Re: [PATCH v2] swiotlb: Adjust SWIOTBL bounce buffer size for SEV guests.

[Konrad Rzeszutek Wilk]

On Tue, Jan 21, 2020 at 08:09:47PM +0000, Ashish Kalra wrote:
> On Thu, Dec 19, 2019 at 08:52:45PM -0500, Konrad Rzeszutek Wilk wrote:
> > On Mon, Dec 09, 2019 at 11:13:46PM +0000, Ashish Kalra wrote:
> > > From: Ashish Kalra <ashish.kalra@xxxxxxx>
> > > 
> > > For SEV, all DMA to and from guest has to use shared
> > > (un-encrypted) pages. SEV uses SWIOTLB to make this happen
> > > without requiring changes to device drivers. However,
> > > depending on workload being run, the default 64MB of SWIOTLB
> > > might not be enough and SWIOTLB may run out of buffers to
> > > use for DMA, resulting in I/O errors.
> > > 
> > > Increase the default size of SWIOTLB for SEV guests using
> > > a minimum value of 128MB and a maximum value of 512MB,
> > > determining on amount of provisioned guest memory.
> > > 
> > > The SWIOTLB default size adjustment is added as an
> > > architecture specific interface/callback to allow
> > > architectures such as those supporting memory encryption
> > > to adjust/expand SWIOTLB size for their use.
> > 
> > What if this was made dynamic? That is if there is a memory
> > pressure you end up expanding the SWIOTLB dynamically?
> 
> As of now we want to keep it as simple as possible and more
> like a stop-gap arrangement till something more elegant is
> available.

That is nice. But past experience has shown that stop-gap arrangments
end up being the defacto solution.

> 
> > 
> >> Also is it worth doing this calculation based on memory or
> >> more on the # of PCI devices + their MMIO ranges size?
> 
> Additional memory calculations based on # of PCI devices and
> their memory ranges will make it more complicated with so
> many other permutations and combinations to explore, it is
> essential to keep this patch as simple as possible by 
> adjusting the bounce buffer size simply by determining it
> from the amount of provisioned guest memory.

Please rework the patch to:

 - Use a log solution instead of the multiplication.
   Feel free to cap it at a sensible value.

 - Also the code depends on SWIOTLB calling in to the
   adjust_swiotlb_default_size which looks wrong.

   You should not adjust io_tlb_nslabs from swiotlb_size_or_default.
   That function's purpose is to report a value.

 - Make io_tlb_nslabs be visible outside of the SWIOTLB code.

 - Can you utilize the IOMMU_INIT APIs and have your own detect which would
   modify the io_tlb_nslabs (and set swiotbl=1?).

   Actually you seem to be piggybacking on pci_swiotlb_detect_4gb - so
   perhaps add in this code ? Albeit it really should be in it's own
   file, not in arch/x86/kernel/pci-swiotlb.c

 - Tweak the code in the swiotlb code to make sure it can deal
   with io_tlb_nslabs being modified outside of the code at
   the start. It should have no trouble, but only testing will
   tell for sure.

> 
> Thanks,
> Ashish