From: konrad.wilk@oracle.com (Konrad Rzeszutek Wilk) Date: Tue, 20 Nov 2018 11:34:16 -0500 Subject: [PATCH 1/2] swiotlb: Make DIRECT_MAPPING_ERROR viable In-Reply-To: <7ea5809e-1e1d-6bac-b723-9f55fafeaaae@arm.com> References: <20181120144918.GH10917@char.us.oracle.com> <7ea5809e-1e1d-6bac-b723-9f55fafeaaae@arm.com> Message-ID: <20181120163416.GC31199@char.us.oracle.com> To: linux-arm-kernel@lists.infradead.org List-Id: linux-arm-kernel.lists.infradead.org On Tue, Nov 20, 2018 at 03:01:33PM +0000, Robin Murphy wrote: > On 20/11/2018 14:49, Konrad Rzeszutek Wilk wrote: > > On Tue, Nov 20, 2018 at 02:09:52PM +0000, Robin Murphy wrote: > > > With the overflow buffer removed, we no longer have a unique address > > > which is guaranteed not to be a valid DMA target to use as an error > > > token. The DIRECT_MAPPING_ERROR value of 0 tries to at least represent > > > an unlikely DMA target, but unfortunately there are already SWIOTLB > > > users with DMA-able memory at physical address 0 which now gets falsely > > > treated as a mapping failure and leads to all manner of misbehaviour. > > > > > > The best we can do to mitigate that is flip DIRECT_MAPPING_ERROR to the > > > commonly-used all-bits-set value, since the last single byte of memory > > > is by far the least-likely-valid DMA target. > > > > Are all the callers checking for DIRECT_MAPPING_ERROR or is it more of > > a comparison (as in if (!ret)) ? > > dma_direct_map_page() and dma_direct_mapping_error() were already doing the > right thing, and external callers must rely on the latter via > dma_mapping_error() rather than trying to inspect the actual value > themselves, since that varies between implementations anyway. AFAICS all the > new return paths from swiotlb_map_page() are also robust in referencing the > macro explicitly, so I think we're good. Cool! Thank you for checking. Acked-by: Konrad Rzeszutek Wilk Thank you! > > Thanks, > Robin. > > > > Fixes: dff8d6c1ed58 ("swiotlb: remove the overflow buffer")] > > > Reported-by: John Stultz > > > Signed-off-by: Robin Murphy > > > --- > > > include/linux/dma-direct.h | 2 +- > > > 1 file changed, 1 insertion(+), 1 deletion(-) > > > > > > diff --git a/include/linux/dma-direct.h b/include/linux/dma-direct.h > > > index bd73e7a91410..9de9c7ab39d6 100644 > > > --- a/include/linux/dma-direct.h > > > +++ b/include/linux/dma-direct.h > > > @@ -5,7 +5,7 @@ > > > #include > > > #include > > > -#define DIRECT_MAPPING_ERROR 0 > > > +#define DIRECT_MAPPING_ERROR ~(dma_addr_t)0 > > > #ifdef CONFIG_ARCH_HAS_PHYS_TO_DMA > > > #include > > > -- > > > 2.19.1.dirty > > >