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Re: [Qemu-devel] [PATCH 15/30] memory: add address_space_valid

From: Peter Maydell
Subject: Re: [Qemu-devel] [PATCH 15/30] memory: add address_space_valid
Date: Sat, 25 May 2013 10:23:32 +0100

On 25 May 2013 04:44, David Gibson <address@hidden> wrote:
> On Fri, May 24, 2013 at 11:52:17AM +0100, Peter Maydell wrote:
>> So when *is* it a good idea to use this API? In real
>> hardware you don't usually get a "tell me whether this
>> access would succeed if I did it" bus operation -- you
>> just do the operation and the memory transaction either
>> succeeds or it doesn't. Are we modelling something that
>> really exists in hardware on spapr here?
> So, as a general rule, you should just attempt the access and handle
> failures - this is a bad interface.  The reason I added it, however,
> is that the PAPR specification mandates that the virtual LAN pre-check
> various buffers when they're registered, and return specific errors if
> they're not mapped to valid memory.  Since we have nothing to read or
> write at that point, adding this interface was the only way I could
> see to implement that requirement.

Would it work to just read and throw away the result of the read?

> Or... a bit more charitably: You should always handle failures at the
> point of read or write, but using this interface can give you an
> earlier, and therefore potentially easier to analyze, error in the
> more common failure cases, even if there are more complex cases where
> the pre-check succeeds but the read/write still fails later.

There's also the converse case where the pre-check fails but
doing the operation at the proper time would succeed, in
which case where we're modelling real hardware we would
be doing it wrong. So the set of cases where it's OK to
pre-check seems a bit limited.

-- PMM

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