due to
> > invalid state with RFLAGS.RF=1 would loop indefinitely due to emulation
> 
> EFLAGS.TF=1

It's always some mundane detail...