On 22/05/2024 01:17, David Woodhouse wrote:
From: David Woodhouse <dwmw@xxxxxxxxxxxx> There was some confusion in kvm_update_guest_time() when software needs to advance the guest TSC. In master clock mode, there are two points of time which need to be taken into account. First there is the master clock reference point, stored in kvm->arch.master_kernel_ns (and associated host TSC ->master_cycle_now). Secondly, there is the time *now*, at the point kvm_update_guest_time() is being called. With software TSC upscaling, the guest TSC is getting further and further ahead of the host TSC as time elapses. So at time "now", the guest TSC should be further ahead of the host, than it was at master_kernel_ns. The adjustment in kvm_update_guest_time() was not taking that into account, and was only advancing the guest TSC by the appropriate amount for master_kernel_ns, *not* the current time. Fix it to calculate them both correctly. Since the KVM clock reference point in master_kernel_ns might actually be *earlier* than the reference point used for the guest TSC (vcpu->last_tsc_nsec), this might lead to a negative delta. Fix the compute_guest_tsc() function to cope with negative numbers, which then means there is no need to force a master clock update when the guest TSC is written. Signed-off-by: David Woodhouse <dwmw@xxxxxxxxxxxx > --- arch/x86/kvm/x86.c | 73 +++++++++++++++++++++++++++++++++++----------- 1 file changed, 56 insertions(+), 17 deletions(-)
Reviewed-by: Paul Durrant <paul@xxxxxxx>
