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Date: Thu, 10 Mar 2011 22:52:25 +0100
From: Carl-Daniel Hailfinger <c-d.hailfinger.devel.2006@gmx.net>
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Subject: Re: [Qemu-devel] Re: RFC: emulation of system flash
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To: Jordan Justen <jljusten@gmail.com>
Cc: Gleb Natapov <gleb@redhat.com>, Stefan Hajnoczi <stefanha@gmail.com>, qemu-devel <qemu-devel@nongnu.org>, Michal Suchanek <hramrach@centrum.cz>, Kevin O'Connor <kevin@koconnor.net>, Avi Kivity <avi@redhat.com>

Auf 10.03.2011 19:43, Jordan Justen schrieb:
> On Thu, Mar 10, 2011 at 01:10, Avi Kivity <avi@redhat.com> wrote:
>   
>> On 03/10/2011 06:51 AM, Jordan Justen wrote:
>>     
>>> http://wiki.qemu.org/Features/System_Flash
>>>
>>>       
>> - make the programming interface the same as an existing device
>>     
> How strongly do you feel about this?
>
> For one thing, real devices are not as flexible as QEMU for flash
> sizes.  QEMU allows for any 64kb multiple bios size.  Real world
> devices generally only support powers of 2 sizes.
>
> Firmware hub devices are somewhat simplistic to emulate, but I think
> they use 16MB of address space, while only providing <= 1MB of flash
> storage.
>   

Up to 4 MB on real hardware, and if you use Parallel flash devices,
there is no limit at all (except cost). The software interface is
identical for read/write/erase/probe.


> SPI devices are available in many sizes, so it might be possible to
> choose a 16MB device to emulate.  But, it would be a lot more complex
> to emulate as it would it involve emulating an SPI contoller + the
> device.
>   

I have written a SPI flash chip emulator (it emulates 3 different
real-world SPI flash chips) and am willing to contribute it to Qemu if
there is interest. The code is pretty small, and adding a SPI host
controller emulator should be a few lines of code extra. Not a big problem.


> I thought this might be a case where deviation from real hardware
> emulation could better serve the VM's needs.
>   

If we have to write the code anyway, and if it can work just fine with
current KVM/Qemu, is there a reason not to use the same interface as
real hardware?

Regards,
Carl-Daniel

-- 
http://www.hailfinger.org/