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

Michael Kelley
2024-12-10T19:58:41Z

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.

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
>