linux-kernel - Re: [PATCH] xen/x86: Adjust stack pointer in xen

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Date:	Mon, 16 Nov 2015 15:31:52 -0500
From:	Boris Ostrovsky <boris.ostrovsky@...cle.com>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	xen-devel <xen-devel@...ts.xen.org>,
	David Vrabel <david.vrabel@...rix.com>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	Borislav Petkov <bp@...en8.de>
Subject: Re: [PATCH] xen/x86: Adjust stack pointer in xen_sysexit

On 11/16/2015 02:03 PM, Andy Lutomirski wrote:
> It's still a waste of effort, though. Also, I'd eventually like the 
> number of places in Xen code in which rsp/esp is invalid to be exactly 
> zero, and this approach makes this harder or even impossible.

That's what PVH is going to do.



> Does PVH hook into the entry asm code at all?  I thought it was just
> boot code and drivers.

Not the current version --- it starts with xen_start_kernel(). But we 
are currently changing it and my plan is to have a small stub executed 
initially (to set bootparams and such) and then jump to startup_{32|64}().

>
> In any case, someone needs to do some serious review and cleanup on
> the whole paravirt op mess.  We have a bunch of paravirt ops that
> serve little purpose.
>
> The paravirt infrastructure is a bit weird, too: it seems to
> effectively have four states for each patch site.  There's:
>
> 1. The initial state, which is unoptimized and works on native.
> Presumably any of these that happen early also need to work, if
> slowly, on Xen.

Not on PV (and as of today, on PVH) --- we start directly from 
xen_start_kernel(). I.e.  from step 2.

>
> 2. The Xen state without text patching.  I'm not actually sure why
> this exists at all.  Are there pvops that need to switch too early for
> us to patch the text?

I don't think so.

-boris
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