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Date: Wed, 21 Apr 2010 23:51:43 +0300
From: Avi Kivity <avi@redhat.com>
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Subject: Re: [Qemu-devel] Re: QEMU-KVM and video performance
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To: Jamie Lokier <jamie@shareable.org>
Cc: Gerhard Wiesinger <lists@wiesinger.com>, qemu-devel@nongnu.org, kvm@vger.kernel.org

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.