linux-kernel - Re: [RFC PATCH V2 1/2] swiotlb: Add Child IO TLB mem support

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Date:   Mon, 16 May 2022 00:34:49 -0700
From:   Christoph Hellwig <hch@...radead.org>
To:     Tianyu Lan <ltykernel@...il.com>
Cc:     hch@...radead.org, m.szyprowski@...sung.com, robin.murphy@....com,
        michael.h.kelley@...rosoft.com, kys@...rosoft.com,
        Tianyu Lan <Tianyu.Lan@...rosoft.com>,
        iommu@...ts.linux-foundation.org, linux-kernel@...r.kernel.org,
        vkuznets@...hat.com, brijesh.singh@....com, konrad.wilk@...cle.com,
        hch@....de, wei.liu@...nel.org, parri.andrea@...il.com,
        thomas.lendacky@....com, linux-hyperv@...r.kernel.org,
        andi.kleen@...el.com, kirill.shutemov@...el.com
Subject: Re: [RFC PATCH V2 1/2] swiotlb: Add Child IO TLB mem support

I don't really understand how 'childs' fit in here.  The code also
doesn't seem to be usable without patch 2 and a caller of the
new functions added in patch 2, so it is rather impossible to review.

Also:

 1) why is SEV/TDX so different from other cases that need bounce
    buffering to treat it different and we can't work on a general
    scalability improvement
 2) per previous discussions at how swiotlb itself works, it is
    clear that another option is to just make pages we DMA to
    shared with the hypervisor.  Why don't we try that at least
    for larger I/O?