RE: [PATCH v6 01/10] arm64: hyperv: Add core Hyper-V include files

[Michael Kelley]

From: Arnd Bergmann <arnd@xxxxxxxx> Sent: Monday, March 16, 2020 1:48 AM
> 
> On Sat, Mar 14, 2020 at 4:36 PM Michael Kelley <mikelley@xxxxxxxxxxxxx> wrote:
> 
> > +
> > +/* Define input and output layout for Get VP Register hypercall */
> > +struct hv_get_vp_register_input {
> > +       u64 partitionid;
> > +       u32 vpindex;
> > +       u8  inputvtl;
> > +       u8  padding[3];
> > +       u32 name0;
> > +       u32 name1;
> > +} __packed;
> 
> Are you sure these need to be made byte-aligned according to the
> specification? If the structure itself is aligned to 64 bit, better mark only
> the individual fields that are misaligned as __packed.
> 
> If the structure is aligned to only 32-bit addresses instead of
> 64-bit, mark it as "__packed __aligned(4)" to let the compiler
> generate better code for accessing it.

None of the fields are misaligned and it will always be aligned to 64-bit
addresses, so there should be no padding needed or added.  There was
a discussion of __packed and the Hyper-V data structures in general on
LKML here:  https://lkml.org/lkml/2018/11/30/848.  Adding __packed was
done as a preventative measure, not because anything was actually
broken.  Marking as __aligned(8) here would indicate the correct intent,
though the use of the structure always ensures 64-bit alignment.

> 
> Also, in order to write portable code, it would be helpful to mark
> all the fields as explicitly little-endian, and use __le32_to_cpu()
> etc for accessing them.

There's an opening comment in this file stating that all data
structures shared between Hyper-V and a guest VM are little
endian.  Is there some other marking to consider using?

We have definitely not allowed for the case of Hyper-V running on
a big endian architecture.  There are a *lot* of messages and data
structures passed between the guest and Hyper-V, and coding
to handle either endianness is a big project.  I'm doubtful
of the value until and unless we actually have a need for it.

> 
> > +/* Define synthetic interrupt controller message flags. */
> > +union hv_message_flags {
> > +       __u8 asu8;
> > +       struct {
> > +               __u8 msg_pending:1;
> > +               __u8 reserved:7;
> > +       } __packed;
> > +};
> 
> For similar reasons, please avoid bit fields and just use a
> bit mask on the first member of the union.

Unfortunately, changing to a bit mask ripples into
architecture independent code and into the x86
implementation.  I'd prefer not to drag that complexity
into this patch set.

> 
> The __packed annotation here is not meaningful,
> as the total size is already only a single byte.

Agreed.

> 
> > +/* Define port identifier type. */
> > +union hv_port_id {
> > +       __u32 asu32;
> > +       struct {
> > +               __u32 id:24;
> > +               __u32 reserved:8;
> > +       }  __packed u;
> > +};
> 
> Here, the __packed annotation is inconsistent with the use
> in the rest of the file: marking only one member of the union
> as __packed means that the union itself is still expected to
> be 4 byte aligned. I would expect that either all of these
> structures have a sensible alignment, or they are all
> completely unaligned.

Agreed.  Looks like it is wrong on the x86 side too.  

> 
> > + * Use the Hyper-V provided stimer0 as the timer that is made
> > + * available to the architecture independent Hyper-V drivers.
> > + */
> > +#define hv_init_timer(timer, tick) \
> > +               hv_set_vpreg(HV_REGISTER_STIMER0_COUNT + (2*timer), tick)
> > +#define hv_init_timer_config(timer, val) \
> > +               hv_set_vpreg(HV_REGISTER_STIMER0_CONFIG + (2*timer), val)
> > +#define hv_get_current_tick(tick) \
> > +               (tick = hv_get_vpreg(HV_REGISTER_TIME_REFCOUNT))
> 
> In general, we prefer inline functions over macros in header files.

I can change the "set" calls to inline functions.  As you can see, the "get"
functions are coded and used in architecture independent code and on
the x86 side in a way that won't convert to inline functions.

> 
> > +#if IS_ENABLED(CONFIG_HYPERV)
> > +#define hv_enable_stimer0_percpu_irq(irq)      enable_percpu_irq(irq, 0)
> > +#define hv_disable_stimer0_percpu_irq(irq)     disable_percpu_irq(irq)
> > +#endif
> 
> Should there be an #else definition here? It helps readability
> to have the two versions (with and without hyperv support) close
> together rather than in different files. If there is no other
> definition, just drop the #if.

Agreed.  I'll figure out a way to handle this better.

> 
>      Arnd