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Date:   Mon, 30 Mar 2020 22:25:51 +0000
From:   Ashish Kalra <ashish.kalra@amd.com>
To:     Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Cc:     Konrad Rzeszutek Wilk <konrad@darnok.org>, hch@lst.de,
        tglx@linutronix.de, mingo@redhat.com, bp@alien8.de, hpa@zytor.com,
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        dave.hansen@linux-intel.com, iommu@lists.linux-foundation.org,
        linux-kernel@vger.kernel.org, brijesh.singh@amd.com,
        Thomas.Lendacky@amd.com
Subject: Re: [PATCH v2] swiotlb: Adjust SWIOTBL bounce buffer size for SEV
 guests.
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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.

Thanks,
Ashish