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

[Ashish Kalra]

Hello Konrad,

Looking fwd. to your feedback regarding support of other memory
encryption architectures such as Power, S390, etc.

Thanks,
Ashish

On Fri, Jan 24, 2020 at 11:00:08PM +0000, Ashish Kalra wrote:
> On Tue, Jan 21, 2020 at 03:54:03PM -0500, Konrad Rzeszutek Wilk wrote:
> > > 
> > > 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.
> 
> Ok.
> 
> >> 
> >>  - 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?).
> 
> This seems to be a nice option, but then IOMMU_INIT APIs are
> x86-specific and this swiotlb buffer size adjustment is also needed
> for other memory encryption architectures like Power, S390, etc.
> 
> >> 
> >>    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
> 
> Actually, we piggyback on pci_swiotlb_detect_override which sets
> swiotlb=1 as x86_64_start_kernel() and invocation of sme_early_init()
> forces swiotlb on, but again this is all x86 architecture specific.
> 
> Thanks,
> Ashish