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

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Date:   Fri, 24 Jan 2020 23:00:08 +0000
From:   Ashish Kalra <ashish.kalra@....com>
To:     Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
Cc:     Konrad Rzeszutek Wilk <konrad@...nok.org>, hch@....de,
        tglx@...utronix.de, mingo@...hat.com, bp@...en8.de, hpa@...or.com,
        x86@...nel.org, luto@...nel.org, peterz@...radead.org,
        dave.hansen@...ux-intel.com, iommu@...ts.linux-foundation.org,
        linux-kernel@...r.kernel.org, brijesh.singh@....com,
        Thomas.Lendacky@....com
Subject: Re: [PATCH v2] swiotlb: Adjust SWIOTBL bounce buffer size for SEV
 guests.

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