Date: Thu, 17 Sep 2020 05:34:22 +0000 From: Wei Hu <weh@microsoft.com> To: Konstantin Belousov <kostikbel@gmail.com> Cc: "freebsd-hackers@freebsd.org" <freebsd-hackers@freebsd.org> Subject: RE: MSR accesses that slows down the hypervisor/host Message-ID: <SG2P153MB0352C010CAA61A46BD28C964BB3E0@SG2P153MB0352.APCP153.PROD.OUTLOOK.COM> In-Reply-To: <20200916135727.GO94807@kib.kiev.ua> References: <SG2P153MB0352138F1A840E8663C039C2BB210@SG2P153MB0352.APCP153.PROD.OUTLOOK.COM> <20200916135727.GO94807@kib.kiev.ua>
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> 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 >=20 > 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. >=20 Yes, it is only accessed in init_amd() at boot time. So it is less concerne= d. 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. >=20 > 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 wo= uld=20 Be a viable solution.=20 Thanks, Wei
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