Re: [edk2-devel] [PATCH 2/5] MdePkg: Allow PcdFSBClock to by Dynamic

["Liming Gao" <liming.gao@intel.com>]

Thanks for your data. Seemly, those data is acceptable on OvmfXen. For this patch, Reviewed-by: Liming Gao <liming.gao@intel.com>

Thanks
Liming
> -----Original Message-----
> From: devel@edk2.groups.io <devel@edk2.groups.io> On Behalf Of Anthony PERARD
> Sent: Tuesday, February 4, 2020 1:26 AM
> To: Gao, Liming <liming.gao@intel.com>
> Cc: Laszlo Ersek <lersek@redhat.com>; devel@edk2.groups.io; Kinney, Michael D <michael.d.kinney@intel.com>; Ard Biesheuvel
> <ard.biesheuvel@linaro.org>; xen-devel@lists.xenproject.org; Justen, Jordan L <jordan.l.justen@intel.com>; Julien Grall
> <julien@xen.org>; Feng, Bob C <bob.c.feng@intel.com>
> Subject: Re: [edk2-devel] [PATCH 2/5] MdePkg: Allow PcdFSBClock to by Dynamic
> 
> On Mon, Feb 03, 2020 at 03:34:07PM +0000, Anthony PERARD wrote:
> > On Mon, Feb 03, 2020 at 01:34:55AM +0000, Gao, Liming wrote:
> > > Anthony:
> > >   This change is OK to me. But if this PCD is configured as Dynamic, its value will be got from PCD service. This operation will take
> some time and cause the inaccurate time delay. Have you measured its impact?
> >
> > No, I haven't. But I don't think it matter in a Xen guest, the APIC timer is
> > emulated anyway, so reading from a register of the APIC is going to be
> > slower than getting the value from the PCD services, I think.
> > (Hopefully, I'm not too wrong.)
> >
> > But I'll give it at measuring the difference, it would be interesting to
> > know.
> 
> Now that I've given a try, having the value as Dynamic doesn't change
> anything in a Xen guest.
> 
> On my test machine, simply running GetPerformanceCounter (); takes
> between 10000 ns and 20000 ns. Reading the dynamic value from PCD on the
> other hand takes about 350ns. (10ns if it's static.)
> 
> When I run NanoSecondDelay() with different values, I have:
>   - with static pcd:
>            63894 ns to delay by 1 ns
>            66611 ns to delay by 10 ns
>            43927 ns to delay by 100 ns
>            71367 ns to delay by 1000 ns
>            55881 ns to delay by 10000 ns
>           147716 ns to delay by 100000 ns
>          1048335 ns to delay by 1000000 ns
>         10041179 ns to delay by 10000000 ns
>   - with a dynamic pcd:
>            40949 ns to delay by 1 ns
>            84832 ns to delay by 10 ns
>            82745 ns to delay by 100 ns
>            59848 ns to delay by 1000 ns
>            52647 ns to delay by 10000 ns
>           137051 ns to delay by 100000 ns
>          1042492 ns to delay by 1000000 ns
>         10036306 ns to delay by 10000000 ns
> 
> So, the kind of PCD used for PcdFSBClock on Xen (with OvmfXen) doesn't
> really matter.
> 
> Anyway, thanks for the feedback.
> 
> --
> Anthony PERARD
> 
>