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Re: [Xen-devel] [RFC 3/4] HVM x86 deprivileged mode: Code for switching into/out of deprivileged mode
- To: "Andrew Cooper" <andrew.cooper3@xxxxxxxxxx>
- From: "Jan Beulich" <JBeulich@xxxxxxxx>
- Date: Wed, 12 Aug 2015 04:10:15 -0600
- Cc: xen-devel@xxxxxxxxxxxxxxxxxxx, keir@xxxxxxx, ian.campbell@xxxxxxxxxx, george.dunlap@xxxxxxxxxxxxx, Tim Deegan <tim@xxxxxxx>, Aravind Gopalakrishnan <Aravind.Gopalakrishnan@xxxxxxx>, Suravee Suthikulpanit <suravee.suthikulpanit@xxxxxxx>, Ben Catterall <Ben.Catterall@xxxxxxxxxx>, Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>
- Delivery-date: Wed, 12 Aug 2015 10:10:28 +0000
- List-id: Xen developer discussion <xen-devel.lists.xen.org>
>>> On 11.08.15 at 19:19, <andrew.cooper3@xxxxxxxxxx> wrote:
> KVM unilaterally reloads the host task register on vmexit, and I suspect
> this is probably the way to go, but have not had time to investigate
> whether there is any performance impact from doing so. Given how little
> of a TSS is actually used in long mode, I wouldn't expect an `ltr` to be
> as expensive as it might have been in legacy modes.
How much of the TSS is being used shouldn't matter at all for
LTR execution time, since the instruction doesn't access the
TSS itself (and iirc there's also no caching documented for TSS
uses).
Jan
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