On 10/03/21 18:05, Marc Zyngier wrote:
On Wed, 10 Mar 2021 16:03:42 +0000,
Paolo Bonzini <pbonzini@xxxxxxxxxx> wrote:
On 10/03/21 16:51, Marc Zyngier wrote:
+ kvm_for_each_vcpu(j, vcpu, kvm) {
+ pdata = data + VM_STAT_COUNT;
+ for (i = 0; i < VCPU_STAT_COUNT; i++, pdata++)
+ *pdata += *((u64 *)&vcpu->stat + i);
Do you really need the in-kernel copy? Why not directly organise the
data structures in a way that would allow a bulk copy using
copy_to_user()?
The result is built by summing per-vCPU counters, so that the counter
updates are fast and do not require a lock. So consistency basically
cannot be guaranteed.
Sure, but I wonder whether there is scope for VM-global counters to be
maintained in parallel with per-vCPU counters if speed/efficiency is
of the essence (and this seems to be how it is sold in the cover
letter).
Maintaining VM-global counters would require an atomic instruction and
would suffer lots of cacheline bouncing even on architectures that have
relaxed atomic memory operations.
Speed/efficiency of retrieving statistics is important, but let's keep
in mind that the baseline for comparison is hundreds of syscalls and
filesystem lookups.
Paolo
