Re: [Xen-devel] [PATCH] Re: I/O port access handling for PVH

[George Dunlap <george.dunlap@xxxxxxxxxxxxx>]

On 11/29/2013 09:19 AM, Jan Beulich wrote:
> > > > On 28.11.13 at 12:44, Tim Deegan <tim@xxxxxxx> wrote:
> > At 11:37 +0000 on 28 Nov (1385635050), George Dunlap wrote:
> > > On 11/28/2013 11:01 AM, Tim Deegan wrote:
> > > > At 15:10 +0100 on 21 Oct (1382364606), Jan Beulich wrote:
> > > > > In particular it would then hopefully be safe to do all that without
> > > > > the on-stack emulation stub, as this ought to be necessary only
> > > > > for Dom0, which ought to always have direct access to such
> > > > > "special" I/O ports. With one apparent caveat: SVM sets
> > > > > GENERAL1_INTERCEPT_SMI (for a reason that escapes my right
> > > > > now), and hence control doesn't transfer directly to SMM when
> > > > > an SMI occurs (and consequently registers aren't expected). But
> > > > > I would hope that this intercept isn't really needed, and hence
> > > > > could be dropped at least for PVH guests.
> > > > (I realise I'm rather late replying to this - I put it aside and then
> > > > only found it again today)
> > > >
> > > > On machines where the BIOS has locked down SMM mode, this
> > > > intercept is in fact ignored by the hardware, and that works fine.
> > > > So we can drop it for all VMs if it's convenient:
> > > >
> > > > commit a842864f3901078e2a5f4d1cca2f01a72c8d7d13
> > > > Author: Tim Deegan <tim@xxxxxxx>
> > > > Date:   Thu Nov 28 10:58:42 2013 +0000
> > > >
> > > >       x86/svm: don't intercept SMI.
> > > >       
> > > >       The SMI intercept is ignored anyway when the BIOS has set the SMMLOCK
> > > >       bit in HWCR (see APM v3.21, volume 2, 15.13.3) and it's convenient for
> > > >       PVH IO processing to have the SMI handled directly with the guest's
> > > >       GPR state (for BIOSes that use SMI as a sort of function call
> > interface).
> > > >       
> > > >       Signed-off-by: Tim Deegan <tim@xxxxxxx>
> > > I take it you're not targeting this for 4.4?
> > Indeed not.  Sorry, I should have said so.
> And even more so considering that PVH support on SVM is going
> to remain a todo item for after 4.4 anyway.
Yes, I had started to write this, but figured there was no point 
discussing the merits of accepting it if Tim wasn't asking for it to be 
accepted. :-)
 -George

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