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Date:   Mon, 27 Apr 2020 17:49:49 +0100
From:   Marc Zyngier <maz@kernel.org>
To:     =?UTF-8?Q?Andr=C3=A9_Przywara?= <andre.przywara@arm.com>
Cc:     kvmarm@lists.cs.columbia.edu, kvm@vger.kernel.org,
        James Morse <james.morse@arm.com>,
        Julien Thierry <julien.thierry.kdev@gmail.com>,
        Suzuki K Poulose <suzuki.poulose@arm.com>,
        Will Deacon <will@kernel.org>,
        Ard Biesheuvel <ardb@kernel.org>,
        Alexandru Elisei <Alexandru.Elisei@arm.com>
Subject: Re: [PATCH][kvmtool] kvm: Request VM specific limits instead of
 system-wide ones
In-Reply-To: <c36c30b1-6017-9c75-e0e9-e643eb348641@arm.com>
References: <20200427141738.285217-1-maz@kernel.org>
 <c36c30b1-6017-9c75-e0e9-e643eb348641@arm.com>
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On 2020-04-27 16:37, André Przywara wrote:
> On 27/04/2020 15:17, Marc Zyngier wrote:
> Hi,
> 
>> On arm64, the maximum number of vcpus is constrained by the type
>> of interrupt controller that has been selected (GICv2 imposes a
>> limit of 8 vcpus, while GICv3 currently has a limit of 512).
>> 
>> It is thus important to request this limit on the VM file descriptor
>> rather than on the one that corresponds to /dev/kvm, as the latter
>> is likely to return something that doesn't take the constraints into
>> account.
> 
> That sounds reasonable, but I fail to find any distinction in the 
> kernel
> code. We don't make any difference between the VM or the system FD in
> the ioctl handler for those two extensions. For arm64 we always return
> max. 512 (max VCPUs on GICv3), and number of online host cores for the
> recommended value. For arm there was a distinction between GICv3 
> support
> compiled in or not, but otherwise the same constant values returned.
> Quickly tested on Juno and N1SDP, the ioctls return the same expected
> values, regardless of sys_fd vs vm_fd.
> 
> So what am I missing here? Is this for some older or even newer 
> kernels?

You're missing this:

https://lore.kernel.org/kvm/20200427141507.284985-1-maz@kernel.org/

which adds the missing bits to the kernel.

Thanks,

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