Re: [PATCH v4 09/15] Drivers: hv: Introduce hv_output_arg_exists in hv_common.c

[Nuno Das Neves]

On 10/2/2023 12:29 PM, Alex Ionescu wrote:
> Hi Nuno,
>
> Is it possible to simply change to always allocating the output page?
> For example, the output page could be needed in scenarios where Linux
> is not running as the root partition, since certain hypercalls that a
> guest can make will still require one (I realize that's not the case
> _today_, but I don't believe this optimization buys much).

I agree - it would indeed simplify the code, and guests will probably
make use of it sooner or later.

Happy to make that change if Hyper-V guest maintainers agree.
Long, Dexuan, Michael, what do you think?

Thanks,
Nuno

> Best regards,
> Alex Ionescu
>
>
> On Fri, Sep 29, 2023 at 2:02 PM Nuno Das Neves
> <nunodasneves@xxxxxxxxxxxxxxxxxxx> wrote:
> > This is a more flexible approach for determining whether to allocate the
> > output page.
> >
> > Signed-off-by: Nuno Das Neves <nunodasneves@xxxxxxxxxxxxxxxxxxx>
> > Acked-by: Wei Liu <wei.liu@xxxxxxxxxx>
> > ---
> >   drivers/hv/hv_common.c | 21 +++++++++++++++++----
> >   1 file changed, 17 insertions(+), 4 deletions(-)
> >
> > diff --git a/drivers/hv/hv_common.c b/drivers/hv/hv_common.c
> > index 39077841d518..3f6f23e4c579 100644
> > --- a/drivers/hv/hv_common.c
> > +++ b/drivers/hv/hv_common.c
> > @@ -58,6 +58,14 @@ EXPORT_SYMBOL_GPL(hyperv_pcpu_input_arg);
> >   void * __percpu *hyperv_pcpu_output_arg;
> >   EXPORT_SYMBOL_GPL(hyperv_pcpu_output_arg);
> >
> > +/*
> > + * Determine whether output arg is needed
> > + */
> > +static inline bool hv_output_arg_exists(void)
> > +{
> > +       return hv_root_partition ? true : false;
> > +}
> > +
> >   static void hv_kmsg_dump_unregister(void);
> >
> >   static struct ctl_table_header *hv_ctl_table_hdr;
> > @@ -342,10 +350,12 @@ int __init hv_common_init(void)
> >          hyperv_pcpu_input_arg = alloc_percpu(void  *);
> >          BUG_ON(!hyperv_pcpu_input_arg);
> >
> > -       /* Allocate the per-CPU state for output arg for root */
> > -       if (hv_root_partition) {
> > +       if (hv_output_arg_exists()) {
> >                  hyperv_pcpu_output_arg = alloc_percpu(void *);
> >                  BUG_ON(!hyperv_pcpu_output_arg);
> > +       }
> > +
> > +       if (hv_root_partition) {
> >                  hv_synic_eventring_tail = alloc_percpu(u8 *);
> >                  BUG_ON(hv_synic_eventring_tail == NULL);
> >          }
> > @@ -375,7 +385,7 @@ int hv_common_cpu_init(unsigned int cpu)
> >          u8 **synic_eventring_tail;
> >          u64 msr_vp_index;
> >          gfp_t flags;
> > -       int pgcount = hv_root_partition ? 2 : 1;
> > +       int pgcount = hv_output_arg_exists() ? 2 : 1;
> >          void *mem;
> >          int ret;
> >
> > @@ -393,9 +403,12 @@ int hv_common_cpu_init(unsigned int cpu)
> >                  if (!mem)
> >                          return -ENOMEM;
> >
> > -               if (hv_root_partition) {
> > +               if (hv_output_arg_exists()) {
> >                          outputarg = (void **)this_cpu_ptr(hyperv_pcpu_output_arg);
> >                          *outputarg = (char *)mem + HV_HYP_PAGE_SIZE;
> > +               }
> > +
> > +               if (hv_root_partition) {
> >                          synic_eventring_tail = (u8 **)this_cpu_ptr(hv_synic_eventring_tail);
> >                          *synic_eventring_tail = kcalloc(HV_SYNIC_SINT_COUNT, sizeof(u8),
> >                                                          flags);
> > --
> > 2.25.1