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

[Paul Durrant <Paul.Durrant@xxxxxxxxxx>]

> -----Original Message-----
> From: Andrew Cooper [mailto:andrew.cooper3@xxxxxxxxxx]
> Sent: 25 June 2015 14:38
> To: Paul Durrant; Jan Beulich
> Cc: xen-devel@xxxxxxxxxxxxxxxxxxxx; Keir (Xen.org)
> Subject: Re: [PATCH v4 07/17] x86/hvm: add length to mmio check op
> 
> On 25/06/15 14:36, Paul Durrant wrote:
> >> -----Original Message-----
> >> From: Andrew Cooper [mailto:andrew.cooper3@xxxxxxxxxx]
> >> Sent: 25 June 2015 14:34
> >> To: Jan Beulich
> >> 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 13:46, Jan Beulich wrote:
> >>>>>> 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.
> >> I discussed this with Paul over lunch.  I had not considered how IO gets
> >> forwarded to the device model for shared implementations.
> >>
> >> Is it reasonable to split a straddled access and direct the halves at
> >> different handlers? This is not in line with how other hardware behaves
> >> (PCIe will reject any straddled access).  Furthermore, given small MMIO
> >> regions and larger registers, there is no guarantee that a single split
> >> will suffice.
> >>
> >> I see in the other thread going on that a domain_crash() is deemed ok
> >> for now, which is fine my me.
> >>
> > I think that also allows me to simplfy the patch since I don't have to 
> > modify
> the mmio_check op any more. I simply call it once for the first byte of the
> access and, if it accepts, verify that it also accepts the last byte of the 
> access.
> 
> At that point, I would say it would be easier to modify the claim check
> to return "yes/straddled/no" rather than calling it twice.

That's excessive code churn, I think. The check functions are generally cheap 
and the second call is only made if the first accepts.

  Paul

> 
> ~Andrew

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