On 2/4/25 18:51, Sean Christopherson wrote:
On Tue, Feb 04, 2025, Naveen N Rao wrote:
On Mon, Feb 03, 2025 at 09:00:05PM -0500, Maxim Levitsky wrote:
On Mon, 2025-02-03 at 22:33 +0530, Naveen N Rao (AMD) wrote:
apicv_update_lock is not required when querying the state of guest
debug in all the vcpus. Remove usage of the same, and switch to
kvm_set_or_clear_apicv_inhibit() helper to simplify the code.
It might be worth to mention that the reason why the lock is not needed,
is because kvm_vcpu_ioctl from which this function is called takes 'vcpu->mutex'
and thus concurrent execution of this function is not really possible.
Looking at this again, that looks to be a vcpu-specific lock, so I guess
it is possible for multiple vcpus to run this concurrently?
Correct.
And this patch is incorrect. Because there is a store and many loads,
you have the typical race when two vCPUs set blockirq at the same time
vcpu 0 vcpu 1
--------------- --------------
set vcpu0->guest_debug
clear vcpu1->guest_debug
read vcpu0->guest_debug
read vcpu1->guest_debug
set inhibit
read stale vcpu0->guest_debug
read vcpu1->guest_debug
clear inhibit
But since this is really a slow path, why even bother optimizing it?
Paolo
