[Qemu-devel] Re: How to map PCI memory into the VM without trapping

[Frank Berreth]

Hi there,

this is just an update if you are interested in the outcome. I turns out that my initial assumption that there would be page faults/trapping on the memory pages was false. The reason the throughput is so low is because the memory was mapped non-cached. The VGA driver and the ivshmem driver use pci_ioremap_bar which will *always* map the PCI bar non-cached (even the resourceX_wc).

Changing the driver(s) to use ioremap_cache or ioremap_wc speeds up things quite a bit. I don't know if VGA framebuffer was always mapped this way -- it appears on a real system that usually graphics memory is mapped WC. Mapping it UC would cause quite a performance degradation. This could be the reason for the reported VGA performance drop in another email thread. IMHO, since QEMU emulates VGA, this could be mapped WB.

Thanks,

Frank.

On Mon, May 3, 2010 at 2:07 PM, Frank Berreth <address@hidden> wrote:

Hello,

I am using a modified patch for the ivshmem device that was posted here recently and discovered some performance issues with it. Essentially I measure write performance into the PCI memory that is mapped into the VM as really slow (about 60mb/sec). I assume that is because there is trapping on each memory access (there are no handlers installed for handling read/write access to that memory though). The device shows up in the guest as a UIO device but I also tried accessing other PCI memory like the vga memory (16mb from vga-pci.c) and it has similar performance (through /sys/device/.../resource...).

My question is how I can map that memory into the VM so it doesn't get trapped on in the normal use case. The mappings (registering PCI bar etc) is done as in the patch submitted recently).

Thanks,

Frank.