linux-kernel - Re: [PATCH RFC v1 5/6] xen-swiotlb: convert variables to arrays

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Date:   Fri, 19 Feb 2021 15:32:15 -0500
From:   Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
To:     Christoph Hellwig <hch@....de>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>, jgross@...e.com
Cc:     Dongli Zhang <dongli.zhang@...cle.com>,
        dri-devel@...ts.freedesktop.org, intel-gfx@...ts.freedesktop.org,
        iommu@...ts.linux-foundation.org, linux-mips@...r.kernel.org,
        linux-mmc@...r.kernel.org, linux-pci@...r.kernel.org,
        linuxppc-dev@...ts.ozlabs.org, nouveau@...ts.freedesktop.org,
        x86@...nel.org, xen-devel@...ts.xenproject.org,
        linux-kernel@...r.kernel.org, adrian.hunter@...el.com,
        akpm@...ux-foundation.org, benh@...nel.crashing.org,
        bskeggs@...hat.com, bhelgaas@...gle.com, bp@...en8.de,
        boris.ostrovsky@...cle.com, chris@...is-wilson.co.uk,
        daniel@...ll.ch, airlied@...ux.ie, hpa@...or.com, mingo@...nel.org,
        mingo@...hat.com, jani.nikula@...ux.intel.com,
        joonas.lahtinen@...ux.intel.com, jgross@...e.com,
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        mpe@...erman.id.au, rppt@...nel.org, paulus@...ba.org,
        peterz@...radead.org, robin.murphy@....com, rodrigo.vivi@...el.com,
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        ulf.hansson@...aro.org, joe.jin@...cle.com, thomas.lendacky@....com
Subject: Re: [PATCH RFC v1 5/6] xen-swiotlb: convert variables to arrays

On Sun, Feb 07, 2021 at 04:56:01PM +0100, Christoph Hellwig wrote:
> On Thu, Feb 04, 2021 at 09:40:23AM +0100, Christoph Hellwig wrote:
> > So one thing that has been on my mind for a while:  I'd really like
> > to kill the separate dma ops in Xen swiotlb.  If we compare xen-swiotlb
> > to swiotlb the main difference seems to be:
> > 
> >  - additional reasons to bounce I/O vs the plain DMA capable
> >  - the possibility to do a hypercall on arm/arm64
> >  - an extra translation layer before doing the phys_to_dma and vice
> >    versa
> >  - an special memory allocator
> > 
> > I wonder if inbetween a few jump labels or other no overhead enablement
> > options and possibly better use of the dma_range_map we could kill
> > off most of swiotlb-xen instead of maintaining all this code duplication?
> 
> So I looked at this a bit more.
> 
> For x86 with XENFEAT_auto_translated_physmap (how common is that?)

Juergen, Boris please correct me if I am wrong, but that XENFEAT_auto_translated_physmap
only works for PVH guests?

> pfn_to_gfn is a nop, so plain phys_to_dma/dma_to_phys do work as-is.
> 
> xen_arch_need_swiotlb always returns true for x86, and
> range_straddles_page_boundary should never be true for the
> XENFEAT_auto_translated_physmap case.

Correct. The kernel should have no clue of what the real MFNs are
for PFNs.
> 
> So as far as I can tell the mapping fast path for the
> XENFEAT_auto_translated_physmap can be trivially reused from swiotlb.
> 
> That leaves us with the next more complicated case, x86 or fully cache
> coherent arm{,64} without XENFEAT_auto_translated_physmap.  In that case
> we need to patch in a phys_to_dma/dma_to_phys that performs the MFN
> lookup, which could be done using alternatives or jump labels.
> I think if that is done right we should also be able to let that cover
> the foreign pages in is_xen_swiotlb_buffer/is_swiotlb_buffer, but
> in that worst case that would need another alternative / jump label.
> 
> For non-coherent arm{,64} we'd also need to use alternatives or jump
> labels to for the cache maintainance ops, but that isn't a hard problem
> either.
> 
>