Re: [Qemu-devel] Re: QEMU-KVM and video performance

Avi Kivity
2010-04-21T20:51:51Z

On 04/21/2010 09:39 PM, Jamie Lokier wrote:
> Avi Kivity wrote:
>
>> Writes to vga in 16-color mode don't change set a memory location to a
>> value, instead they change multiple memory locations.
>>
> While code is just writing to the VGA memory, not reading(*) and not
> touching the VGA I/O register that control the write latches, is it
> possible in principle to swizzle the format around in memory to make
> regular writes work?
>

Not in software. We can map pages, not cross address lines.

> (*) Reading should be ok for some settings of the write latches, I
> think.
>
> I wonder if guests of interest behave like that.
>

Guests that use 16 color vga are usually of little interest.

>>> Is this a case where TCG would run significantly faster for code blocks
>>> that have been detected to access the VGA memory?
>>>
>> Yes.
>>
> $ date
> Wed Apr 21 19:37:38 2015
> $ modprobe ktcg
>

That's why the vmware software vmm was faster than the hardware vmm for
the initial iterations of vmx.

--
I have a truly marvellous patch that fixes the bug which this
signature is too narrow to contain.