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From:   Thiago Jung Bauermann <bauerman@linux.ibm.com>
To:     Christoph Hellwig <hch@lst.de>
Cc:     iommu@lists.linux-foundation.org, linuxppc-dev@lists.ozlabs.org,
        linux-kernel@vger.kernel.org,
        Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>,
        Marek Szyprowski <m.szyprowski@samsung.com>,
        Robin Murphy <robin.murphy@arm.com>,
        Michael Ellerman <mpe@ellerman.id.au>,
        Ram Pai <linuxram@us.ibm.com>,
        Satheesh Rajendran <sathnaga@linux.vnet.ibm.com>
Subject: Re: [PATCH v2] powerpc/pseries/svm: Allocate SWIOTLB buffer anywhere in memory
In-reply-to: <20200818065911.GA2324@lst.de>
Date:   Tue, 18 Aug 2020 16:51:52 -0300
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Christoph Hellwig <hch@lst.de> writes:

> On Mon, Aug 17, 2020 at 06:46:58PM -0300, Thiago Jung Bauermann wrote:
>> POWER secure guests (i.e., guests which use the Protection Execution
>> Facility) need to use SWIOTLB to be able to do I/O with the hypervisor, but
>> they don't need the SWIOTLB memory to be in low addresses since the
>> hypervisor doesn't have any addressing limitation.
>> 
>> This solves a SWIOTLB initialization problem we are seeing in secure guests
>> with 128 GB of RAM: they are configured with 4 GB of crashkernel reserved
>> memory, which leaves no space for SWIOTLB in low addresses.
>> 
>> To do this, we use mostly the same code as swiotlb_init(), but allocate the
>> buffer using memblock_alloc() instead of memblock_alloc_low().
>> 
>> We also need to add swiotlb_set_no_iotlb_memory() in order to set the
>> no_iotlb_memory flag if initialization fails.
>
> Do you really need the helper?  As far as I can tell the secure guests
> very much rely on swiotlb for all I/O, so you might as well panic if
> you fail to allocate it.

That is true. Ok, I will do that.

-- 
Thiago Jung Bauermann
IBM Linux Technology Center