From: Eduardo Habkost <ehabkost@redhat.com>
To: Paolo Bonzini <pbonzini@redhat.com>
Cc: "Borislav Petkov" <bp@alien8.de>, "Jörg Rödel" <joro@8bytes.org>,
"Andre Przywara" <andre.przywara@arm.com>,
"kvm ML" <kvm@vger.kernel.org>,
lkml <linux-kernel@vger.kernel.org>
Subject: Re: kvm: RDTSCP on AMD
Date: Wed, 6 Jul 2016 14:34:40 -0300 [thread overview]
Message-ID: <20160706173440.GU4131@thinpad.lan.raisama.net> (raw)
In-Reply-To: <b7e01819-f604-7e2b-fd6d-6323bd05587e@redhat.com>
On Wed, Jul 06, 2016 at 03:01:04PM +0200, Paolo Bonzini wrote:
> On 06/07/2016 14:44, Borislav Petkov wrote:
> > Hi guys,
> >
> > how about this below to enable RDTSCP emulation on AMD? IOW, I'm staring
> > at
> >
> > 33b5e8c03ae7 ("target-i386: Disable rdtscp on Opteron_G* CPU models")
> >
> > in the qemu repo.
> >
> > It seems to work here, RDTSCP in the guest gives me node and cpu as
> > vsyscall_set_cpu() in the guest kernel has set them.
> >
> > Thoughts?
> >
> > (Below is the simple qemu diff reenabling RDTSCP)
> >
> > @@ -3919,6 +3935,7 @@ static int (*const svm_exit_handlers[])(struct vcpu_svm *svm) = {
> > [SVM_EXIT_STGI] = stgi_interception,
> > [SVM_EXIT_CLGI] = clgi_interception,
> > [SVM_EXIT_SKINIT] = skinit_interception,
> > + [SVM_EXIT_RDTSCP] = rdtscp_interception,
> > [SVM_EXIT_WBINVD] = wbinvd_interception,
> > [SVM_EXIT_MONITOR] = monitor_interception,
> > [SVM_EXIT_MWAIT] = mwait_interception,
>
> Nothing is needed in the kernel actually. You can skip the intercept
> by running the guest with MSR_TSC_AUX set to the guest's expected value.
> Which KVM does, except that it's botched so I need to apply the
> patch in https://lkml.org/lkml/2016/4/13/802.
Do you mean -cpu Opteron_G*,+rdtscp will be buggy on Linux v4.5?
(v4.5 reports rdtscp as supported in GET_SUPPORTED_CPUID)
Can we do something to make QEMU detect the buggy kernel before
allowing rdtscp to be enabled, or should we just tell people to
upgrade their kernel?
>
> > ---
> >
> > qemu diff:
> >
> > ---
> > diff --git a/target-i386/cpu.c b/target-i386/cpu.c
> > index 3bd3cfc3ad16..aa6d0d027d00 100644
> > --- a/target-i386/cpu.c
> > +++ b/target-i386/cpu.c
>
> This is not enough because it's missing some backwards compatibility
> gunk (similar to the include/hw/i386/pc.h parts of 33b5e8c03ae), but
> it's enough for a proof of concept and to discuss it.
>
> The main issue with this is that it would force a lockstep update of
> QEMU and kernel, which we try to avoid. I'm not sure if we have a
> solution for this problem. Eduardo?
We don't. Either we make QEMU require a newer kernel, or we need
a new CPU model. :(
--
Eduardo
next prev parent reply other threads:[~2016-07-06 17:35 UTC|newest]
Thread overview: 18+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-07-06 12:44 kvm: RDTSCP on AMD Borislav Petkov
2016-07-06 13:01 ` Paolo Bonzini
2016-07-06 17:34 ` Eduardo Habkost [this message]
2016-07-06 21:27 ` Paolo Bonzini
2018-12-10 18:13 Borislav Petkov
2018-12-10 18:37 ` Eduardo Habkost
2018-12-10 18:41 ` Borislav Petkov
2018-12-10 19:06 ` Eduardo Habkost
2018-12-10 19:42 ` Borislav Petkov
2018-12-10 20:08 ` Eduardo Habkost
2018-12-10 20:39 ` Borislav Petkov
2018-12-11 10:38 ` Daniel P. Berrangé
2018-12-11 11:55 ` Eduardo Habkost
2018-12-11 14:35 ` Daniel P. Berrangé
2018-12-11 15:23 ` Paolo Bonzini
2018-12-11 15:30 ` Daniel P. Berrangé
2018-12-10 19:47 ` Eduardo Habkost
2018-12-10 20:16 ` Borislav Petkov
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