linux-kernel - Re: [PATCH V2 1/2] Swiotlb: Add swiotlb_alloc_from_low

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Date:   Mon, 14 Feb 2022 14:58:34 +0100
From:   Christoph Hellwig <hch@....de>
To:     Tianyu Lan <ltykernel@...il.com>
Cc:     Christoph Hellwig <hch@....de>, kys@...rosoft.com,
        haiyangz@...rosoft.com, sthemmin@...rosoft.com, wei.liu@...nel.org,
        decui@...rosoft.com, tglx@...utronix.de, mingo@...hat.com,
        bp@...en8.de, dave.hansen@...ux.intel.com, x86@...nel.org,
        hpa@...or.com, hch@...radead.org, m.szyprowski@...sung.com,
        robin.murphy@....com, michael.h.kelley@...rosoft.com,
        Tianyu Lan <Tianyu.Lan@...rosoft.com>,
        iommu@...ts.linux-foundation.org, linux-hyperv@...r.kernel.org,
        linux-kernel@...r.kernel.org, vkuznets@...hat.com,
        brijesh.singh@....com, konrad.wilk@...cle.com,
        parri.andrea@...il.com, thomas.lendacky@....com
Subject: Re: [PATCH V2 1/2] Swiotlb: Add swiotlb_alloc_from_low_pages switch

On Mon, Feb 14, 2022 at 07:28:40PM +0800, Tianyu Lan wrote:
> On 2/14/2022 4:19 PM, Christoph Hellwig wrote:
>> Adding a function to set the flag doesn't really change much.  As Robin
>> pointed out last time you should fine a way to just call
>> swiotlb_init_with_tbl directly with the memory allocated the way you
>> like it.  Or given that we have quite a few of these trusted hypervisor
>> schemes maybe add an argument to swiotlb_init that specifies how to
>> allocate the memory.
>
> Thanks for your suggestion. I will try the first approach first approach.

Take a look at the SWIOTLB_ANY flag in this WIP branch:

   http://git.infradead.org/users/hch/misc.git/shortlog/refs/heads/swiotlb-init-cleanup

That being said I'm not sure that either this flag or the existing powerpc
code iѕ actually the right thing to do.  We still need the 4G limited
buffer to support devices with addressing limitations.  So I think we need
an additional io_tlb_mem instance for the devices without addressing
limitations instead.