linux-kernel - Re: [PATCH v5 23/23] KVM: arm64: GICv4.1: Expose HW-based SGIs in debugfs

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Date:   Thu, 19 Mar 2020 17:17:48 +0100
From:   Auger Eric <eric.auger@...hat.com>
To:     Marc Zyngier <maz@...nel.org>
Cc:     linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.cs.columbia.edu,
        kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        Lorenzo Pieralisi <lorenzo.pieralisi@....com>,
        Jason Cooper <jason@...edaemon.net>,
        Robert Richter <rrichter@...vell.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Zenghui Yu <yuzenghui@...wei.com>,
        James Morse <james.morse@....com>,
        Julien Thierry <julien.thierry.kdev@...il.com>,
        Suzuki K Poulose <suzuki.poulose@....com>
Subject: Re: [PATCH v5 23/23] KVM: arm64: GICv4.1: Expose HW-based SGIs in
 debugfs

Hi,

On 3/19/20 5:16 PM, Marc Zyngier wrote:
> Hi Eric,
> 
> On 2020-03-19 15:43, Auger Eric wrote:
>> Hi Marc,
>>
>> On 3/19/20 4:21 PM, Marc Zyngier wrote:
>>> Hi Eric,
> 
> [...]
> 
>>>> The patch looks good to me but I am now lost about how we retrieve the
>>>> pending stat of other hw mapped interrupts. Looks we use
>>>> irq->pending_latch always. Is that correct?
>>>
>>> Correct. GICv4.0 doesn't give us an architectural way to look at the
>>> vLPI pending state (there isn't even a guarantee about when the GIC
>>> will stop writing to memory, if it ever does).
>>>
>>> With GICv4.1, you can introspect the HW state for SGIs. You can also
>>> look at the vLPI state by peeking at the virtual pending table, but
>>> you'd need to unmap the VPE first, which I obviously don't want to do
>>> for this debug interface, specially as it can be used whilst the guest
>>> is up and running.
>> OK for vLPIs, what about other HW mapped IRQs (arch timer?)
> 
> Different kind of HW. With those, the injection is still virtual, so the
> SW pending bit is still very much valid. You can actually try and make
> the timer interrupt pending, it should show up.
> 
> What the irq->hw bit means is "this virtual interrupt is somehow related
> to the host_irq". How this is interpreted is completely context-dependent.
OK thank you for refreshing my memories ;-)

Eric
> 
> Thanks,
> 
>         M.