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Date:   Tue, 18 Aug 2020 08:59:11 +0200
From:   Christoph Hellwig <hch@lst.de>
To:     Thiago Jung Bauermann <bauerman@linux.ibm.com>
Cc:     iommu@lists.linux-foundation.org, linuxppc-dev@lists.ozlabs.org,
        linux-kernel@vger.kernel.org,
        Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>,
        Christoph Hellwig <hch@lst.de>,
        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
Message-ID: <20200818065911.GA2324@lst.de>
References: <20200817214658.103093-1-bauerman@linux.ibm.com>
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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.