Re: [Xen-devel] [PATCH v2 5/6] xen/arm: traps: Avoid unnecessary VA -> IPA translation in abort handlers

[Julien Grall <julien.grall@xxxxxxx>]

On 17/08/16 21:08, Shanker Donthineni wrote:
> On 08/17/2016 06:11 AM, Julien Grall wrote:
> > On 17/08/16 03:19, Shanker Donthineni wrote:
> > > Hi Julien,
> > Hello Shanker,
> >
> > > On 07/27/2016 12:09 PM, Julien Grall wrote:
> > > > Translating a VA to a IPA is expensive. Currently, Xen is assuming that
> > > > HPFAR_EL2 is only valid when the stage-2 data/instruction abort
> > > > happened
> > > > during a translation table walk of a first stage translation (i.e S1PTW
> > > > is set).
> > > >
> > > > However, based on the ARM ARM (D7.2.34 in DDI 0487A.j), the register is
> > > > also valid when the data/instruction abort occured for a translation
> > > > fault.
> > > >
> > > > With this change, the VA -> IPA translation will only happen for
> > > > permission faults that are not related to a translation table of a
> > > > first stage translation.
> > > >
> > > > Signed-off-by: Julien Grall <julien.grall@xxxxxxx>
> > > >
> > > > ---
> > > >      Changes in v2:
> > > >          - Use fsc in the switch in do_trap_data_abort_guest
> > > > ---
> > > >   xen/arch/arm/traps.c | 24 ++++++++++++++++++++----
> > > >   1 file changed, 20 insertions(+), 4 deletions(-)
> > > >
> > > > diff --git a/xen/arch/arm/traps.c b/xen/arch/arm/traps.c
> > > > index ea105f2..83a30fa 100644
> > > > --- a/xen/arch/arm/traps.c
> > > > +++ b/xen/arch/arm/traps.c
> > > > @@ -2382,13 +2382,28 @@ static inline paddr_t get_faulting_ipa(vaddr_t
> > > > gva)
> > > >       return ipa;
> > > >   }
> > > >
> > > > +static inline bool hpfar_is_valid(bool s1ptw, uint8_t fsc)
> > > > +{
> > > > +    /*
> > > > +     * HPFAR is valid if one of the following cases are true:
> > > > +     *  1. the stage 2 fault happen during a stage 1 page table walk
> > > > +     *  (the bit ESR_EL2.S1PTW is set)
> > > > +     *  2. the fault was due to a translation fault
> > > > +     *
> > > > +     * Note that technically HPFAR is valid for other cases, but they
> > > > +     * are currently not supported by Xen.
> > > > +     */
> > > > +    return s1ptw || (fsc == FSC_FLT_TRANS);
> > > Yes, XEN is not supporting the stage 2 access flag but we should handle
> > > a stage 2 address size fault.
> > The function hpfar_is_valid indicates whether the register HPFAR is
> > valid. If the function returns false, Xen will use the hardware do the
> > translation.
> >
> > It will only lead to a waste of cycle but this is fine as the address
> > size fault is not a hot path for now.
> >
> > > I think we should do some thing like to below to match ARM ARM.
> > >
> > > return s1ptw || (fsc != FSC_FLT_PERM);
> > This does not match the ARM ARM, with this check you consider that
> > HPFAR will be valid for all the fault but permission ones which is not
> > true.
> >
> > I purposefully choose a white list because it is safer to use the
> > hardware to do the translation more often than the invert.
> >
> > So I don't see why we should handle stage 2 access flag with the
> > current Xen. If you still disagree, please explain why with a concrete
> > example.
> Agree with you, I have suggested the above change because I saw the same
> check in Linux KVM.
> As per ARM ARM, it should be 'return s1ptw || (fsc == FSC_FLT_TRANS) ||
> (fsc == FSC_FLT_ACCESS) || (fsc == 0x00)';
Feel free to send a patch for this explaining why we would need to have 
the full check. But I don't think it is worth to test every case and 
therefore increasing the number of cycles of this function for faults we 
don't care so far.
Regards,

--
Julien Grall

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