linux-kernel - Re: [PATCH 1/2] swiotlb: Remove alloc_size argument to swiotlb_tbl_map

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <20240415145023.78e7ce97@meshulam.tesarici.cz>
Date: Mon, 15 Apr 2024 14:50:23 +0200
From: Petr Tesařík <petr@...arici.cz>
To: Michael Kelley <mhklinux@...look.com>
Cc: "mhkelley58@...il.com" <mhkelley58@...il.com>, "robin.murphy@....com"
 <robin.murphy@....com>, "joro@...tes.org" <joro@...tes.org>,
 "will@...nel.org" <will@...nel.org>, "jgross@...e.com" <jgross@...e.com>,
 "sstabellini@...nel.org" <sstabellini@...nel.org>,
 "oleksandr_tyshchenko@...m.com" <oleksandr_tyshchenko@...m.com>,
 "hch@....de" <hch@....de>, "m.szyprowski@...sung.com"
 <m.szyprowski@...sung.com>, "iommu@...ts.linux.dev"
 <iommu@...ts.linux.dev>, "linux-kernel@...r.kernel.org"
 <linux-kernel@...r.kernel.org>, "xen-devel@...ts.xenproject.org"
 <xen-devel@...ts.xenproject.org>, "roberto.sassu@...weicloud.com"
 <roberto.sassu@...weicloud.com>
Subject: Re: [PATCH 1/2] swiotlb: Remove alloc_size argument to
 swiotlb_tbl_map_single()

On Mon, 15 Apr 2024 12:23:22 +0000
Michael Kelley <mhklinux@...look.com> wrote:

> From: Petr Tesařík <petr@...arici.cz> Sent: Monday, April 15, 2024 4:46 AM
> > 
> > Hi Michael,
> > 
> > sorry for taking so long to answer. Yes, there was no agreement on the
> > removal of the "dir" parameter, but I'm not sure it's because of
> > symmetry with swiotlb_sync_*(), because the topic was not really
> > discussed.
> > 
> > The discussion was about the KUnit test suite and whether direction is
> > a property of the bounce buffer or of each sync operation. Since DMA API
> > defines associates each DMA buffer with a direction, the direction
> > parameter passed to swiotlb_sync_*() should match what was passed to
> > swiotlb_tbl_map_single(), because that's how it is used by the generic
> > DMA code. In other words, if the parameter is kept, it should be kept
> > to match dma_map_*().
> > 
> > However, there is also symmetry with swiotlb_tbl_unmap_single(). This
> > function does use the parameter for the final sync. I believe there
> > should be a matching initial sync in swiotlb_tbl_map_single(). In
> > short, the buffer sync for DMA non-coherent devices should be moved from
> > swiotlb_map() to swiotlb_tbl_map_single(). If this sync is not needed,
> > then the caller can (and should) include DMA_ATTR_SKIP_CPU_SYNC in
> > the flags parameter.
> > 
> > To sum it up:
> > 
> > * Do *NOT* remove the "dir" parameter.
> > * Let me send a patch which moves the initial buffer sync.
> >   
> 
> I'm not seeing the need to move the initial buffer sync.  All
> callers of swiotlb_tbl_map_single() already have a subsequent
> check for a non-coherent device, and a call to 
> arch_sync_dma_for_device().  And the Xen code has some 
> special handling that probably shouldn't go in
> swiotlb_tbl_map_single().  Or am I missing something?

Oh, sure, there's nothing broken ATM. It's merely a cleanup. The API is
asymmetric and thus confusing. You get a final sync by default if you
call swiotlb_tbl_unmap_single(), but you don't get an initial sync by
default if you call swiotlb_tbl_map_single(). This is difficult to
remember, so potential new users of the API may incorrectly assume that
an initial sync is done, or that a final sync is not done.

And yes, when moving the code, all current users of
swiotlb_tbl_map_single() should specify DMA_ATTR_SKIP_CPU_SYNC.

Petr T