Re: [Xen-devel] PCI passthrough issue

[Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>]

> > Ping tests work whatever can be packet size.
> > 
> > If I understood well what you explain to me, it's now clear that the
> > problem is somewhat Xen related, isn't it ?
> 
> It certainly seems that way. I'm not 100% sure that swiotlb=force will
> have actually made the driver use the swiotlb, it may just have forced

It should have.

> the swiotlb to be allocated. Konrad?

Both. It would allocate it and force all DMA operations to go through
the bounce buffer.

Let me take a look at the driver itself. Ah, so I had a similar card: 3c59x
which hit the same type of failure way back in 2.6.31 time. It had the
pci_dma_sync_single_for_cpu(.. PCI_DMA_FROMDEVICE) turned around so it would 
never
copy properly. But in my tree (2.6.38) the sky driver looks to be doing just
right.. It has PCI_DMA_FROMDEVICE..

Well this looks odd:

2330         skb = netdev_alloc_skb_ip_align(sky2->netdev, length);
2331         if (likely(skb)) {
2332                 pci_dma_sync_single_for_cpu(sky2->hw->pdev, re->data_addr,
2333                                             length, PCI_DMA_FROMDEVICE); 
=======>             skb_copy_from_linear_data(re->skb, skb->data, length); 
<====
2335                 skb->ip_summed = re->skb->ip_summed;
2336                 skb->csum = re->skb->csum;
2337                 pci_dma_sync_single_for_device(sky2->hw->pdev, 
re->data_addr,
2338                                                length, 
PCI_DMA_FROMDEVICE); 
2339                 re->skb->ip_summed = CHECKSUM_NONE;
2340                 skb_put(skb, length);
2341         }

It copies from skb->data (just allocated) to re->skb (seems that the 
re->skb->data
has been earlier DMA-mapped).

But there is no data in skb->data as we just allocated it? Shouldn't this
be the other way around? Like this:


diff --git a/drivers/net/sky2.c b/drivers/net/sky2.c
index 7d85a38..37c0631 100644
--- a/drivers/net/sky2.c
+++ b/drivers/net/sky2.c
@@ -2331,7 +2331,7 @@ static struct sk_buff *receive_copy(struct sky2_port 
*sky2,
        if (likely(skb)) {
                pci_dma_sync_single_for_cpu(sky2->hw->pdev, re->data_addr,
                                            length, PCI_DMA_FROMDEVICE);
-               skb_copy_from_linear_data(re->skb, skb->data, length);
+               skb_copy_from_linear_data(skb, re->skb->data, length);
                skb->ip_summed = re->skb->ip_summed;
                skb->csum = re->skb->csum;
                pci_dma_sync_single_for_device(sky2->hw->pdev, re->data_addr,

> 
> > I'll be able to continue any tests you want tomorrow.
> > Do you still need me to compile domU kernel with "CONFIG_DMA_API_DEBUG"
> > enabled ?
> 
> Yes please.
> 
> > If so, what tools will I need to get debug informations ?
> 
> Uh, Konrad?

Here, try this patch. Run it under baremetal and Xen and we can compare the
outputs .. but I think the issue here is not off mis-using the DMA API but
rather a driver bug.


/*
 *
 * This program is free software; you can redistribute it and/or modify
 * it under the terms of the GNU General Public License v2.0 as published by
 * the Free Software Foundation
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 */

#include <linux/module.h>
#include <linux/string.h>
#include <linux/types.h>
#include <linux/init.h>
#include <linux/stat.h>
#include <linux/err.h>
#include <linux/ctype.h>
#include <linux/slab.h>
#include <linux/limits.h>
#include <linux/device.h>
#include <linux/pci.h>
#include <linux/blkdev.h>
#include <linux/device.h>

#include <linux/init.h>
#include <linux/mm.h>
#include <linux/fcntl.h>
#include <linux/slab.h>
#include <linux/kmod.h>
#include <linux/major.h>
#include <linux/smp_lock.h>
#include <linux/highmem.h>
#include <linux/blkdev.h>
#include <linux/module.h>
#include <linux/blkpg.h>
#include <linux/buffer_head.h>
#include <linux/mpage.h>
#include <linux/mount.h>
#include <linux/uio.h>
#include <linux/namei.h>
#include <asm/uaccess.h>

#include <linux/pagemap.h>
#include <linux/pagevec.h>

#include <linux/dma-debug.h>

#define DUMP_DMA_FUN  "0.1"

MODULE_AUTHOR("Konrad Rzeszutek Wilk <konrad@virtualiron>");
MODULE_DESCRIPTION("dump dma");
MODULE_LICENSE("GPL");
MODULE_VERSION(DUMP_DMA_FUN);

static int __init dump_dma_init(void)
{
        debug_dma_dump_mappings(NULL);
        return 0;
}

static void __exit dump_dma_exit(void)
{
}

module_init(dump_dma_init);
module_exit(dump_dma_exit);
> 
> Ian.
> 
> 
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