Re: [LSF/MM TOPIC NOTES] x86 ZONE_DMA love
From: Luis R. Rodriguez
Date: Fri Apr 27 2018 - 12:15:08 EST
On Thu, Apr 26, 2018 at 10:35:56PM -0700, Christoph Hellwig wrote:
> On Thu, Apr 26, 2018 at 09:54:06PM +0000, Luis R. Rodriguez wrote:
> > In practice if you don't have a floppy device on x86, you don't need ZONE_DMA,
> I call BS on that,
I did not explain though that it was not me who claimed this though.
The list displayed below is the result of trying to confirm/deny this,
and what could be done, and also evaluating if there is *any* gain
about doing something about it.
But curious, on a standard qemu x86_x64 KVM guest, which of the
drivers do we know for certain *are* being used from the ones
What about Xen guests, I wonder?
> and you actually explain later why it it BS due
> to some drivers using it more explicitly.
Or implicitly. The list I showed is the work to show that the users
of GFP_DMA on x86 is *much* more wide spread than expected from the
I however did not also answer the above qemu x86_64 question, but
would be good to know. Note I stated that the claim was *in practice*.
> But even more importantly
> we have plenty driver using it through dma_alloc_* and a small DMA
> mask, and they are in use
Do we have a list of users for x86 with a small DMA mask?
Or, given that I'm not aware of a tool to be able to look
for this in an easy way, would it be good to find out which
x86 drivers do have a small mask?
> - we actually had a 4.16 regression due to them.
Ah what commit was the culprit? Is that fixed already? If so what
> > SCSI is *severely* affected:
> Not really. We have unchecked_isa_dma to support about 4 drivers,
Ah very neat:
* CONFIG_CHR_DEV_OSST - "SCSI OnStream SC-x0 tape support"
* CONFIG_SCSI_ADVANSYS - "AdvanSys SCSI support"
* CONFIG_SCSI_AHA1542 - "Adaptec AHA1542 support"
* CONFIG_SCSI_ESAS2R - "ATTO Technology's ExpressSAS RAID adapter driver"
> and less than a hand ful of drivers doing stupid things, which can
> be fixed easily, and just need a volunteer.
Care to list what needs to be done? Can an eager beaver student do it?
> > That's the end of the review of all current explicit callers on x86.
> > # dma_alloc_coherent_gfp_flags() and dma_generic_alloc_coherent()
> > dma_alloc_coherent_gfp_flags() and dma_generic_alloc_coherent() set
> > GFP_DMA if if (dma_mask <= DMA_BIT_MASK(24))
> All that code is long gone and replaced with dma-direct. Which still
> uses GFP_DMA based on the dma mask, though - see above.
And that's mostly IOMMU code, on the alloc() dma_map_ops.