RE: [Xen-devel] swiotlb=force in Konrad's xen-pcifront-0.8.2 pvops domU kernel with PCI passthrough

["Lin, Ray" <Ray.Lin@xxxxxxx>]

-----Original Message-----
From: Konrad Rzeszutek Wilk [mailto:konrad.wilk@xxxxxxxxxx] 
Sent: Friday, November 12, 2010 8:56 AM
To: Lin, Ray
Cc: Dante Cinco; Xen-devel
Subject: Re: [Xen-devel] swiotlb=force in Konrad's xen-pcifront-0.8.2 pvops 
domU kernel with PCI passthrough

> That does not sound right. You should be able to use the PCI passthrough 
> without the IOMMU. Since it is an interrupt issue it sounds like that you are 
> using x2APIC and that is enabled without the IOMMU.
> Had you tried disabling IOMMU and x2apic? (this is all on the 
> hypervisor line?)
> 
> Konrad,
> It's unlikely the interrupt issue but DMA issue. Here is the sequence 
> how the tachyon device generates the DMA/interrupts,
> - the tachyon device does the DMA to update the memory which indicates the 
> source of interrupt.
> - After the DMA is done, the tachyon device trigger an interrupt.
> - The interrupt service routine of software driver is invoked due to 
> the interrupt
> - The interrupt service routine checks the source of interrupts by examining 
> the memory which is supposed to be updated by previous DMA.
> - Even though the interrupt happens, the driver code can't find the source of 
> interrupt since the DMA doesn't work properly.

That sounds like the tachyon device is updating the wrong memory location. How 
are you programming the memory location where thetachyon device is suppose to 
touch? Are you using the value from pci_map_page or are you using virt_to_phys? 
The virt_to_phys should be different from the pci_map_page.. unless you 
allocated a coherent DMA pool using pci_alloc_coherent in which case the 
virt_to_phys() values for that pool should be the right MFNs.

Our driver uses pci_map_single to get the physical addr to program the chip.


One way you can figure this is doing something like this to make sure you got 
the right MFN:

add these two:
#include <xen/page.h>
#include <asm/xen/page.h>

          phys_addr_t phys = page_to_phys(mem->pages[i]);
+               if (xen_pv_domain()) {
+                       phys_addr_t xen_phys = PFN_PHYS(pfn_to_mfn(
+                                       page_to_pfn(mem->pages[i])));
+                       if (phys != xen_phys) {
+                               printk(KERN_ERR "Fixing up: (0x%lx->0x%lx)." \
+                                       " CODE UNTESTED!\n",
+                                       (unsigned long)phys,
+                                       (unsigned long)xen_phys);
+                               WARN_ON_ONCE(phys != xen_phys);
+                               phys = xen_phys;
+                       }
+               }
        and using the 'phys' value from now.



If this sounds like black magic, here is a short writeup 
http://wiki.xensource.com/xenwiki/XenPVOPSDRM

look at "Why those patches" section.

Lastly, are you using unsigned long for or the phys_addr_t typedefs?

The driver uses dma_addr_t for physical address. 

The more I think about your problem the more it sounds like a truncating issue. 
You said that it works just right (albeit slow) if you use 'swiotlb=force'. The 
slowness could be due to not using the pci_sync_* APIs to sync the DMA 
buffers.. But irregardless using bounce buffers will slow the DMA operations 
down.

The driver do use pci_dma_sync_single_for_cpu or pci_dma_sync_single_for_device 
to sync the DMA buffers. Without these syncs, the driver would not work at all.

Using the bounce buffers limits the DMA operations to under 32-bit. So could it 
be that you are using some casting macro that casts a PFN to unsigned long or 
vice-versa and we end up truncating it to 32-bit? (I've seen this issue 
actually with InfiniBand drivers back in RHEL5 days..). Lastly, do you set your 
DMA mask on the device to 32BIT?

The tachyon chip supports both 32-bit & 45-bit dma. Some features need to set 
32-bit physical addr to chip. Others need to set 45-bit physical addr to chip. 
The driver doesn't set DMA mask on the device to 32 bit.

I'm looking at the driver code to see anything wrong or not. We appreciate your 
help Konrad.


_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxxxxxxxx
http://lists.xensource.com/xen-devel