linux-kernel - Re: [RFC v2 10/10] KVM: arm/arm64: Emulate the EL1 phys timer register access

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Message-ID: <ba9de0f8-4b35-2bb8-cb4b-cc97434f5183@arm.com>
Date:   Mon, 30 Jan 2017 17:35:28 +0000
From:   Marc Zyngier <marc.zyngier@....com>
To:     Peter Maydell <peter.maydell@...aro.org>,
        Jintack Lim <jintack@...columbia.edu>
Cc:     KVM General <kvm@...r.kernel.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will.deacon@....com>, linux@...linux.org.uk,
        lkml - Kernel Mailing List <linux-kernel@...r.kernel.org>,
        arm-mail-list <linux-arm-kernel@...ts.infradead.org>,
        Andre Przywara <andre.przywara@....com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        "kvmarm@...ts.cs.columbia.edu" <kvmarm@...ts.cs.columbia.edu>
Subject: Re: [RFC v2 10/10] KVM: arm/arm64: Emulate the EL1 phys timer
 register access

On 30/01/17 17:26, Peter Maydell wrote:
> On 30 January 2017 at 17:08, Jintack Lim <jintack@...columbia.edu> wrote:
>> On Sun, Jan 29, 2017 at 10:44 AM, Marc Zyngier <marc.zyngier@....com> wrote:
>>> Shouldn't we take the ENABLE bit into account? The ARMv8 ARM version I
>>> have at hand (version h) seems to indicate that we should, but we should
>>> check with the latest and greatest...
>>
>> Thanks! I was not clear about this. I have ARM ARM version k, and it
>> says that 'When the value of the ENABLE bit is 0, the ISTATUS field is
>> UNKNOWN.' So I thought the istatus value doesn't matter if ENABLE is
>> 0, and just set istatus bit regardless of ENABLE bit. If this is not
>> what the manual meant, then I'm happy to fix this.
> 
> It looks like the spec has been relaxed between the doc version
> that Marc was looking at and the current one. So it's OK for
> an implementation to either (a) set ISTATUS to 0 if ENABLE
> is 0, or (b) do what you've done and set ISTATUS according
> to the timer comparison whether ENABLE is clear or not
> (or even (c) set ISTATUS to a random value if ENABLE is clear,
> and other less likely choices).
> I think we should add a comment to note that it's architecturally
> UNKNOWN and we've made a choice for our implementation convenience.

In that case, the proposed implementation is perfectly fine. I'll retire
the old ARMv8 ARM from my laptop (funnily enough, I didn't fancy
downloading version k while on the train and having my phone as my link
to the outside world... ;-).

Thanks,

	M.
-- 
Jazz is not dead. It just smells funny...