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

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Date:   Tue, 30 Apr 2019 16:32:01 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Perr Zhang <strongbox8@...o.com>
Cc:     pbonzini@...hat.com, rkrcmar@...hat.com, tglx@...utronix.de,
        stable@...r.kernel.org, mingo@...hat.com, x86@...nel.org,
        kvm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] KVM: x86: revert the order of calls in kvm_fast_pio()

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.

> 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
> 
> 
>