linux-kernel - Re: [PATCH v4 16/20] KVM: arm64: GICv4.1: Allow SGIs to switch between HW and SW interrupts

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Message-ID: <7aa668a5920b8deb8c2ee2fec3ef69b3@kernel.org>
Date:   Fri, 28 Feb 2020 19:16:42 +0000
From:   Marc Zyngier <maz@...nel.org>
To:     Zenghui Yu <yuzenghui@...wei.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>,
        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 v4 16/20] KVM: arm64: GICv4.1: Allow SGIs to switch
 between HW and SW interrupts

Hi Zenghui,

On 2020-02-20 03:55, Zenghui Yu wrote:
> Hi Marc,
> 
> On 2020/2/14 22:57, Marc Zyngier wrote:
>> In order to let a guest buy in the new, active-less SGIs, we
>> need to be able to switch between the two modes.
>> 
>> Handle this by stopping all guest activity, transfer the state
>> from one mode to the other, and resume the guest.
>> 
>> Signed-off-by: Marc Zyngier <maz@...nel.org>
> 
> [...]
> 
>> diff --git a/virt/kvm/arm/vgic/vgic-v3.c b/virt/kvm/arm/vgic/vgic-v3.c
>> index 1bc09b523486..2c9fc13e2c59 100644
>> --- a/virt/kvm/arm/vgic/vgic-v3.c
>> +++ b/virt/kvm/arm/vgic/vgic-v3.c
>> @@ -540,6 +540,8 @@ int vgic_v3_map_resources(struct kvm *kvm)
>>   		goto out;
>>   	}
>>   +	if (kvm_vgic_global_state.has_gicv4_1)
>> +		vgic_v4_configure_vsgis(kvm);
>>   	dist->ready = true;
>>     out:
> 
> Is there any reason to invoke vgic_v4_configure_vsgis() here?
> This is called on the first VCPU run, through kvm_vgic_map_resources().
> Shouldn't the vSGI configuration only driven by a GICD_CTLR.nASSGIreq
> writing (from guest, or from userspace maybe)?

What I'm trying to catch here is the guest that has been restored with
nASSGIreq set. At the moment, we don't do anything on the userspace
side, because the vmm could decide to write that particular bit
multiple times, and switching between the two modes is expensive (not
to mention that all the vcpus may not have been created yet).

Moving it to the first run makes all these pitfalls go away (we have the
final nASSSGIreq value, and all the vcpus are accounted for).

Does this make sense to you?

Thanks,

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