Date: Thu, 17 Sep 2020 21:38:25 +0300 From: Konstantin Belousov <kostikbel@gmail.com> To: Wei Hu <weh@microsoft.com>, avg@freebsd.org Cc: "freebsd-hackers@freebsd.org" <freebsd-hackers@freebsd.org> Subject: Re: MSR accesses that slows down the hypervisor/host Message-ID: <20200917183825.GU94807@kib.kiev.ua> In-Reply-To: <SG2P153MB0352C010CAA61A46BD28C964BB3E0@SG2P153MB0352.APCP153.PROD.OUTLOOK.COM> References: <SG2P153MB0352138F1A840E8663C039C2BB210@SG2P153MB0352.APCP153.PROD.OUTLOOK.COM> <20200916135727.GO94807@kib.kiev.ua> <SG2P153MB0352C010CAA61A46BD28C964BB3E0@SG2P153MB0352.APCP153.PROD.OUTLOOK.COM>
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On Thu, Sep 17, 2020 at 05:34:22AM +0000, Wei Hu wrote: > > From: Konstantin Belousov <kostikbel@gmail.com> > > Sent: Wednesday, September 16, 2020 9:57 PM > > To: Wei Hu <weh@microsoft.com> > > Cc: freebsd-hackers@freebsd.org > > Subject: Re: MSR accesses that slows down the hypervisor/host > > > > Where do you see accesses to MSR_LS_CFG ? I can only find manipulations of > > that MSR in init_amd(), and then it is all under check that we are not > > virtualized. > > > Yes, it is only accessed in init_amd() at boot time. So it is less concerned. MSR_AMDK8_IPM > is accessed in cpu_idle() all the time, so it is the key place to optimize. > > > For MSR_AMDK8_IPM access in cpu_idle(), it seems that the workaround was > > applied too wide. It might be that we do not need to do it on recent CPUs, but I > > need to spent more time looking at datasheets to confirm/deny. > > > > But, do you (hypervisor) indeed allow guest to initiate C1 or deeper idle state ? > > If not, perhaps as the first measure, we can avoid manipulating > > MSR_AMDK8_IPM under hypervisor at all. > > You are right a guest cannot initiate C1 or deeper idle state when running on Hyper-V. > So skipping the read of MSR_AMDK8_IPM when running under this hypervisor would > Be a viable solution. https://reviews.freebsd.org/D26470
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