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

[George Dunlap <George.Dunlap@xxxxxxxxxxxxx>]

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.  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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