[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] Re: [Xen-devel] [PATCH] xen/swiotlb: Exchange to contiguous memory for map_sg hook
On Wed, Dec 12, 2012 at 01:03:55AM +0000, Xu, Dongxiao wrote: > > -----Original Message----- > > From: Konrad Rzeszutek Wilk [mailto:konrad.wilk@xxxxxxxxxx] > > Sent: Wednesday, December 12, 2012 1:07 AM > > To: Xu, Dongxiao > > Cc: xen-devel@xxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx > > Subject: Re: [PATCH] xen/swiotlb: Exchange to contiguous memory for map_sg > > hook > > > > On Tue, Dec 11, 2012 at 06:39:35AM +0000, Xu, Dongxiao wrote: > > > > -----Original Message----- > > > > From: Konrad Rzeszutek Wilk [mailto:konrad.wilk@xxxxxxxxxx] > > > > Sent: Friday, December 07, 2012 10:09 PM > > > > To: Xu, Dongxiao > > > > Cc: xen-devel@xxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx > > > > Subject: Re: [PATCH] xen/swiotlb: Exchange to contiguous memory for > > > > map_sg hook > > > > > > > > On Thu, Dec 06, 2012 at 09:08:42PM +0800, Dongxiao Xu wrote: > > > > > While mapping sg buffers, checking to cross page DMA buffer is > > > > > also needed. If the guest DMA buffer crosses page boundary, Xen > > > > > should exchange contiguous memory for it. > > > > > > > > So this is when we cross those 2MB contingous swatch of buffers. > > > > Wouldn't we get the same problem with the 'map_page' call? If the > > > > driver tried to map say a 4MB DMA region? > > > > > > Yes, it also needs such check, as I just replied to Jan's mail. > > > > > > > > > > > What if this check was done in the routines that provide the > > > > software static buffers and there try to provide a nice DMA contingous > > swatch of pages? > > > > > > Yes, this approach also came to our mind, which needs to modify the driver > > itself. > > > If so, it requires driver not using such static buffers (e.g., from > > > kmalloc) to do > > DMA even if the buffer is continuous in native. > > > > I am bit loss here. > > > > Is the issue you found only with drivers that do not use DMA API? > > Can you perhaps point me to the code that triggered this fix in the first > > place? > > Yes, we met this issue on a specific SAS device/driver, and it calls into > libata-core code, you can refer to function ata_dev_read_id() called from > ata_dev_reread_id() in drivers/ata/libata-core.c. > > In the above function, the target buffer is (void > *)dev->link->ap->sector_buf, which is 512 bytes static buffer and > unfortunately it across the page boundary. Hm, that looks like a DMA API violation. If you ran the code with CONFIG_DEBUG_DMA_API did it complain about this? I recall the floppy driver doing something similar and the Debug DMA API was quite verbose in pointing this out. > > > > Is this acceptable by kernel/driver upstream? > > > > I am still not completely clear on what you had in mind. The one method I > > thought about that might help in this is to have Xen-SWIOTLB track which > > memory ranges were exchanged (so xen_swiotlb_fixup would save the *buf > > and the size for each call to xen_create_contiguous_region in a list or > > array). > > > > When xen_swiotlb_map/xen_swiotlb_map_sg_attrs care called they would > > consult said array/list to see if the region they retrieved crosses said 2MB > > chunks. If so.. and here I am unsure of what would be the best way to > > proceed. > > We thought we can solve the issue in several ways: > > 1) Like the previous patch I sent out, we check the DMA region in > xen_swiotlb_map_page() and xen_swiotlb_map_sg_attr(), and if DMA region > crosses page boundary, we exchange the memory and copy the content. However > it has race condition that when copying the memory content (we introduced two > memory copies in the patch), some other code may also visit the page, which > may encounter incorrect values. > 2) Mostly the same as item 1), the only difference is that we put the memory > content copy inside Xen hypervisor but not in Dom0. This requires we add > certain flags to indicating memory moving in the XENMEM_exchange hypercall. > 3) As you also mentioned, this is not a common case, it is only triggered by > some specific devices/drivers. we can fix it in certain driver to avoid DMA > into static buffers. Like (void *)dev->link->ap->sector_buf in the above > case. But I am not sure whether it is acceptable by kernel/driver upstream. > > Thanks, > Dongxiao > > _______________________________________________ Xen-devel mailing list Xen-devel@xxxxxxxxxxxxx http://lists.xen.org/xen-devel
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