Re: [Xen-devel] [PATCH 5/3] x86/shadow: emulate_gva_to_mfn() should respect p2m_ioreq_server

[Tim Deegan <tim@xxxxxxx>]

At 04:22 -0700 on 13 Nov (1542082936), Jan Beulich wrote:
> >>> On 13.11.18 at 11:59, <Paul.Durrant@xxxxxxxxxx> wrote:
> >> Subject: [PATCH 5/3] x86/shadow: emulate_gva_to_mfn() should respect
> >> p2m_ioreq_server
> >> 
> >> Writes to such pages would need to be handed to the emulator, which we're
> >> not prepared to do at this point.
> >> 
> >> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
> >> 
> >> --- a/xen/arch/x86/mm/shadow/hvm.c
> >> +++ b/xen/arch/x86/mm/shadow/hvm.c
> >> @@ -338,7 +338,7 @@ static mfn_t emulate_gva_to_mfn(struct v
> >>      {
> >>          return _mfn(BAD_GFN_TO_MFN);
> >>      }
> >> -    if ( p2m_is_discard_write(p2mt) )
> >> +    if ( p2m_is_discard_write(p2mt) || p2mt == p2m_ioreq_server )
> >>      {
> >>          put_page(page);
> >>          return _mfn(READONLY_GFN);
> > 
> > Is this what we want here? I would have thought we want to return 
> > BAD_GFN_TO_MFN in the p2m_ioreq_server case so that the caller treats this 
> > in 
> > the same way it would MMIO.
> 
> I'm not sure which behavior is better; I'm certainly fine with switching
> as you say, but I'd first like to see Tim's opinion as well.

I'm not clear on what behaviour you want for this kind of page in
general -- I suspect I have missed or forgotten some context.  If the
guest's not supposed to write to it, then IMO you should add it to
P2M_DISCARD_WRITE_TYPES rather than special-casing it here.

Cheers,

Tim.


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