linux-kernel - Re: [PATCH 2/2] x86/ACPI: Set swiotlb area according to the number of lapic entry in MADT

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Date:   Wed, 6 Jul 2022 16:57:33 +0800
From:   Tianyu Lan <ltykernel@...il.com>
To:     Christoph Hellwig <hch@...radead.org>
Cc:     corbet@....net, rafael@...nel.org, len.brown@...el.com,
        pavel@....cz, tglx@...utronix.de, mingo@...hat.com, bp@...en8.de,
        dave.hansen@...ux.intel.com, x86@...nel.org, hpa@...or.com,
        m.szyprowski@...sung.com, robin.murphy@....com, paulmck@...nel.org,
        akpm@...ux-foundation.org, keescook@...omium.org,
        songmuchun@...edance.com, rdunlap@...radead.org,
        damien.lemoal@...nsource.wdc.com, michael.h.kelley@...rosoft.com,
        kys@...rosoft.com, Tianyu Lan <Tianyu.Lan@...rosoft.com>,
        iommu@...ts.linux-foundation.org, linux-doc@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-pm@...r.kernel.org,
        vkuznets@...hat.com, wei.liu@...nel.org, parri.andrea@...il.com,
        thomas.lendacky@....com, linux-hyperv@...r.kernel.org,
        kirill.shutemov@...el.com, andi.kleen@...el.com,
        Andi Kleen <ak@...ux.intel.com>
Subject: Re: [PATCH 2/2] x86/ACPI: Set swiotlb area according to the number of
 lapic entry in MADT

On 7/6/2022 4:00 PM, Christoph Hellwig wrote:
> On Fri, Jul 01, 2022 at 01:02:21AM +0800, Tianyu Lan wrote:
>>> Can we reorder that initialization?  Because I really hate having
>>> to have an arch hook in every architecture.
>>
>> How about using "flags" parameter of swiotlb_init() to pass area number
>> or add new parameter for area number?
>>
>> I just reposted patch 1 since there is just some coding style issue and area
>> number may also set via swiotlb kernel parameter. We still need figure out a
>> good solution to pass area number from architecture code.
> 
> What is the problem with calling swiotlb_init after nr_possible_cpus()
> works?

Swiotlb_init() is called in the mem_init() of different architects and
memblock free pages are released to the buddy allocator just after
calling swiotlb_init() via memblock_free_all().

The mem_init() is called before smp_init(). If calling swiotlb_init()
after smp_init(), that means we can't allocate large chunk low end
memory via memblock_alloc() in the swiotlb(). Swiotlb_init() needs
to rework to allocate memory from the buddy allocator and just like
swiotlb_init_late() does. This will limit the bounce buffer size.
Otherwise We need to do the reorder for all achitectures and there maybe
some other unknown issues.

swiotlb flags parameter of swiotlb_init() seems to be a good place to
pass the area number in current code. If not set the swiotlb_area
number/flag, the area number will be one and keep the original behavior
of one single global spinlock protecting io tlb data structure.