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

[Paul Durrant <Paul.Durrant@xxxxxxxxxx>]

> -----Original Message-----
> From: Jan Beulich [mailto:JBeulich@xxxxxxxx]
> Sent: 25 June 2015 13:46
> To: Andrew Cooper
> Cc: Paul Durrant; xen-devel@xxxxxxxxxxxxxxxxxxxx; Keir (Xen.org)
> Subject: Re: [PATCH v4 07/17] x86/hvm: add length to mmio check op
> 
> >>> 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.

That's not a trivial thing to do. Could we, for now, have the check claim based 
on address but domain_crash() if length does not fit?

  Paul

> 
> Jan


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