linux-kernel - Re: [PATCH v2] KVM: x86: Fix poll command

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Message-ID: <ZDmSWIEOTYo3qHf7@google.com>
Date:   Fri, 14 Apr 2023 17:50:16 +0000
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 v2] KVM: x86: Fix poll command

On Thu, Apr 13, 2023, alexjlzheng@...il.com wrote:
> From: Jinliang Zheng <alexjlzheng@...cent.com>
> 
> According to the hardware manual, when the Poll command is issued, the

Please add "8259", i.e. "According to the 8259 hardware manual".

> byte returned by the I/O read is 1 in Bit 7 when there is an interrupt,
> and the highest priority binary code in Bits 2:0. The current pic
> simulation code is not implemented strictly according to the above
> expression.
> 
> Fix the implementation of pic_poll_read():
> 1. Set Bit 7 when there is an interrupt
> 2. Return 0 when there is no interrupt

I don't think #2 is justified.  The spec says:

  The interrupt requests are ordered in priority from 0 through 7 (0 highest).

I.e. the current code enumerates the _lowest_ priority when there is no interrupt,
which seems more correct than reporting the highest priority possible.