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 16:16:03 +0000
From:   Marc Zyngier <maz@...nel.org>
To:     Auger Eric <eric.auger@...hat.com>
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 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.

Thanks,

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