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Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.10.0 MIME-Version: 1.0 In-Reply-To: <0d7201d6ce09$e13dce80$a3b96b80$@xen.org> Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 8bit Content-Language: en-US Hi Paul. >>>>>> On 30.11.2020 11:31, Oleksandr Tyshchenko wrote: >>>>>>> --- a/xen/include/xen/ioreq.h >>>>>>> +++ b/xen/include/xen/ioreq.h >>>>>>> @@ -55,6 +55,20 @@ struct ioreq_server { >>>>>>> uint8_t bufioreq_handling; >>>>>>> }; >>>>>>> +/* >>>>>>> + * This should only be used when d == current->domain and it's not >>>>>>> paused, >>>>>> Is the "not paused" part really relevant here? Besides it being rare >>>>>> that the current domain would be paused (if so, it's in the process >>>>>> of having all its vCPU-s scheduled out), does this matter at all?do >>>>>> any extra actionsdo any extra actions >>>>> No, it isn't relevant, I will drop it. >>>>> >>>>> >>>>>> Apart from this the patch looks okay to me, but I'm not sure it >>>>>> addresses Paul's concerns. Iirc he had suggested to switch back to >>>>>> a list if doing a swipe over the entire array is too expensive in >>>>>> this specific case. >>>>> We would like to avoid to do any extra actions in >>>>> leave_hypervisor_to_guest() if possible. >>>>> But not only there, the logic whether we check/set >>>>> mapcache_invalidation variable could be avoided if a domain doesn't >>>>> use IOREQ server... >>>> Are you OK with this patch (common part of it)? >>> How much of a performance benefit is this? The array is small to simply counting the non-NULL >> entries should be pretty quick. >> I didn't perform performance measurements on how much this call consumes. >> In our system we run three domains. The emulator is in DomD only, so I >> would like to avoid to call vcpu_ioreq_handle_completion() for every >> Dom0/DomU's vCPUs >> if there is no real need to do it. > This is not relevant to the domain that the emulator is running in; it's concerning the domains which the emulator is servicing. How many of those are there? Err, yes, I wasn't precise when providing an example. Single emulator is running in DomD and servicing DomU. So with the helper in place the vcpu_ioreq_handle_completion() gets only called for DomU vCPUs (as expected). Without an optimization the vcpu_ioreq_handle_completion() gets called for _all_ vCPUs, and I see it as an extra action for Dom0, DomD vCPUs. > >> On Arm vcpu_ioreq_handle_completion() >> is called with IRQ enabled, so the call is accompanied with >> corresponding irq_enable/irq_disable. >> These unneeded actions could be avoided by using this simple one-line >> helper... >> > The helper may be one line but there is more to the patch than that. I still think you could just walk the array in the helper rather than keeping a running occupancy count. OK, is the implementation below close to what you propose? If yes, I will update a helper and drop nr_servers variable. bool domain_has_ioreq_server(const struct domain *d) {     const struct ioreq_server *s;     unsigned int id;     FOR_EACH_IOREQ_SERVER(d, id, s)         return true;     return false; } -- Regards, Oleksandr Tyshchenko