Re: [Xen-devel] [PATCH] x86: Control CR0 TS behavior using dev_na_ts_allowed

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 17.03.14 at 13:42, George Dunlap <George.Dunlap@xxxxxxxxxxxxx> wrote:
> On Mon, Mar 17, 2014 at 8:38 AM, Jan Beulich <JBeulich@xxxxxxxx> wrote:
>>>>> On 17.03.14 at 04:30, Sarah Newman <srn@xxxxxxxxx> wrote:
>>
>> Not being convinced at all that this is the right approach (in
>> particular it remains unclear how an affected guest should deal with
>> running on a hypervisor not supporting the new interface)
> 
> It looks like the intention of this patch was that if the dom0
> administrator enables the new option, then it will be on by default,
> *but* the guest can disable the new behavior.  That way, if an admin
> knows that she's running all PVOPS kernels (no "classic Xen" kernels),
> she can enable it system-wide.  Older PVOPS kernels will behave
> correctly (but a bit slowly), and newer PVOPS kernels will switch to
> the PVABI behavior and reap the performance benefit.
> 
> Newer PVOPS kernels running on older hypervisors will simply use the
> PVABI behavior.

But if that works correctly, then there's no hypervisor/tools
change needed in the first place.

Jan

>  Older PVOPS kernels (without the kernel-side fix
> backported) running on older hypervisors (without this patch
> backported) will be buggy, but there's nothing we can do about that.
> :-)
> 
>  -George




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