linux-kernel - Re: [PATCH 2/2] KVM: x86: Adjust return value of pic_poll

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Message-ID: <ZD605NFbjiCBX9jW@google.com>
Date:   Tue, 18 Apr 2023 08:19:00 -0700
From:   Sean Christopherson <seanjc@...gle.com>
To:     alexjlzheng@...il.com
Cc:     pbonzini@...hat.com, tglx@...utronix.de, mingo@...hat.com,
        bp@...en8.de, dave.hansen@...ux.intel.com, x86@...nel.org,
        hpa@...or.com, kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        Jinliang Zheng <alexjlzheng@...cent.com>
Subject: Re: [PATCH 2/2] KVM: x86: Adjust return value of pic_poll_read()

On Tue, Apr 18, 2023, alexjlzheng@...il.com wrote:
> From: Jinliang Zheng <alexjlzheng@...cent.com>
> 
> Returning 0x07 raises ambiguity when no interrupt is in pic_poll_read().
> Although it will not cause a functional exception (Bit 7 is 0 means no

>From KVM's perspective, it's a functional change.  It _shouldn't_ impact the
overall functionality of the guest, but we have no idea what guest code exists
in the wild.

> interrupt), it will easily make developers mistakenly think that a
> spurious interrupt (IRQ 7) has been returned.
> 
> Return 0x00 instread of 0x07.

Again, I do not want to introduce a functional change in this code without evidence
that the change fixes something for a real world guest.  Based on your response[*],
that is not the case.

A comment explaining the KVM behavior would be very welcome, but I'm not taking
this patch.

[*] https://lore.kernel.org/all/20230418075923.752113-1-alexjlzheng@tencent.com