Re: [PATCH 1/7] KVM: userspace interface

[Anthony Liguori <aliguori@us.ibm.com>]

Avi Kivity wrote:
> Anthony Liguori wrote:
>>>
>>>  writel(dst_x_reg, x);
>>>  writel(dst_y_reg, y)
>>>  writel(width_reg, w);
>>>  writel(height_reg, h);
>>>  writel(blt_cmd_reg, fill);
>>>
>>> then kvm would cache the first four in a mmap()able memory area and 
>>> only exit to userspace on the fifth.  Userspace would then read the 
>>> cached registers from memory and emulate the command.
>>
>> Letting QEMU do a certain amount of emulation after every transition 
>> would the problem in a more elegant and generic way.
>>
>
> But what amount?  A basic block, or several?
>
> Emulation has its costs.  You need to marshal the registers to and 
> fro.  You need to reset qemu's cached translations.  You need to throw 
> away shadow page tables and qemu's softmmu.  You increase the time 
> spent in single threaded code.

Admittedly still a research topic.  If you're interested in what we're 
doing in Xen, check out:

http://xenbits.xensource.com/ext/xen-unstable-hvm.hg (sorry, xenbits is 
down right now but hopefully it will be fixed quickly).

Regards,

Anthony Liguori