Re: [Xen-devel] [PATCH v4] x86/AMD: Add support for AMD's OSVW feature in guests

[Boris Ostrovsky <boris.ostrovsky@xxxxxxx>]

On 02/02/12 08:22, Jan Beulich wrote:
> > > > On 01.02.12 at 17:30, Boris Ostrovsky<boris.ostrovsky@xxxxxxx>  wrote:
> > # HG changeset patch
> > # User Boris Ostrovsky<boris.ostrovsky@xxxxxxx>
> > # Date 1328108207 -3600
> > # Node ID 789bbf4f478b0e81d71240a1f1147ef66a7c8848
> > # Parent  e2722b24dc0962de37215320b05d1bb7c4c42864
> > x86/AMD: Add support for AMD's OSVW feature in guests.
> >
> > In some cases guests should not provide workarounds for errata even when the
> > physical processor is affected. For example, because of erratum 400 on
> > family
> > 10h processors a Linux guest will read an MSR (resulting in VMEXIT) before
> > going to idle in order to avoid getting stuck in a non-C0 state. This is not
> > necessary: HLT and IO instructions are intercepted and therefore there is no
> > reason for erratum 400 workaround in the guest.
> >
> > This patch allows us to present a guest with certain errata as fixed,
> > regardless of the state of actual hardware.
> As I was about to apply this to my local tree to give it a try, I had
> to realize that the microcode integration is still not correct: There
> is (at least from an abstract perspective) no guarantee for
> cpu_request_microcode() to be called on all CPUs, yet you want
> svm_host_osvw_init() to be re-run on all of them. If you choose
> to not deal with this in a formally correct way, it should be stated
> so in a code comment (to lower the risk of surprises when someone
> touches that code) that this is not possible in practice because
> collect_cpu_info() won't currently fail for CPUs of interest.
What if svm_host_osvw_init() is called from collect_cpu_info()? There 
may be cases when svm_host_osvw_init() is called multiple times for the 
same cpu but that should be harmless (and the routine will be renamed to 
svm_host_osvw_update()).
This would change a bit the scope of things that collect_cpu_info() is 
expected to do though. But then one could argue that stashing OSVW bits 
is to some extent also collecting CPU info, albeit for a different purpose.

-boris


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