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

[Ashish Kalra]

Hello Konrad,

On Mon, Mar 30, 2020 at 10:25:51PM +0000, Ashish Kalra wrote:
> Hello Konrad,
> 
> On Tue, Mar 03, 2020 at 12:03:53PM -0500, Konrad Rzeszutek Wilk wrote:
> > On Tue, Feb 04, 2020 at 07:35:00PM +0000, Ashish Kalra wrote:
> > > 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.
> > 
> > Oh dear. That I hadn't considered.
> > > > 
> > > > >> 
> > > > >>    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.
> > 
> > Then it looks like the best bet is to do it from within swiotlb_init?
> > We really can't do it from swiotlb_size_or_default - that function
> > should just return a value and nothing else.
> > 
> 
> Actually, we need to do it in swiotlb_size_or_default() as this gets called by
> reserve_crashkernel_low() in arch/x86/kernel/setup.c and used to
> reserve low crashkernel memory. If we adjust swiotlb size later in
> swiotlb_init() which gets called later than reserve_crashkernel_low(),
> then any swiotlb size changes/expansion will conflict/overlap with the
> low memory reserved for crashkernel.
> 
and will also potentially cause SWIOTLB buffer allocation failures.

Do you have any feedback, comments on the above ?

As such i feel, this patch is complete otherwise and can be included as
it is. 

Thanks,
Ashish