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

["Lin, Ray" <Ray.Lin@lsi.com>]

 

-----Original Message-----
From: Konrad Rzeszutek Wilk [mailto:konrad.wilk@oracle.com] 
Sent: Friday, November 12, 2010 2:34 PM
To: Lin, Ray
Cc: Xen-devel; Dante Cinco
Subject: Re: [Xen-devel] swiotlb=force in Konrad's xen-pcifront-0.8.2 pvops domU kernel with PCI passthrough

>> 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.

OK. Good.
> 
> 
> 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. 

Excellent.
> 
> 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.

<nods> That makes sense.
> 
> 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. 

Oh boy. That complicates it. 

> The driver doesn't set DMA mask on the device to 32 bit.

Is it set then to 45bit?
The driver doesn't use pci_set_dma_mask to set the HW_DMA_MASK.
The tachyon chip should support 64-bit dma transfer even though dma programmable address is limited to 32-bit/45-bit address. The chip should fill the upper address with 0. I'm confirming it with their fae now. In the mean time, I try to manipulate pci_set_dma_mask to see it make the difference or not.