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

Wei Liu
2024-12-11T00:14:52Z

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.

Thanks,
Wei.

>
> Thanks,
>
> Michael
>
> [2] https://lore.kernel.org/linux-hyperv/20241004100742.GO18071@noisy.programming.kicks-ass.net/
> [3] https://lore.kernel.org/linux-hyperv/yq15xnsjlc1.fsf@ca-mkp.ca.oracle.com/
> [4] https://lore.kernel.org/linux-hyperv/172808404024.2772330.2975585273609596688.git-patchwork-notify@kernel.org/
>
> >
> > I tested the changes by hacking the construction of cpu_possible_mask
> > to include a hole on x86. With a configuration set to demonstrate the
> > problem, a Hyper-V guest kernel eventually crashes due to memory
> > corruption. After the patches in this series, the crash does not occur.
> >
> > [1] https://lore.kernel.org/lkml/SN6PR02MB4157210CC36B2593F8572E5ED4692@SN6PR02MB4157.namprd02.prod.outlook.com/
> >
> > Michael Kelley (5):
> > x86/hyperv: Don't assume cpu_possible_mask is dense
> > Drivers: hv: Don't assume cpu_possible_mask is dense
> > iommu/hyper-v: Don't assume cpu_possible_mask is dense
> > scsi: storvsc: Don't assume cpu_possible_mask is dense
> > hv_netvsc: Don't assume cpu_possible_mask is dense
> >
> > arch/x86/hyperv/hv_init.c | 2 +-
> > drivers/hv/hv_common.c | 4 ++--
> > drivers/iommu/hyperv-iommu.c | 4 ++--
> > drivers/net/hyperv/netvsc_drv.c | 2 +-
> > drivers/scsi/storvsc_drv.c | 13 ++++++-------
> > 5 files changed, 12 insertions(+), 13 deletions(-)
> >
> > --
> > 2.25.1
> >