[Qemu-devel] Use of -machine when running via qemu-system-arm

[Qemu-devel] Use of -machine when running via qemu-system-arm

[Ajay Garg]

Hi All.

Not sure if this is purely an ARM-related question, but I will be
grateful if someone could point to some literature that explains what
difference choosing a  machine makes (when we don't need to have any
such flag for qemu-system-i386 or qemu-system-x86_64).

I am sorry for the noob question.


Thanks and Regards,
Ajay

Re: [Qemu-devel] [Qemu-arm] Use of -machine when running via qemu-system-arm

[Peter Maydell]

On 9 April 2018 at 14:35, Ajay Garg <ajaygargnsit@gmail.com> wrote:
> Hi All.
>
> Not sure if this is purely an ARM-related question, but I will be
> grateful if someone could point to some literature that explains what
> difference choosing a  machine makes (when we don't need to have any
> such flag for qemu-system-i386 or qemu-system-x86_64).

Hi; this is a pretty common question, which we talk about on our
wiki page here:
 https://wiki.qemu.org/Documentation/Platforms/ARM

The underlying reason why you need this on Arm but not on x86
is that for x86 every single piece of x86 hardware is pretty
much identical. For Arm (as part of its history in the embedded
space) the general rule is that every board is different.

thanks
-- PMM

Re: [Qemu-devel] [Qemu-arm] Use of -machine when running via qemu-system-arm

[Ajay Garg]

Hi Peter,

Thanks for the reply ..

On Mon, Apr 9, 2018 at 7:08 PM, Peter Maydell <peter.maydell@linaro.org> wrote:
> On 9 April 2018 at 14:35, Ajay Garg <ajaygargnsit@gmail.com> wrote:
>> Hi All.
>>
>> Not sure if this is purely an ARM-related question, but I will be
>> grateful if someone could point to some literature that explains what
>> difference choosing a  machine makes (when we don't need to have any
>> such flag for qemu-system-i386 or qemu-system-x86_64).
>
> Hi; this is a pretty common question, which we talk about on our
> wiki page here:
>  https://wiki.qemu.org/Documentation/Platforms/ARM

Yep, had already been through it :)

>
> The underlying reason why you need this on Arm but not on x86
> is that for x86 every single piece of x86 hardware is pretty
> much identical. For Arm (as part of its history in the embedded
> space) the general rule is that every board is different.

Yep, perhaps I am needing a more layman-kinda example-cum-explanation
for this, so would be grateful for some help in this regard.


>
> thanks
> -- PMM



-- 
Regards,
Ajay

Re: [Qemu-devel] [Qemu-arm] Use of -machine when running via qemu-system-arm

[Peter Maydell]

On 9 April 2018 at 14:43, Ajay Garg <ajaygargnsit@gmail.com> wrote:
> On Mon, Apr 9, 2018 at 7:08 PM, Peter Maydell <peter.maydell@linaro.org> wrote:
>> On 9 April 2018 at 14:35, Ajay Garg <ajaygargnsit@gmail.com> wrote:
>>> Hi All.
>>>
>>> Not sure if this is purely an ARM-related question, but I will be
>>> grateful if someone could point to some literature that explains what
>>> difference choosing a  machine makes (when we don't need to have any
>>> such flag for qemu-system-i386 or qemu-system-x86_64).
>>
>> Hi; this is a pretty common question, which we talk about on our
>> wiki page here:
>>  https://wiki.qemu.org/Documentation/Platforms/ARM
>
> Yep, had already been through it :)
>
>>
>> The underlying reason why you need this on Arm but not on x86
>> is that for x86 every single piece of x86 hardware is pretty
>> much identical. For Arm (as part of its history in the embedded
>> space) the general rule is that every board is different.
>
> Yep, perhaps I am needing a more layman-kinda example-cum-explanation
> for this, so would be grateful for some help in this regard.

Well, the wiki text is the best explanation I have; perhaps
you could suggest what part of it is confusing, and we could
improve it?

thanks
-- PMM

Re: [Qemu-devel] [Qemu-arm] Use of -machine when running via qemu-system-arm

[Ajay Garg]

Hi Peter.

The link is pretty good, just that I am a complete noob.

Maybe a more concrete example on the following, clearly specifying why
things work on x86 without the need of -machine flag, will make things
crystal clear :

Because ARM systems differ so much and in fundamental ways, typically
operating system or firmware images intended to run on one machine
will not run at all on any other. This is often surprising for new
users who are used to the x86 world where every system looks like a
standard PC. (Once the kernel has booted, most userspace software
cares much less about the detail of the hardware.)


Sorry again for my zero knowledge on the ARM world.




On Mon, Apr 9, 2018 at 7:31 PM, Peter Maydell <peter.maydell@linaro.org> wrote:
> On 9 April 2018 at 14:43, Ajay Garg <ajaygargnsit@gmail.com> wrote:
>> On Mon, Apr 9, 2018 at 7:08 PM, Peter Maydell <peter.maydell@linaro.org> wrote:
>>> On 9 April 2018 at 14:35, Ajay Garg <ajaygargnsit@gmail.com> wrote:
>>>> Hi All.
>>>>
>>>> Not sure if this is purely an ARM-related question, but I will be
>>>> grateful if someone could point to some literature that explains what
>>>> difference choosing a  machine makes (when we don't need to have any
>>>> such flag for qemu-system-i386 or qemu-system-x86_64).
>>>
>>> Hi; this is a pretty common question, which we talk about on our
>>> wiki page here:
>>>  https://wiki.qemu.org/Documentation/Platforms/ARM
>>
>> Yep, had already been through it :)
>>
>>>
>>> The underlying reason why you need this on Arm but not on x86
>>> is that for x86 every single piece of x86 hardware is pretty
>>> much identical. For Arm (as part of its history in the embedded
>>> space) the general rule is that every board is different.
>>
>> Yep, perhaps I am needing a more layman-kinda example-cum-explanation
>> for this, so would be grateful for some help in this regard.
>
> Well, the wiki text is the best explanation I have; perhaps
> you could suggest what part of it is confusing, and we could
> improve it?
>
> thanks
> -- PMM



-- 
Regards,
Ajay

Re: [Qemu-devel] [Qemu-arm] Use of -machine when running via qemu-system-arm

[Peter Maydell]

On 9 April 2018 at 18:41, Ajay Garg <ajaygargnsit@gmail.com> wrote:
> Hi Peter.
>
> The link is pretty good, just that I am a complete noob.
>
> Maybe a more concrete example on the following, clearly specifying why
> things work on x86 without the need of -machine flag, will make things
> crystal clear :
>
> Because ARM systems differ so much and in fundamental ways, typically
> operating system or firmware images intended to run on one machine
> will not run at all on any other. This is often surprising for new
> users who are used to the x86 world where every system looks like a
> standard PC. (Once the kernel has booted, most userspace software
> cares much less about the detail of the hardware.)

For instance, on x86 you can always find the UART for serial
output at IO port 0x3f8, and it's always an 8250 or compatible.
On Arm, the memory address where the UART is will be likely
different for every board. And once you know the address of the
UART, each SoC likely may well have its own implementation,
which has different registers that do different things.
And there's no way to probe for what's there -- you have to
know in advance. So you can't even usefully print bootup
messages without knowing what you're running on.

thanks
-- PMM

Re: [Qemu-devel] [Qemu-arm] Use of -machine when running via qemu-system-arm

[Ajay Garg]

Thanks a ton Peter, that cleared things up a great deal !!!
Thanks again for the quick help !!


Thanks and Regards,
Ajay

On Mon, Apr 9, 2018 at 11:17 PM, Peter Maydell <peter.maydell@linaro.org> wrote:
> On 9 April 2018 at 18:41, Ajay Garg <ajaygargnsit@gmail.com> wrote:
>> Hi Peter.
>>
>> The link is pretty good, just that I am a complete noob.
>>
>> Maybe a more concrete example on the following, clearly specifying why
>> things work on x86 without the need of -machine flag, will make things
>> crystal clear :
>>
>> Because ARM systems differ so much and in fundamental ways, typically
>> operating system or firmware images intended to run on one machine
>> will not run at all on any other. This is often surprising for new
>> users who are used to the x86 world where every system looks like a
>> standard PC. (Once the kernel has booted, most userspace software
>> cares much less about the detail of the hardware.)
>
> For instance, on x86 you can always find the UART for serial
> output at IO port 0x3f8, and it's always an 8250 or compatible.
> On Arm, the memory address where the UART is will be likely
> different for every board. And once you know the address of the
> UART, each SoC likely may well have its own implementation,
> which has different registers that do different things.
> And there's no way to probe for what's there -- you have to
> know in advance. So you can't even usefully print bootup
> messages without knowing what you're running on.
>
> thanks
> -- PMM