Re: [Xen-devel] [PATCH 6/7] vm-event/arm: move hvm_event_cr->common vm_event_monitor_cr

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 21.06.16 at 17:22, <tamas@xxxxxxxxxxxxx> wrote:
> On Jun 21, 2016 01:20, "Jan Beulich" <JBeulich@xxxxxxxx> wrote:
>>
>> >>> On 21.06.16 at 09:08, <czuzu@xxxxxxxxxxxxxxx> wrote:
>> > On 6/17/2016 11:25 AM, Corneliu ZUZU wrote:
>> >> On 6/16/2016 6:16 PM, Jan Beulich wrote:
>> >>>>>> On 16.06.16 at 16:12, <czuzu@xxxxxxxxxxxxxxx> wrote:
>> >>>> Prepare for ARM implementation of control-register write vm-events
>> >>>> by moving
>> >>>> X86-specific hvm_event_cr to the common-side.
>> >>>>
>> >>>> Signed-off-by: Corneliu ZUZU <czuzu@xxxxxxxxxxxxxxx>
>> >>>> ---
>> >>>>   xen/arch/x86/hvm/event.c        | 30 ------------------------------
>> >>>>   xen/arch/x86/hvm/hvm.c          |  2 +-
>> >>>>   xen/arch/x86/monitor.c          | 37
>> >>>> -------------------------------------
>> >>>>   xen/arch/x86/vm_event.c         |  2 +-
>> >>>>   xen/common/monitor.c            | 40
>> >>>> ++++++++++++++++++++++++++++++++++++++++
>> >>>>   xen/common/vm_event.c           | 31
> +++++++++++++++++++++++++++++++
>> >>>>   xen/include/asm-x86/hvm/event.h | 13 ++++---------
>> >>>>   xen/include/asm-x86/monitor.h   |  2 --
>> >>>>   xen/include/xen/monitor.h       |  4 ++++
>> >>>>   xen/include/xen/vm_event.h      | 10 ++++++++++
>> >>>>   10 files changed, 91 insertions(+), 80 deletions(-)
>> >>> Considering that there's no ARM file getting altered here at all,
>> >>> mentioning ARM in the subject is a little odd.
>> >>
>> >> This patch and the following one should be meld together.
>> >> I only split them to ease reviewing, sorry I forgot to mention that in
>> >> the cover letter.
>> >>
>> >>>
>> >>>> --- a/xen/common/monitor.c
>> >>>> +++ b/xen/common/monitor.c
>> >>>> @@ -62,6 +62,46 @@ int monitor_domctl(struct domain *d, struct
>> >>>> xen_domctl_monitor_op *mop)
>> >>>>         switch ( mop->event )
>> >>>>       {
>> >>>> +#if CONFIG_X86
>> >>> #ifdef please.
>> >> Ack.
>> >>>> +    case XEN_DOMCTL_MONITOR_EVENT_WRITE_CTRLREG:
>> >>>> +    {
>> >>>> +        struct arch_domain *ad = &d->arch;
>> >>> Peeking into the next patch I see that this stays there. Common code,
>> >>> however, shouldn't access ->arch sub-structures - respective fields
>> >>> should be moved out.
>> >>
>> >> Then we need to find a resolution that avoids code duplication.
>> >> The code is the same, but those bits that are currently on the arch
>> >> side (arch.monitor.write_ctrlreg_*) cannot be moved to common as they
>> >> are, since their -number- might differ from arch-to-arch.
>> >> But we could:
>> >> - in public/vm_event.h, besides the VM_EVENT_X86_* and VM_EVENT_ARM_*
>> >> defines (wcr index), also have
>> >>     #define VM_EVENT_X86_CR_COUNT        4
>> >>     #define VM_EVENT_ARM_CR_COUNT      4
>> >> - move the 3 write_ctrlreg_{enabled,sync,onchangeonly} fields from
>> >> arch_domain to domain (common) and make them 8-bits wide each for now
>> >> (widened more in the future if the need arises)
>> >> - let monitor_ctrlreg_bitmask() macro to be architecture-dependent and
>> >> use the introduced VM_EVENT_<arch>_CR_COUNT
>> >>
>> >> Tamas, we also talked on this matter @ some point (when I sent the
>> >> patches that moved vm-event parts to common). What do you think of the
>> >> above?
>>
>> I don't really care about the specifics, my only request is what I
>> already voiced: Common code should not access arch-specific
>> fields. Having the field to hold the control register bits common,
>> but the definitions for the individual bits arch-specific is perfectly
>> fine for this (assuming that these per-arch definitions then again
>> don't get used in common code).
> 
> As Jan says it would be fine to have the holder field on the common struct
> but IMHO it wouldn't be easier to follow the logic that way and the only
> benefit is reducing code duplication a little bit. I think for now it is
> acceptable to just rather have some code duplication.

Code duplication isn't the main issue here. Inviting further
conceptually wrong code additions (accessing per-arch fields from
common code), by setting a(nother) bad precedent, is what I want
to avoid from the beginning.

Jan

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