Re: [RFC PATCH V1 01/12] hvm/ioreq: Make x86's IOREQ feature common

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

On Wed, 5 Aug 2020, Julien Grall wrote:
> On 04/08/2020 20:11, Stefano Stabellini wrote:
> > On Tue, 4 Aug 2020, Julien Grall wrote:
> > > On 04/08/2020 12:10, Oleksandr wrote:
> > > > On 04.08.20 10:45, Paul Durrant wrote:
> > > > > > +static inline bool hvm_ioreq_needs_completion(const ioreq_t *ioreq)
> > > > > > +{
> > > > > > +    return ioreq->state == STATE_IOREQ_READY &&
> > > > > > +           !ioreq->data_is_ptr &&
> > > > > > +           (ioreq->type != IOREQ_TYPE_PIO || ioreq->dir !=
> > > > > > IOREQ_WRITE);
> > > > > > +}
> > > > > I don't think having this in common code is correct. The short-cut of
> > > > > not
> > > > > completing PIO reads seems somewhat x86 specific.
> > > 
> > > Hmmm, looking at the code, I think it doesn't wait for PIO writes to
> > > complete
> > > (not read). Did I miss anything?
> > > 
> > > > Does ARM even
> > > > > have the concept of PIO?
> > > > 
> > > > I am not 100% sure here, but it seems that doesn't have.
> > > 
> > > Technically, the PIOs exist on Arm, however they are accessed the same way
> > > as
> > > MMIO and will have a dedicated area defined by the HW.
> > > 
> > > AFAICT, on Arm64, they are only used for PCI IO Bar.
> > > 
> > > Now the question is whether we want to expose them to the Device Emulator
> > > as
> > > PIO or MMIO access. From a generic PoV, a DM shouldn't have to care about
> > > the
> > > architecture used. It should just be able to request a given IOport
> > > region.
> > > 
> > > So it may make sense to differentiate them in the common ioreq code as
> > > well.
> > > 
> > > I had a quick look at QEMU and wasn't able to tell if PIOs and MMIOs
> > > address
> > > space are different on Arm as well. Paul, Stefano, do you know what they
> > > are
> > > doing?
> > 
> > On the QEMU side, it looks like PIO (address_space_io) is used in
> > connection with the emulation of the "in" or "out" instructions, see
> > ioport.c:cpu_inb for instance. Some parts of PCI on QEMU emulate PIO
> > space regardless of the architecture, such as
> > hw/pci/pci_bridge.c:pci_bridge_initfn.
> > 
> > However, because there is no "in" and "out" on ARM, I don't think
> > address_space_io can be accessed. Specifically, there is no equivalent
> > for target/i386/misc_helper.c:helper_inb on ARM.
> 
> So how PCI I/O BAR are accessed? Surely, they could be used on Arm, right?

PIO is also memory mapped on ARM and it seems to have its own MMIO
address window.