Re: [Xen-devel] [PATCH] xen: Add __GFP_DMA flag when xen_swiotlb_init gets free pages.

[Ian Campbell]

On Tue, 2015-04-21 at 11:36 +0100, Stefano Stabellini wrote:
> On Tue, 21 Apr 2015, Ian Campbell wrote:
> > On Mon, 2015-04-20 at 18:54 +0100, Stefano Stabellini wrote:
> > > This should definitely be done only on ARM and ARM64, as on x86 PVH
> > > assumes the presence of an IOMMU. We need an ifdef.
> > > 
> > > Also we need to figure out a way to try without GFP_DMA in case no ram
> > > under 4g is available at all, as some arm64 platforms don't have any. Of
> > > course in those cases we don't need to worry about devices and their dma
> > > masks.  Maybe we could use memblock for that?
> > 
> > It's pretty ugly, but I've not got any better ideas.
> > 
> > It would perhaps be less ugly as a an arch-specific
> > get_me_a_swiotlb_region type function, with the bare __get_free_pages as
> > the generic fallback.
> 
> We could do that, but even open code like this isn't too bad: it might
> be ugly but at least is very obvious.
> 
> 
> > >  Something like:
> > > 
> > >     struct memblock_region *reg;
> > >     gfp_t flags = __GFP_NOWARN;
> > > 
> > > #if defined(CONFIG_ARM) || defined(CONFIG_ARM64)
> > > 	for_each_memblock(memory, reg) {
> > > 		unsigned long start = memblock_region_memory_base_pfn(reg);
> > > 
> > >         if (start < 4G) {
> > >             flags |= __GFP_DMA;
> > >             break;
> > >         }
> > >     }
> > > #endif
> > > 
> > >     [...]
> > > 
> > >     xen_io_tlb_start = (void *)__get_free_pages(flags, order);
> > >     
> > > 
> > > 
> > > 
> > > 
> > > > This is also conceptually wrong since it doesn't matter where the pages
> > > > are in PFN space,  but where they are in bus address (MFN) space (which
> > > > is what the subsequent hypercall is required to sort out).
> > > 
> > > Actually on ARM dom0 is mapped 1:1, so it is the same thing.
> > 
> > On a system with a fully functional SMMU dom0 may not be 1:1 mapped, but
> > I think that dom0 can still treat that as 1:1 mapped for these purposes,
> > since the SMMU will provide that illusion.
> > 
> > Dumb question, and this might affect PVH too, if you have an IOMMU and a
> > device with a limited DMA range, I suppose you need to provide DMA
> > addresses in the <4G for the input to the IOMMU (i.e. PFN) and not the
> > output (i.e. MFN) space (since the device only sees PFNs).
> 
> I think you mean "for the input to the device (PFN)", but I presume the
> same.

Yes, either the numbers the driver programs into the device, or the ones
the device itself puts on the bus (i.e. inputs to the IOMMU). They are
in the same address space.

> > So even for x86 PVH isn't something required here to ensure that the
> > swiotlb has suitable pages under 4GB in PFN space too?
> > 
> > (On ARM PFN==IPA and MFN==PA)
> 
> I guess that is true. PVH people, any thoughts?
> 
> 
> > Second dumb question, on x86 PVH or ARM with an SMMU, would we even hit
> > the Xen swiotlb code, or would we want to arrange to go via the native
> > swiotlb paths? I initially thought the latter, but does e.g. grant
> > mappings still need some special handling?
> 
> In both cases I think it would be simpler to just go through the normal
> swiotlb.

No special grant table magic in this case == great!

> FYI at the moment there is no knowledge about SMMUs availability on ARM.
> The code is still missing.

Right, I just want to avoid painting ourselves into a corner before that
stuff lands.

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