Glauber Costa wrote: > On Tue, Dec 1, 2009 at 10:17 AM, Jan Kiszka <jan.kis...@siemens.com> wrote: >> Avi Kivity wrote: >>> On 11/30/2009 07:30 PM, Jan Kiszka wrote: >>>>>> No. But what's wrong with on_vcpu? >>>>>> >>>>> intrinsically racy. signal passing slow down things, etc. >>>>> >>>>> That said, as I've stated many times: I don't believe there's anything >>>>> fundamentally wrong with on_vcpu. But we might get benefits from a >>>>> re-design >>>>> of things to avoid it whenever possible. (just like the vcpu_reset() >>>>> I've just posted) >>>>> >>>>> >>>> If you don't want immediate execution of update_guest_debug, save the >>>> state that shall be transferred, set some flag, and run the transfer >>>> before guest entry inside the vcpu threads (after putting the registers >>>> as older kernels may otherwise overwrite the flags register). Should >>>> work, may even avoid redundant calls during a gdb session. >>>> >>> There's no guarantee the vcpu will ever exit to qemu, so you have to >>> signal the vcpu thread anyway. When you do that, you might as well load >>> the new state. >> Debugging is special here as it involves vmstop before you start playing >> with the debug settings. But I may also oversee some corner case right now. >> > > I imagined so. In this case, it might be better to set a flag before > vmstop, and then > honor it on vcpu entry. I had this feeling that in most cases where we > current signal the > vcpu, the VM will be already stopped anyway, so we don't need to resignal. >
Do not set the flag before the machine stop, otherwise you risk to race with vcpus that exit and re-enter briefly before that. This manipulation should really be done under vmstop protection. Jan -- Siemens AG, Corporate Technology, CT T DE IT 1 Corporate Competence Center Embedded Linux
