linux-kernel - Re: [PATCH v5 20/23] KVM: arm64: GICv4.1: Plumb SGI implementation selection in the distributor

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Date:   Fri, 20 Mar 2020 11:53:10 +0800
From:   Zenghui Yu <yuzenghui@...wei.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>,
        "Eric Auger" <eric.auger@...hat.com>,
        James Morse <james.morse@....com>,
        "Julien Thierry" <julien.thierry.kdev@...il.com>,
        Suzuki K Poulose <suzuki.poulose@....com>
Subject: Re: [PATCH v5 20/23] KVM: arm64: GICv4.1: Plumb SGI implementation
 selection in the distributor

Hi Marc,

On 2020/3/19 20:10, Marc Zyngier wrote:
>> But I wonder that should we use nassgireq to *only* keep track what
>> the guest had written into the GICD_CTLR.nASSGIreq.  If not, we may
>> lose the guest-request bit after migration among hosts with different
>> has_gicv4_1 settings.
> 
> I'm unsure of what you're suggesting here. If userspace tries to set
> GICD_CTLR.nASSGIreq on a non-4.1 host, this bit will not latch.

This is exactly what I *was* concerning about.

> Userspace can check that at restore time. Or we could fail the
> userspace write, which is a bit odd (the bit is otherwise RES0).
> 
> Could you clarify your proposal?

Let's assume two hosts below. 'has_gicv4_1' is true on host-A while
it is false on host-B because of lack of HW support or the kernel
parameter "kvm-arm.vgic_v4_enable=0".

If we migrate guest through A->B->A, we may end-up lose the initial
guest-request "nASSGIreq=1" and don't use direct vSGI delivery for
this guest when it's migrated back to host-A.

This can be "fixed" by keep track of what guest had written into
nASSGIreq. And we need to evaluate the need for using direct vSGI
for a specified guest by 'has_gicv4_1 && nassgireq'.

But if it's expected that "if userspace tries to set nASSGIreq on
a non-4.1 host, this bit will not latch", then this shouldn't be
a problem at all.

Anyway this is not a big deal to me and I won't complain about it
in the future ;-) Either way, for this patch:

Reviewed-by: Zenghui Yu <yuzenghui@...wei.com>

Thanks