linux-kernel - Re: [Xen-devel] [RFC][PATCH 03/10] xen/hybrid: Xen Hybrid Extension initialization

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Message-ID: <4AB1496B.9070705@goop.org>
Date:	Wed, 16 Sep 2009 13:24:11 -0700
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	Sheng Yang <sheng@...ux.intel.com>
CC:	Keir Fraser <keir.fraser@...citrix.com>,
	Jeremy Fitzhardinge <jeremy.fitzhardinge@...rix.com>,
	xen-devel <xen-devel@...ts.xensource.com>,
	Eddie Dong <eddie.dong@...el.com>,
	linux-kernel@...r.kernel.org, Jun Nakajima <jun.nakajima@...el.com>
Subject: Re: [Xen-devel] [RFC][PATCH 03/10] xen/hybrid: Xen Hybrid Extension
 initialization

On 09/16/09 01:42, Sheng Yang wrote:
> As we know that PV guest have performance issue with x86_64 that guest kernel
> and userspace resistent in the same ring, then the necessary TLB flushes when
> switch between guest userspace and guest kernel cause overhead, and much more
> syscall overhead is also introduced. The Hybrid Extension estimated these
> overhead by putting guest kernel back in (non-root) ring0 then achieve the
> better performance than PV guest.
>
> The Hybrid Extension is started from real mode like HVM guest, but also with a
> component based PV feature selection(e.g. PV halt, PV timer, event channel,
> then PV drivers). So guest with Hybrid extension feature can takes the
> advantages of both H/W virtualization and Para-Virtualization.
>
> This patch introduced the Hybrid Extension guest initialization.
>
> Guest would detect Hybrid capability using CPUID 0x40000002.edx, then call
> HVMOP_enable_hybrid hypercall to enable hybrid support in hypervisor.
>   

I think having an option to put PV guests into an HVM container is a
good one, but as I mentioned in the other mail, I don't think this is
the right approach.

It would be much better to make it so that an unmodified guest works in
such a mode; even with no specific optimisations the guest would get
benefit from faster kernel<->usermode switches.

Then we can add specific optimisations to take advantage of, say,
running in ring 0 (=fast syscalls) and having access to HAP hardware
(=direct pagetable updates, no pinning).


    J
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