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

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> 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.

Jan


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