linux-kernel - Re: kvm: RDTSCP on AMD

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Message-ID: <0b830bbe-182a-3bd3-bab0-0c2fd823edd4@redhat.com>
Date:	Wed, 6 Jul 2016 23:27:16 +0200
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	Eduardo Habkost <ehabkost@...hat.com>
Cc:	Borislav Petkov <bp@...en8.de>,
	Jörg Rödel <joro@...tes.org>,
	Andre Przywara <andre.przywara@....com>,
	kvm ML <kvm@...r.kernel.org>,
	lkml <linux-kernel@...r.kernel.org>
Subject: Re: kvm: RDTSCP on AMD



On 06/07/2016 19:34, Eduardo Habkost wrote:
>> > 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?

We usually just tell people to use the latest stable kernel.

Adding new CPU models is not a big deal, in fact it's almost easier than
getting compat properties right. :)

Paolo