RE: [PATCH v3 07/34] x86/hyperv: Introduce HV_MAX_SPARSE_VCPU_BANKS/HV_VCPUS_PER_SPARSE_BANK constants

[Michael Kelley (LINUX)]

From: Wei Liu <wei.liu@xxxxxxxxxx> Sent: Monday, April 25, 2022 8:47 AM

> On Thu, Apr 14, 2022 at 03:19:46PM +0200, Vitaly Kuznetsov wrote:
> > It may not come clear from where the magical '64' value used in
> > __cpumask_to_vpset() come from. Moreover, '64' means both the maximum
> > sparse bank number as well as the number of vCPUs per bank. Add defines
> > to make things clear. These defines are also going to be used by KVM.
> >
> > No functional change.
> >
> > Signed-off-by: Vitaly Kuznetsov <vkuznets@xxxxxxxxxx>
> > ---
> >  include/asm-generic/hyperv-tlfs.h |  5 +++++
> >  include/asm-generic/mshyperv.h    | 11 ++++++-----
> >  2 files changed, 11 insertions(+), 5 deletions(-)
> >
> > diff --git a/include/asm-generic/hyperv-tlfs.h b/include/asm-generic/hyperv-tlfs.h
> > index fdce7a4cfc6f..020ca9bdbb79 100644
> > --- a/include/asm-generic/hyperv-tlfs.h
> > +++ b/include/asm-generic/hyperv-tlfs.h
> > @@ -399,6 +399,11 @@ struct hv_vpset {
> >  	u64 bank_contents[];
> >  } __packed;
> >
> > +/* The maximum number of sparse vCPU banks which can be encoded by 'struct
> hv_vpset' */
> > +#define HV_MAX_SPARSE_VCPU_BANKS (64)
> > +/* The number of vCPUs in one sparse bank */
> > +#define HV_VCPUS_PER_SPARSE_BANK (64)
> 
> I think replacing the magic number with a macro is a good thing.
> 
> Where do you get these names? Did you make them up yourself?
> 
> I'm trying to dig into internal code to find the most appropriate names,
> but I couldn't find any so far. Michael, do you have insight here?
> 
> Thanks,
> Wei.

These names look good to me.  The "sparse" and "bank" terminology
comes from the Hyper-V TLFS, sections 7.8.7.3 thru 7.8.7.5.  The TLFS
uses the constant "64", but for two different purposes as Vitaly
points out.  But in both cases the "64" accrues from the use of
a uint64 value as a bitmap.

Michael