Re: [Xen-devel] [PATCH v4 07/17] x86/hvm: add length to mmio check op

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 25.06.15 at 14:21, <andrew.cooper3@xxxxxxxxxx> wrote:
> On 24/06/15 12:24, Paul Durrant wrote:
>> When memory mapped I/O is range checked by internal handlers, the length
>> of the access should be taken into account.
>>
>> Signed-off-by: Paul Durrant <paul.durrant@xxxxxxxxxx>
>> Cc: Keir Fraser <keir@xxxxxxx>
>> Cc: Jan Beulich <jbeulich@xxxxxxxx>
>> Cc: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
>>
> 
> For what purpose?  The length of the access doesn't affect which handler
> should accept the IO.
> 
> This length check now causes an MMIO handler to not claim an access
> which straddles the upper boundary.
> 
> It is probably fine to terminate such an access early, but it isn't fine
> to pass such a straddled access to the default ioreq server.

No, without involving the length in the check we can end up with
check() saying "Yes, mine" but read() or write() saying "Not me".
What I would agree with is for the generic handler to split the
access if the first byte fits, but the final byte doesn't.

Jan


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