Re: kvm: RDTSCP on AMD

[Eduardo Habkost <ehabkost@redhat.com>]

On Mon, Dec 10, 2018 at 07:13:28PM +0100, Borislav Petkov wrote:
> Reviving an old thread here.
> 
> On Wed, Jul 06, 2016 at 11:27:16PM +0200, Paolo Bonzini wrote:
> > 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. :)
> 
> Ok, can we finally revert
> 
>   33b5e8c03ae7 ("target-i386: Disable rdtscp on Opteron_G* CPU models")
> 
> in the qemu tree?
> 
> Three years should be enough by now for
> 
>   46896c73c1a4 ("KVM: svm: add support for RDTSCP")
> 
> to have percolated downstream.

That's Linux v4.5, released in March 2016.

It isn't as simply as reverting commit 33b5e8c03ae7, but we can
surely re-add RDTSCP on pc-*-4.0 and newer.

I thought we added documentation mentioning the minimum kernel
version required by QEMU, but I can't find it.  In either case,
it seems reasonable to require Linux 4.5 or newer on newer
machine-types.

-- 
Eduardo