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

[Tim Deegan <tim@xxxxxxx>]

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.

Cheers,

Tim.

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