linux-kernel - Re: [PATCH] swiotlb: Allow allocating buffer anywhere in memory

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Message-ID: <87o8n96j0e.fsf@morokweng.localdomain>
Date:   Mon, 17 Aug 2020 18:48:49 -0300
From:   Thiago Jung Bauermann <bauerman@...ux.ibm.com>
To:     Christoph Hellwig <hch@....de>
Cc:     iommu@...ts.linux-foundation.org, linuxppc-dev@...ts.ozlabs.org,
        linux-kernel@...r.kernel.org,
        Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
        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>,
        Mike Rapoport <rppt@...nel.org>
Subject: Re: [PATCH] swiotlb: Allow allocating buffer anywhere in memory


Hello Christoph,

Christoph Hellwig <hch@....de> writes:

> On Sat, Aug 15, 2020 at 05:45:36PM -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.
>
> What about just open coding the allocation and using
> swiotlb_init_with_tbl?

Yes, that works too. I just sent a v2 implementing that change. I just
had to add a small accessor function so that I could set no_iotlb_memory
from outside swiotlb.c.

Thank you for the quick review.

-- 
Thiago Jung Bauermann
IBM Linux Technology Center