linux-kernel - Can avic_vm_id_bitmap be made smaller? 2 mbytes feels too large

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Date:   Wed, 21 Jun 2017 20:28:48 +0200
From:   Denys Vlasenko <dvlasenk@...hat.com>
To:     Suravee Suthikulpanit <suravee.suthikulpanit@....com>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Radim Krčmář <rkrcmar@...hat.com>
Subject: Can avic_vm_id_bitmap be made smaller? 2 mbytes feels too large

#define AVIC_VM_ID_BITS                        24
#define AVIC_VM_ID_NR                  (1 << AVIC_VM_ID_BITS)
...
static DECLARE_BITMAP(avic_vm_id_bitmap, AVIC_VM_ID_NR);

The above results in a data object which is 2 megabytes large.

avic_vm_init() -> avic_get_next_vm_id() allocates a new vm_id
by looking for a free bit there. That's the whole purpose
for that bitmap existing.

Is there a way to do this less wastefully? Say, such as
iterating over all existing VMs and picking an id which is
not taken.

I imagine VM creation operation is not that frequent, no need
to make new vm_id selection super fast.