Re: [Xen-devel] Question about alignment check in sh_x86_emulate

To:	"Tim Deegan" <Tim.Deegan@xxxxxxxxxx>
Subject:	Re: [Xen-devel] Question about alignment check in sh_x86_emulate_write
From:	"Trolle Selander" <trolle.selander@xxxxxxxxx>
Date:	Tue, 4 Dec 2007 14:52:41 +0100
Cc:	"Su, Disheng" <disheng.su@xxxxxxxxx>, xen-devel@xxxxxxxxxxxxxxxxxxx
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To expand on my hastily posted comment before - OS/2 does get snagged on the check because of unaligned page table writes.

On Dec 4, 2007 2:33 PM, Tim Deegan < Tim.Deegan@xxxxxxxxxx> wrote:

At 21:03 +0800 on 04 Dec (1196802182), Su, Disheng wrote:
> It does not write to pagetable. I am doing CR0.wp emulation(don't let
> guest turn off CR0.wp, but emulate the trapped instructions in shadow).
> Some applications like Anti-virus tools will turn off CR0.wp to change
> guest IDT etc.

Oh, I see.

> >> I am wordering about the reason for this check. Is it safe to get
> >> rid of it?
> >
> > Not really. Among other things, self-aligned writes never cross page
> > boundaries.
> >
> Is it Ok to add the checking for cross page boundaries instead?

Yes, but you'll probably find you need page-crossing support as well if
you're emulating arbitrary memory writes. I believe we have a patch
somewhere that does that; I'll dig it out and polish it up for -unstable.

Cheers,

Tim.

--
Tim Deegan <Tim.Deegan@xxxxxxxxxx >
Principal Software Engineer, Citrix Systems.
[Company #5334508: XenSource UK Ltd, reg'd c/o EC2Y 5EB, UK.]

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Re: [Xen-devel] Question about alignment check in sh_x86_emulate_write