linux-kernel - Re: [PATCH 2/5] swiotlb: Add is_swiotlb

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Message-ID: <20190124150000.GN32526@8bytes.org>
Date:   Thu, 24 Jan 2019 16:00:00 +0100
From:   Joerg Roedel <joro@...tes.org>
To:     Christoph Hellwig <hch@....de>
Cc:     "Michael S . Tsirkin" <mst@...hat.com>,
        Jason Wang <jasowang@...hat.com>,
        Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
        Jens Axboe <axboe@...nel.dk>,
        virtualization@...ts.linux-foundation.org,
        linux-block@...r.kernel.org, linux-kernel@...r.kernel.org,
        iommu@...ts.linux-foundation.org, jfehlig@...e.com,
        jon.grimm@....com, brijesh.singh@....com, jroedel@...e.de
Subject: Re: [PATCH 2/5] swiotlb: Add is_swiotlb_active() function

On Thu, Jan 24, 2019 at 09:41:07AM +0100, Christoph Hellwig wrote:
> On Thu, Jan 24, 2019 at 09:29:23AM +0100, Joerg Roedel wrote:
> > > As I've just introduced and fixed a bug in this area in the current
> > > cycle - I don't think no_iotlb_memory is what your want (and maybe
> > > not useful at all): if the arch valls swiotlb_exit after previously
> > > initializing a buffer it won't be set.  You probably want to check
> > > for non-zero io_tlb_start and/or io_tlb_end.
> > 
> > Okay, but that requires that I also set io_tlb_start and friends back to
> > zero in the failure path of swiotlb_init(). Otherwise it could be left
> > non-zero in case swiotlb_init_with_tbl() returns an error.
> 
> Indeed, and we'll need to do that anyway as otherwise the dma mapping
> path might cause problems similar to the one when swiotlb_exit is
> called that I fixed.

Turns out the the error path in swiotlb_init() is redundant because it
will never be executed. If the function returns it will always return 0
because in case of failure it will just panic (through memblock_alloc).

I'll clean that up in a separate patch-set. There are more users of that
function and all of them panic when the function fails.


	Joerg