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Drivers: hv: Don't assume cpu_possible_mask is dense 

JIRA: https://issues.redhat.com/browse/RHEL-80096

commit 16b18fdf6bc7292ae0edbf33d2d693af3240e49d
Author: Michael Kelley <mhklinux@outlook.com>
Date:   Wed Oct 2 20:53:30 2024 -0700

    Drivers: hv: Don't assume cpu_possible_mask is dense

    Current code allocates the hv_vp_index array with size
    num_possible_cpus(). This code assumes cpu_possible_mask is dense,
    which is not true in the general case per [1]. If cpu_possible_mask
    is sparse, the array might be indexed by a value beyond the size of
    the array.

    However, the configurations that Hyper-V provides to guest VMs on x86
    and ARM64 hardware, in combination with how architecture specific code
    assigns Linux CPU numbers, *does* always produce a dense cpu_possible_mask.
    So the dense assumption is not currently causing failures. But for
    robustness against future changes in how cpu_possible_mask is populated,
    update the code to no longer assume dense.

    The correct approach is to allocate and initialize the array using size
    "nr_cpu_ids". While this leaves unused 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.

    Using nr_cpu_ids also reduces initialization time, in that the loop to
    initialize the array currently rescans cpu_possible_mask on each
    iteration. This is n-squared in the number of CPUs, which could be
    significant for large CPU counts.

    [1] https://lore.kernel.org/lkml/SN6PR02MB4157210CC36B2593F8572E5ED4692@SN6PR02MB4157.namprd02.prod.outlook.com/

    Signed-off-by: Michael Kelley <mhklinux@outlook.com>
    Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
    Link: https://lore.kernel.org/r/20241003035333.49261-3-mhklinux@outlook.com
    Signed-off-by: Wei Liu <wei.liu@kernel.org>
    Message-ID: <20241003035333.49261-3-mhklinux@outlook.com>

Signed-off-by: Maxim Levitsky <mlevitsk@redhat.com>
This commit is contained in:
Maxim Levitsky 2025-05-28 20:09:00 -04:00
parent cdd19f3949
commit a2554bb802
1 changed files with 2 additions and 2 deletions
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4
drivers/hv/hv_common.c
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@ -346,14 +346,14 @@ int __init hv_common_init(void)
BUG_ON(!hyperv_pcpu_output_arg);
}
hv_vp_index = kmalloc_array(num_possible_cpus(), sizeof(*hv_vp_index),
hv_vp_index = kmalloc_array(nr_cpu_ids, sizeof(*hv_vp_index),
GFP_KERNEL);
if (!hv_vp_index) {
hv_common_free();
return -ENOMEM;
}
for (i = 0; i < num_possible_cpus(); i++)
for (i = 0; i < nr_cpu_ids; i++)
hv_vp_index[i] = VP_INVAL;
return 0;