linux-kernel - Re: [PATCH v2 12/14] arm64: realm: Support nonsecure ITS emulation shared

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Date: Thu, 23 May 2024 10:57:37 +0100
From: Steven Price <steven.price@....com>
To: Catalin Marinas <catalin.marinas@....com>
Cc: kvm@...r.kernel.org, kvmarm@...ts.linux.dev, Marc Zyngier
 <maz@...nel.org>, Will Deacon <will@...nel.org>,
 James Morse <james.morse@....com>, Oliver Upton <oliver.upton@...ux.dev>,
 Suzuki K Poulose <suzuki.poulose@....com>, Zenghui Yu
 <yuzenghui@...wei.com>, linux-arm-kernel@...ts.infradead.org,
 linux-kernel@...r.kernel.org, Joey Gouly <joey.gouly@....com>,
 Alexandru Elisei <alexandru.elisei@....com>,
 Christoffer Dall <christoffer.dall@....com>, Fuad Tabba <tabba@...gle.com>,
 linux-coco@...ts.linux.dev,
 Ganapatrao Kulkarni <gankulkarni@...amperecomputing.com>
Subject: Re: [PATCH v2 12/14] arm64: realm: Support nonsecure ITS emulation
 shared

On 22/05/2024 18:05, Catalin Marinas wrote:
> On Wed, May 22, 2024 at 04:52:45PM +0100, Steven Price wrote:
>> On 15/05/2024 12:01, Catalin Marinas wrote:
>>> On Fri, Apr 12, 2024 at 09:42:11AM +0100, Steven Price wrote:
>>>> @@ -3432,7 +3468,16 @@ static struct its_device *its_create_device(struct its_node *its, u32 dev_id,
>>>>  	nr_ites = max(2, nvecs);
>>>>  	sz = nr_ites * (FIELD_GET(GITS_TYPER_ITT_ENTRY_SIZE, its->typer) + 1);
>>>>  	sz = max(sz, ITS_ITT_ALIGN) + ITS_ITT_ALIGN - 1;
>>>> -	itt = kzalloc_node(sz, GFP_KERNEL, its->numa_node);
>>>> +	itt_order = get_order(sz);
>>>> +	page = its_alloc_shared_pages_node(its->numa_node,
>>>> +					   GFP_KERNEL | __GFP_ZERO,
>>>> +					   itt_order);
>>>
>>> How much do we waste by going for a full page always if this is going to
>>> be used on the host?
>>
>> sz is a minimum of ITS_ITT_ALIGN*2-1 - which is 511 bytes. So
>> potentially PAGE_SIZE-512 bytes could be wasted here (minus kmalloc
>> overhead).
> 
> That I figured out as well but how many times is this path called with a
> size smaller than a page?
> 

That presumably depends on the number of devices in the guest. For my
test guest setup (using kvmtool) this is called 3 times each with sz=511.

Steve