Re: [Qemu-devel] [PATCH v7 1/8] virtio: endian-ambivalent targets using legacy virtio

[Greg Kurz]

On Thu, 17 Apr 2014 11:00:26 +0300
"Michael S. Tsirkin" <address@hidden> wrote:

> On Thu, Apr 17, 2014 at 08:54:12AM +0200, Greg Kurz wrote:
> > On Wed, 16 Apr 2014 20:32:07 +0300
> > "Michael S. Tsirkin" <address@hidden> wrote:
> > 
> > > On Wed, Apr 16, 2014 at 05:42:22PM +0100, Peter Maydell wrote:
> > > > On 16 April 2014 17:34, Michael S. Tsirkin <address@hidden> wrote:
> > > > > so it looks like virtio is currently compiled per-target.
> > > > > So why isn't it reasonable to keep it per-target for
> > > > > purpose of this enhancement?
> > > > > What am I missing?
> > > > 
> > > > "virtio" is more than one C file. Currently per-target:
> > > > hw/virtio/virtio.c
> > > > hw/virtio/virtio-balloon.c
> > > > hw/scsi/virtio-scsi.c
> > > > hw/block/virtio-blk.c
> > > > hw/char/virtio-serial-bus.c
> > > > hw/net/virtio-net.c
> > > > 
> > > > Currently built once:
> > > > hw/char/virtio-console.c
> > > > hw/virtio/virtio-bus.c
> > > > hw/virtio/virtio-mmio.c
> > > > hw/virtio/virtio-pci.c
> > > > hw/virtio/virtio-rng.c
> > > > 
> > > > If we can move files from the first group to the second
> > > > that's cool.
> > > 
> > > But doesn't have to be part of this series, yes?
> > > I would say let's at least have virtio 1.0 out
> > > and implemented in qemu and linux guests, then
> > > we can start deprecate virtio 0.X in favor of it.
> > > 
> > > > Moving files from the second to the first is
> > > > what I'd like to avoid...
> > > > 
> > > > thanks
> > > > -- PMM
> > > 
> > > Absolutely.
> > > 
> > > Looks like we are in agreement after all.
> > > 
> > > So as far as I could see the only issue is with config access
> > > e.g. in virtio-pci?
> > > That one already has a branch around virtio_is_big_endian,
> > > so it's not a problem, there's no need to optimize that.
> > > 
> > 
> > It is because you haven't looked at the other patches... When
> > the whole serie is applied, all virtio files use inlined helpers
> > from virtio-access.h to access memory and fix endianness. That is
> > why we'd rather not add target dependency here...
> 
> Right, so what I'm saying is very simple: there are two types of helpers:
> - per target ones for guest memory access
> - generic ones for config access
> 
> 
> One way to implement generic ones is as out of line wrappers
> for inline per target ones, another way is a separate implementation.
> 

Then you want something like this ?

static inline void virtio_stl_p(VirtIODevice *vdev, void *ptr, uint32_t v)
{
#ifdef TARGET_BIENDIAN
    if (virtio_is_big_endian(vdev)) {
        stl_be_p(ptr, v);
    } else {
        stl_le_p(ptr, v);
    }
#else
    stl_p(ptr, v);
#endif
}

> 
> > This being said, if you have an insight to have branch-less code
> > for fixed endian targets, and dedicated code for ambivalent endian
> > targets, please send something.
> > 
> > Cheers.
> > 
> > --
> > Greg
> 
> Sure but I'm a bit busy otherwise - if above hints are insufficient,
> expect something by Tuesday.
> 
> 

-- 
Gregory Kurz                                     address@hidden
                                                 address@hidden
Software Engineer @ IBM/Meiosys                  http://www.ibm.com
Tel +33 (0)562 165 496

"Anarchy is about taking complete responsibility for yourself."
        Alan Moore.