RE: [PATCH v2 01/12] viridian: don't blindly write to 32-bit registers is 'mode' is invalid

["Durrant, Paul" <pdurrant@xxxxxxxxxxxx>]

> -----Original Message-----
> From: Jan Beulich <jbeulich@xxxxxxxx>
> Sent: 20 November 2020 14:20
> To: Paul Durrant <paul@xxxxxxx>
> Cc: Durrant, Paul <pdurrant@xxxxxxxxxxxx>; Wei Liu <wl@xxxxxxx>; Andrew Cooper
> <andrew.cooper3@xxxxxxxxxx>; Roger Pau Monné <roger.pau@xxxxxxxxxx>; 
> xen-devel@xxxxxxxxxxxxxxxxxxxx
> Subject: RE: [EXTERNAL] [PATCH v2 01/12] viridian: don't blindly write to 
> 32-bit registers is 'mode'
> is invalid
> 
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> 
> On 20.11.2020 10:48, Paul Durrant wrote:
> > From: Paul Durrant <pdurrant@xxxxxxxxxx>
> >
> > If hvm_guest_x86_mode() returns something other than 8 or 4 then
> > viridian_hypercall() will return immediately but, on the way out, will write
> > back status as if 'mode' was 4. This patch simply makes it leave the 
> > registers
> > alone.
> 
> IOW 16-bit protected mode and real mode aren't allowed to make
> hypercalls (supported also be the earlier switch() in the
> function)?

I don't think running enlightened versions of OS/2 1.3 is really a use case :-)

> But then, to achieve what you want, wouldn't it be
> more direct to simply "return HVM_HCALL_completed;" straight
> from that earlier switch()'s default case? At which point the
> switch() you modify could become if/else? Anyway - you're the
> maintainer, I'm just wondering ...
> 

It could be done that way but I prefer the exit path via goto.

  Paul

> Jan