Re: direct mmio for passthrough - kernel part

[Avi Kivity <avi@qumranet.com>]

benami@il.ibm.com wrote:
> This patch for PCI passthrough devices enables a guest to access a device's
> memory mapped I/O regions directly, without requiring the host to trap and
> emulate every MMIO access. 
>
> Updated from last version: we create a memory slot for each MMIO region of the
> guest's devices, and then use the /sys/bus/pci/.../resource# mapping to find the
> hfn for that MMIO region. The kernel part and the userspace part of this
> patchset apply to Amit's pv-dma tree.  Tested on a Lenovo M57p with an e1000 NIC
> assigned directly to an FC8 guest.
>
> Comments are appreciated. 
>   

I see no support for cache attributes in the page attributes table or 
mtrr.  I guess for most devices this will work (as they will be set as 
uncachable by the mtrrs), but for display cards we'd need to set the 
vram as write-combining to get reasonable performance.  This requires 
mtrr and pat emulation in kvm so we detect the guest's intentions.

-- 
Any sufficiently difficult bug is indistinguishable from a feature.


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