Re: [Xen-devel] [PATCH RFC] ioreq-server: handle the lack of a default emulator properly

[Paul Durrant <Paul.Durrant@xxxxxxxxxx>]

> -----Original Message-----
> From: Jan Beulich [mailto:JBeulich@xxxxxxxx]
> Sent: 26 September 2014 17:23
> To: Paul Durrant
> Cc: xen-devel@xxxxxxxxxxxxx; Keir (Xen.org)
> Subject: Re: [PATCH RFC] ioreq-server: handle the lack of a default emulator
> properly
> 
> >>> On 26.09.14 at 17:31, <paul.durrant@xxxxxxxxxx> wrote:
> > This fix should probably go in 4.5 but this patch is RFC for the moment
> > because of uncertainty about what to do for unemulated MMIO accesses.
> > Originally I forced a domain crash in this case, but hvmloader actually
> > hit the crash because the code that deals with building the ACPI TPM
> > info reads from 0xFED40F00 looking for a signature value and there is
> > nothing backing this access in my configuration. So, the question is
> > whether to whitelist this access in some way or make building that
> > table optional in some way so that it is only invoked if an emulated
> > TPM is definitely present.
> 
> I think in the sense of acting like real hardware, having a definite
> point to complete such I/O is quite desirable, as opposed to killing
> the guest.

Ok. The doubt was around the fact that on bare metal an unhandled MMIO would 
normally cause an MCE (by way of a bus error).

> 
> > --- a/xen/arch/x86/hvm/hvm.c
> > +++ b/xen/arch/x86/hvm/hvm.c
> > @@ -2386,8 +2386,7 @@ static struct hvm_ioreq_server
> *hvm_select_ioreq_server(struct domain *d,
> >      if ( list_empty(&d->arch.hvm_domain.ioreq_server.list) )
> >          return NULL;
> >
> > -    if ( list_is_singular(&d->arch.hvm_domain.ioreq_server.list) ||
> > -         (p->type != IOREQ_TYPE_COPY && p->type != IOREQ_TYPE_PIO) )
> > +    if ( p->type != IOREQ_TYPE_COPY && p->type != IOREQ_TYPE_PIO )
> 
> I'm having some trouble understanding the reason for this change.
> 

The implication that list_is_singular -> 'singular ioreq server is default' is 
wrong; it was an 'optimization' too far. Now that I have QEMU ported, in my 
tests it is the only emulator but it is not a default emulator (because it 
registers explicit ranges).

  Paul



> Jan


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