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Re: [Qemu-devel] [PATCH v3 0/7] Let boards state maximum RAM limits in Q

From: Peter Maydell
Subject: Re: [Qemu-devel] [PATCH v3 0/7] Let boards state maximum RAM limits in QEMUMachine struct
Date: Mon, 4 Apr 2011 15:42:27 +0100

On 4 April 2011 15:29, Jes Sorensen <address@hidden> wrote:
> On 03/30/11 16:07, Peter Maydell wrote:
>> On 30 March 2011 14:56, Anthony Liguori <address@hidden> wrote:
>>> On 03/30/2011 08:22 AM, Peter Maydell wrote:
>>>> Not really, typically they're just filled up in some particular
>>>> order (main RAM in one place and expansion RAM elsewhere).
>>>> Since the board init function is only passed a single "ram_size"
>>>> parameter that's all you can do anyhow.
>>> FWIW, I don't think any static data is going to be perfect here.  A lot of
>>> boards have strict requirements on ram_size based on plausible combinations
>>> of DIMMs.  Arbitrary values up to ram_size is not good enough.
>>> ram_size ought to be viewed as a hint, not a mechanism to allow common code
>>> to completely validate the passed in ram size parameter.  It's still up to
>>> the board to validate that the given ram size makes sense.
>> Yes, I agree, so we shouldn't try to specify some complicated
>> set of static data that still won't be good enough.
>> I'm trying to make it easy for boards to avoid crashing horribly
>> when the user passes a bad value; that's all.
> If you don't validate properly, is there really a point in introducing
> that value anyway? From what you write, it sounds like it can still fail
> for some limits of the memory valid if the config is wrong?

For the boards I care about (the ARM ones), the only validation
requirement is that we don't allow the user to specify so much
ram that we overlap physical RAM with I/O space. So ram_size is
good enough. For the sun4m boards we can assume that the only
validation they need is a ram_size check, because that's all they
do at the moment and nobody's complaining that I know of.

As far as I know Anthony is suggesting that some boards might in theory
have stricter memory size requirements. I agree that this is a possibility,
but if you have a rare board which has an idiosyncratic requirement then
the correct way to handle that is to add extra checks in the board init
functions. I don't see a huge queue of people with patches to add
complex checks to board init functions that would indicate that we need
a generic mechanism for handling this. What we do have is simple ram_size
limit checks (in sun4m and we need them for the arm devboards) -- which
is a demonstrated need that justifies the common code framework.

> It still seems to me it would be better to have the boards present a
> table of valid memory ranges so we can do a proper validation of the valud?

If you have a concrete example of multiple boards which we currently model
and which require this level of flexibility to avoid odd misbehaviour trying
to run a guest, then please point them out and I'll look at expanding the
patch to cover their requirements.

If this is just a theoretical issue, then I think we should only add the
extra generic framework code if and when we turn out to need it.

-- PMM

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