linux-kernel - Re: [PATCH v3] powerpc/pseries/svm: Allocate SWIOTLB buffer anywhere in memory

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Message-ID: <20200819044351.GA19391@lst.de>
Date:   Wed, 19 Aug 2020 06:43:51 +0200
From:   Christoph Hellwig <hch@....de>
To:     Thiago Jung Bauermann <bauerman@...ux.ibm.com>
Cc:     iommu@...ts.linux-foundation.org, linuxppc-dev@...ts.ozlabs.org,
        linux-kernel@...r.kernel.org,
        Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
        Christoph Hellwig <hch@....de>,
        Marek Szyprowski <m.szyprowski@...sung.com>,
        Robin Murphy <robin.murphy@....com>,
        Michael Ellerman <mpe@...erman.id.au>,
        Ram Pai <linuxram@...ibm.com>,
        Satheesh Rajendran <sathnaga@...ux.vnet.ibm.com>
Subject: Re: [PATCH v3] powerpc/pseries/svm: Allocate SWIOTLB buffer
 anywhere in memory

On Tue, Aug 18, 2020 at 07:11:26PM -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().
> 
> Signed-off-by: Thiago Jung Bauermann <bauerman@...ux.ibm.com>

Looks fine to me (except for the pointlessly long comment lines, but I've
been told that's the powerpc way).