Re: [PATCH 0/5] hyper-v: Don't assume cpu_possible_mask is dense

Wei Liu
2024-12-17T19:21:58Z

On Wed, Dec 11, 2024 at 12:14:50AM +0000, Wei Liu wrote:
> On Tue, Dec 10, 2024 at 07:58:34PM +0000, Michael Kelley wrote:
> > From: mhkelley58@gmail.com <mhkelley58@gmail.com> Sent: Wednesday, October 2, 2024 8:53 PM
> > >
> > > Code specific to Hyper-V guests currently assumes the cpu_possible_mask
> > > is "dense" -- i.e., all bit positions 0 thru (nr_cpu_ids - 1) are set,
> > > with no "holes". Therefore, num_possible_cpus() is assumed to be equal
> > > to nr_cpu_ids.
> > >
> > > Per a separate discussion[1], this assumption is not valid in the
> > > general case. For example, the function setup_nr_cpu_ids() in
> > > kernel/smp.c is coded to assume cpu_possible_mask may be sparse,
> > > and other patches have been made in the past to correctly handle
> > > the sparseness. See bc75e99983df1efd ("rcu: Correctly handle sparse
> > > possible cpu") as noted by Mark Rutland.
> > >
> > > The general case notwithstanding, the configurations that Hyper-V
> > > provides to guest VMs on x86 and ARM64 hardware, in combination
> > > with the algorithms currently used by architecture specific code
> > > to assign Linux CPU numbers, *does* always produce a dense
> > > cpu_possible_mask. So the invalid assumption is not currently
> > > causing failures. But in the interest of correctness, and robustness
> > > against future changes in the code that populates cpu_possible_mask,
> > > update the Hyper-V code to no longer assume denseness.
> > >
> > > The typical code pattern with the invalid assumption is as follows:
> > >
> > > array = kcalloc(num_possible_cpus(), sizeof(<some struct>),
> > > GFP_KERNEL);
> > > ....
> > > index into "array" with smp_processor_id()
> > >
> > > In such as case, the array might be indexed by a value beyond the size
> > > of the array. The correct approach is to allocate the array with size
> > > "nr_cpu_ids". While this will probably leave unused any array entries
> > > corresponding to holes in cpu_possible_mask, the holes are assumed to
> > > be minimal and hence the amount of memory wasted by unused entries is
> > > minimal.
> > >
> > > Removing the assumption in Hyper-V code is done in several patches
> > > because they touch different kernel subsystems:
> > >
> > > Patch 1: Hyper-V x86 initialization of hv_vp_assist_page (there's no
> > > hv_vp_assist_page on ARM64)
> > > Patch 2: Hyper-V common init of hv_vp_index
> > > Patch 3: Hyper-V IOMMU driver
> > > Patch 4: storvsc driver
> > > Patch 5: netvsc driver
> >
> > Wei --
> >
> > Could you pick up Patches 1, 2, and 3 in this series for the hyperv-next
> > tree? Peter Zijlstra acked the full series [2], and Patches 4 and 5 have
> > already been picked by the SCSI and net maintainers respectively [3][4].
> >
> > Let me know if you have any concerns.
>
> Michael, I will take a look later after I finish dealing with the
> hyperv-fixes branch.

Patch 1 to 3 have been applied to the hyperv-next branch.

Wei.