linux-kernel - Re: [PATCH] KVM: x86: revert the order of calls in kvm_fast

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Message-ID: <16a6ec0afd8.10d96b0bf79755.8849251462286427289@zoho.com>
Date:   Tue, 30 Apr 2019 22:58:02 +0800
From:   Perr Zhang <strongbox8@...o.com>
To:     "Peter Zijlstra" <peterz@...radead.org>
Cc:     "pbonzini" <pbonzini@...hat.com>, "rkrcmar" <rkrcmar@...hat.com>,
        "tglx" <tglx@...utronix.de>, "stable" <stable@...r.kernel.org>,
        "mingo" <mingo@...hat.com>, "x86" <x86@...nel.org>,
        "kvm" <kvm@...r.kernel.org>,
        "linux-kernel" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] KVM: x86: revert the order of calls in kvm_fast_pio()


 ---- On Tue, 30 Apr 2019 22:32:01 +0800 Peter Zijlstra <peterz@...radead.org> wrote ----
 > On Tue, Apr 30, 2019 at 10:24:23PM +0800, Perr Zhang wrote:
 > > In commit 45def77ebf79, the order of function calls in kvm_fast_pio()
 > > was changed. This causes that the vm(XP,and also XP's iso img) failed
 > > to boot. This doesn't happen with win10 or ubuntu.
 > > 
 > > After revert the order, the vm(XP) succeedes to boot. In addition, the
 > > change of calls's order of kvm_fast_pio() in commit 45def77ebf79 has no
 > > obvious reason.
 > 
 > This Changelog fails to explain why the order is important and equally
 > fails to inform the future reader of that code. So this very same thing
 > will happen again in 6 months time or thereabout.

 I'm not familiar with KVM,  don't know the particular reason.

 > 
 > > Fixes: 45def77ebf79 ("KVM: x86: update %rip after emulating IO")
 > > Cc: <stable@...r.kernel.org>
 > > Signed-off-by: Perr Zhang <strongbox8@...o.com>
 > > ---
 > >  arch/x86/kvm/x86.c | 7 +++----
 > >  1 file changed, 3 insertions(+), 4 deletions(-)
 > > 
 > > diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
 > > index a0d1fc80ac5a..248753cb94a1 100644
 > > --- a/arch/x86/kvm/x86.c
 > > +++ b/arch/x86/kvm/x86.c
 > > @@ -6610,13 +6610,12 @@ static int kvm_fast_pio_in(struct kvm_vcpu *vcpu, int size,
 > >  
 > >  int kvm_fast_pio(struct kvm_vcpu *vcpu, int size, unsigned short port, int in)
 > >  {
 > > -    int ret;
 > > +    int ret = kvm_skip_emulated_instruction(vcpu);
 > >  
 > >      if (in)
 > > -        ret = kvm_fast_pio_in(vcpu, size, port);
 > > +        return kvm_fast_pio_in(vcpu, size, port) && ret;
 > >      else
 > > -        ret = kvm_fast_pio_out(vcpu, size, port);
 > > -    return ret && kvm_skip_emulated_instruction(vcpu);
 > > +        return kvm_fast_pio_out(vcpu, size, port) && ret;
 > >  }
 > >  EXPORT_SYMBOL_GPL(kvm_fast_pio);
 > >  
 > > -- 
 > > 2.21.0
 > > 
 > > 
 > > 
 >