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Re: [Qemu-devel] [PATCH RFC] ps2: set the keybord output buffer size as

From: Gerd Hoffmann
Subject: Re: [Qemu-devel] [PATCH RFC] ps2: set the keybord output buffer size as the same as kernel
Date: Wed, 23 Apr 2014 11:37:57 +0200

On Mi, 2014-04-23 at 08:06 +0000, Gonglei (Arei) wrote:
> Hi, Gerd and Juan.
> Thanks for your guides about the confuse live migration about changing the 
> keyboard buffer size. 
> According your suggestion, I got two solutions to address the issue:
> - Keep the data array 256 bytes long, change the rptr/wptr/count/data array 
> at post_load(), both
>  Ps/2 keyboard and mouse. This solution can be compatible with older qemu 
> versions, which can 
>  do live migration each other.

> -Change the data array to 16 bytes, still save as PS2_QUEUE_SIZE. Reset the 
> rptr/wptr/count at 
> post_load(), both ps/2 keyboard and mouse. Add VMSTATE_UNUSED(256-16) in 
> struct vmstate_ps2_common.
> This solution just save the 16 bytes buffer and drop the rest, So we can't 
> migrate vm to older qemu versions.
> But migration from old qemu to new qemu is ok.

I think long term we want #2, but using #1 as step inbetween for a few
releases until 16 byte buffer size is widely used will might be useful.

Completely separate question:  Have you figured what the root cause for
the bug is?  While wading through the code I've figured the queue size
isn't (directly) exposed to the guest.  And it's used for both mouse and
kbd.  For the kbd 16 byte buffer is probably ok, but for mouse events we
probably want keep some more space to buffer events ...


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