From: Will Deacon <will@kernel.org>
To: Marc Zyngier <maz@kernel.org>
Cc: Peter Maydell <peter.maydell@linaro.org>,
James Morse <james.morse@arm.com>,
Julien Thierry <julien.thierry.kdev@gmail.com>,
Suzuki K Poulose <suzuki.poulose@arm.com>,
Zenghui Yu <yuzenghui@huawei.com>,
qemu-arm@nongnu.org, kvmarm@lists.cs.columbia.edu,
linux-arm-kernel@lists.infradead.org, kvm@vger.kernel.org
Subject: Re: [PATCH] KVM: arm/arm64: vgic: Allow more than 256 vcpus for KVM_IRQ_LINE
Date: Mon, 19 Aug 2019 08:41:50 +0100 [thread overview]
Message-ID: <20190819074150.jv3dyyxqazoawgds@willie-the-truck> (raw)
In-Reply-To: <20190818140710.23920-1-maz@kernel.org>
On Sun, Aug 18, 2019 at 03:07:10PM +0100, Marc Zyngier wrote:
> While parts of the VGIC support a large number of vcpus (we
> bravely allow up to 512), other parts are more limited.
>
> One of these limits is visible in the KVM_IRQ_LINE ioctl, which
> only allows 256 vcpus to be signalled when using the CPU or PPI
> types. Unfortunately, we've cornered ourselves badly by allocating
> all the bits in the irq field.
>
> Since the irq_type subfield (8 bit wide) is currently only taking
> the values 0, 1 and 2 (and we have been careful not to allow anything
> else), let's reduce this field to only 4 bits, and allocate the
> remaining 4 bits to a vcpu2_index, which acts as a multiplier:
>
> vcpu_id = 256 * vcpu2_index + vcpu_index
>
> With that, and a new capability (KVM_CAP_ARM_IRQ_LINE_LAYOUT_2)
> allowing this to be discovered, it becomes possible to inject
> PPIs to up to 4096 vcpus. But please just don't.
Do you actually need a new capability for this? Older kernels reject
non-zero upper bits in the 'irq_type', so isn't that enough to probe
for this directly?
Will
next prev parent reply other threads:[~2019-08-19 7:41 UTC|newest]
Thread overview: 6+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-08-18 14:07 [PATCH] KVM: arm/arm64: vgic: Allow more than 256 vcpus for KVM_IRQ_LINE Marc Zyngier
2019-08-19 7:41 ` Will Deacon [this message]
2019-08-19 8:46 ` Marc Zyngier
2019-08-22 9:00 ` Auger Eric
2019-08-29 2:46 ` Zenghui Yu
2019-09-03 11:33 ` Peter Maydell
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