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From: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
To: Christoph Hellwig <hch@lst.de>,
	Boris Ostrovsky <boris.ostrovsky@oracle.com>,
	jgross@suse.com
Cc: ulf.hansson@linaro.org, airlied@linux.ie,
	benh@kernel.crashing.org, joonas.lahtinen@linux.intel.com,
	dri-devel@lists.freedesktop.org, linux-kernel@vger.kernel.org,
	paulus@samba.org, hpa@zytor.com, mingo@kernel.org,
	m.szyprowski@samsung.com, sstabellini@kernel.org,
	adrian.hunter@intel.com, Dongli Zhang <dongli.zhang@oracle.com>,
	x86@kernel.org, joe.jin@oracle.com, peterz@infradead.org,
	mingo@redhat.com, bskeggs@redhat.com, linux-pci@vger.kernel.org,
	xen-devel@lists.xenproject.org, matthew.auld@intel.com,
	thomas.lendacky@amd.com, intel-gfx@lists.freedesktop.org,
	jani.nikula@linux.intel.com, bp@alien8.de,
	rodrigo.vivi@intel.com, bhelgaas@google.com,
	boris.ostrovsky@oracle.com, chris@chris-wilson.co.uk,
	jgross@suse.com, tsbogend@alpha.franken.de,
	nouveau@lists.freedesktop.org, robin.murphy@arm.com,
	linux-mmc@vger.kernel.org, linux-mips@vger.kernel.org,
	iommu@lists.linux-foundation.org, tglx@linutronix.de,
	bauerman@linux.ibm.com, daniel@ffwll.ch, mpe@ellerman.id.au,
	akpm@linux-foundation.org, linuxppc-dev@lists.ozlabs.org,
	rppt@kernel.org
Subject: Re: [Nouveau] [PATCH RFC v1 5/6] xen-swiotlb: convert variables to arrays
Date: Fri, 19 Feb 2021 15:32:15 -0500
Message-ID: <YDAgT2ZIdncNwNlf@Konrads-MacBook-Pro.local> (raw)
In-Reply-To: <20210207155601.GA25111@lst.de>

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.
> 
> 
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Thread overview: 15+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-02-03 23:37 [Nouveau] [PATCH RFC v1 0/6] swiotlb: 64-bit DMA buffer Dongli Zhang
2021-02-03 23:37 ` [Nouveau] [PATCH RFC v1 1/6] swiotlb: define new enumerated type Dongli Zhang
2021-02-03 23:37 ` [Nouveau] [PATCH RFC v1 2/6] swiotlb: convert variables to arrays Dongli Zhang
2021-02-04  7:29   ` Christoph Hellwig
2021-02-04 11:49     ` Robin Murphy
2021-02-04 19:31       ` Konrad Rzeszutek Wilk
2021-02-03 23:37 ` [Nouveau] [PATCH RFC v1 3/6] swiotlb: introduce swiotlb_get_type() to calculate swiotlb buffer type Dongli Zhang
2021-02-03 23:37 ` [Nouveau] [PATCH RFC v1 4/6] swiotlb: enable 64-bit swiotlb Dongli Zhang
2021-02-03 23:37 ` [Nouveau] [PATCH RFC v1 5/6] xen-swiotlb: convert variables to arrays Dongli Zhang
2021-02-04  8:40   ` Christoph Hellwig
2021-02-07 15:56     ` Christoph Hellwig
2021-02-19 20:32       ` Konrad Rzeszutek Wilk [this message]
2021-02-19 23:59         ` Boris Ostrovsky
2021-02-23  1:22         ` Stefano Stabellini
2021-02-03 23:37 ` [Nouveau] [PATCH RFC v1 6/6] xen-swiotlb: enable 64-bit xen-swiotlb Dongli Zhang

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