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Date:   Mon, 9 Jan 2023 10:57:24 -0600
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Subject: Re: [PATCH v2] arm64: gic: increase the number of IRQ descriptors
To:     Marc Zyngier <maz@kernel.org>
Cc:     Catalin Marinas <catalin.marinas@arm.com>,
        Will Deacon <will@kernel.org>,
        James Morse <james.morse@arm.com>,
        linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org
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From:   Shanker Donthineni <sdonthineni@nvidia.com>
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On 1/9/23 10:41, Marc Zyngier wrote:
> External email: Use caution opening links or attachments
> 
> 
> On Thu, 05 Jan 2023 14:47:44 +0000,
> Shanker Donthineni <sdonthineni@nvidia.com> wrote:
>>
>>
>>
>> On 1/5/23 04:59, Marc Zyngier wrote:
>>> External email: Use caution opening links or attachments
>>>
>>>
>>> On Wed, 04 Jan 2023 13:47:03 +0000,
>>> Shanker Donthineni <sdonthineni@nvidia.com> wrote:
>>>>
>>>> Hi Marc,
>>>>
>>>> On 1/4/23 03:14, Marc Zyngier wrote:
>>>>> External email: Use caution opening links or attachments
>>>>>
>>>>>
>>>>> On Wed, 04 Jan 2023 02:37:38 +0000,
>>>>> Shanker Donthineni <sdonthineni@nvidia.com> wrote:
>>>>>>
>>>>>> The default value of NR_IRQS is not sufficient to support GICv4.1
>>>>>> features and ~56K LPIs. This parameter would be too small for certain
>>>>>> server platforms where it has many IO devices and is capable of
>>>>>> direct injection of vSGI and vLPI features.
>>>>>>
>>>>>> Currently, maximum of 64 + 8192 (IRQ_BITMAP_BITS) IRQ descriptors
>>>>>> are allowed. The vCPU creation fails after reaching count ~400 with
>>>>>> kvm-arm.vgic_v4_enable=1.
>>>>>>
>>>>>> This patch increases NR_IRQS to 1^19 to cover 56K LPIs and 262144
>>>>>> vSGIs (16K vPEs x 16).
>>>>>>
>>>>>> Signed-off-by: Shanker Donthineni <sdonthineni@nvidia.com>
>>>>>> ---
>>>>>> Changes since v1:
>>>>>>     -create from v6.2-rc1 and edit commit text
>>>>>>
>>>>>>     arch/arm64/include/asm/irq.h | 4 ++++
>>>>>>     1 file changed, 4 insertions(+)
>>>>>>
>>>>>> diff --git a/arch/arm64/include/asm/irq.h b/arch/arm64/include/asm/irq.h
>>>>>> index fac08e18bcd5..3fffc0b8b704 100644
>>>>>> --- a/arch/arm64/include/asm/irq.h
>>>>>> +++ b/arch/arm64/include/asm/irq.h
>>>>>> @@ -4,6 +4,10 @@
>>>>>>
>>>>>>     #ifndef __ASSEMBLER__
>>>>>>
>>>>>> +#if defined(CONFIG_ARM_GIC_V3_ITS)
>>>>>> +#define  NR_IRQS  (1 << 19)
>>>>>> +#endif
>>>>>> +
>>>>>>     #include <asm-generic/irq.h>
>>>>>>
>>>>>>     struct pt_regs;
>>>>>
>>>>> Sorry, but I don't think this is an acceptable change. This is a large
>>>>> overhead that affects *everyone*, and that will eventually be too
>>>>> small anyway with larger systems and larger interrupt spaces.
>>>>>
>>>>> A better way to address this would be to move to a more dynamic
>>>>> allocation, converting the irqdesc rb-tree into an xarray, getting rid
>>>>> of the bitmaps (the allocation bitmap and the resend one), and track
>>>>> everything in the xarray.
>>>>
>>>> The actual memory allocation for IRQ descriptors is still dynamic for ARM64.
>>>> This change increases static memory for variable 'allocated_irqs' by 64KB,
>>>> feel not a noticeable overhead.
>>>
>>> 64kB for each bitmap, so that's already 128kB (you missed the
>>> irqs_resend bitmap). And that's for a number of IRQs that is still way
>>> below what the GIC architecture supports today.
>>>
>>> The architecture supports 32bit INTIDs, and that's 1GB worth of
>>> bitmaps, only for the physical side. Add the virtual stuff for which
>>> we create host-side descriptors, and we can go way beyond that.
>>>
>>> So what happens next, once you exceed the arbitrary limit that only
>>> satisfies your own use case? We will bump it up again, and again,
>>> bloating the kernel with useless static data that *nobody* needs.
>>> Specially not the VMs that you plan to run.
>>>
>>> So I'm putting my foot down right now, and saying that it needs to be
>>> fixed once and for all. The current scheme was OK for small interrupt
>>> spaces, but it isn't fit for purpose anymore, certainly not with
>>> things like the GICv4 architecture.
>>>
>>> I'm happy to help with it, but I'm certainly not willing to accept any
>>> sort of new compile-time limit.
>>
>> Thanks for helping with a scalable solution instead of static
>> allocation.  Please include me whenever patches posted to LKML. I'm
>> happy to verify on NVIDIA server platforms and provide test
>> feedback.
>>
> 
> I offered to help you. I didn't offer to do the work for you! ;-)
> 

I've looked at the IDR/IDA API. There is no suitable function for
allocating contiguous IDs to replace bitmap API.

__irq_alloc_descs():

	mutex_lock(&sparse_irq_lock);

	start = bitmap_find_next_zero_area(allocated_irqs, IRQ_BITMAP_BITS,
					   from, cnt, 0);
	ret = -EEXIST;