Re: [Xen-devel] [PATCH 04/10] xen/arm: vgic-v3: Don't check the size when we ignore the write/read as zero

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

On Mon, 2015-01-19 at 16:29 +0000, Julien Grall wrote:
> In general, it's not necessary/important to check the size.

Only if the docs say this register can be accessed by a partial
read/write, or if it is implementation defined what the result would be
(and RAZ/WI is within the set of allowable actions).

Do you have a reference for the behaviour of GICR accesses which aren't
of the register's natural size?

>  It's better
> to log it to let know the guest that its access will have no effect.
> 
> Note: On debug build it may happen to see some of these messages during
> domain boot.

We should only print if the guest has done something wrong, and reading
a RAZ register (or one which we have implemented that way) is not
inherently wrong.

IOW read_as_zero* should be silent, and a different code path used for
"guest did something wrong".

IOW I think the current distinction between bad_width and read_as_zero*
is correct currently, although perhaps the goto's which target them need
adjusting in some cases.

Perhaps you want to add a read_as_zero_32 which has the check, making
read_as_zero accept either 32- or 64-bit accesses, and goto the correct
label for each register?

Ian.


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