Re: [RFC PATCH V1 10/12] libxl: Add support for virtio-disk configuration

[Stefano Stabellini <sstabellini@kernel.org>]

On Thu, 6 Aug 2020, Oleksandr wrote:
> On 05.08.20 02:23, Stefano Stabellini wrote:
> > On Mon, 3 Aug 2020, Oleksandr Tyshchenko wrote:
> > > From: Oleksandr Tyshchenko <oleksandr_tyshchenko@epam.com>
> > > 
> > > This patch adds basic support for configuring and assisting virtio-disk
> > > backend (emualator) which is intended to run out of Qemu and could be run
> > > in any domain.
> > > 
> > > Xenstore was chosen as a communication interface for the emulator running
> > > in non-toolstack domain to be able to get configuration either by reading
> > > Xenstore directly or by receiving command line parameters (an updated 'xl
> > > devd'
> > > running in the same domain would read Xenstore beforehand and call backend
> > > executable with the required arguments).
> > > 
> > > An example of domain configuration (two disks are assigned to the guest,
> > > the latter is in readonly mode):
> > > 
> > > vdisk = [ 'backend=DomD, disks=rw:/dev/mmcblk0p3;ro:/dev/mmcblk1p3' ]
> > > 
> > > Where per-disk Xenstore entries are:
> > > - filename and readonly flag (configured via "vdisk" property)
> > > - base and irq (allocated dynamically)
> > > 
> > > Besides handling 'visible' params described in configuration file,
> > > patch also allocates virtio-mmio specific ones for each device and
> > > writes them into Xenstore. virtio-mmio params (irq and base) are
> > > unique per guest domain, they allocated at the domain creation time
> > > and passed through to the emulator. Each VirtIO device has at least
> > > one pair of these params.
> > > 
> > > TODO:
> > > 1. An extra "virtio" property could be removed.
> > > 2. Update documentation.
> > > 
> > > Signed-off-by: Oleksandr Tyshchenko <oleksandr_tyshchenko@epam.com>
> > > ---
> > >   tools/libxl/Makefile                 |   4 +-
> > >   tools/libxl/libxl_arm.c              |  63 +++++++++++++++----
> > >   tools/libxl/libxl_create.c           |   1 +
> > >   tools/libxl/libxl_internal.h         |   1 +
> > >   tools/libxl/libxl_types.idl          |  15 +++++
> > >   tools/libxl/libxl_types_internal.idl |   1 +
> > >   tools/libxl/libxl_virtio_disk.c      | 109
> > > +++++++++++++++++++++++++++++++++
> > >   tools/xl/Makefile                    |   2 +-
> > >   tools/xl/xl.h                        |   3 +
> > >   tools/xl/xl_cmdtable.c               |  15 +++++
> > >   tools/xl/xl_parse.c                  | 115
> > > +++++++++++++++++++++++++++++++++++
> > >   tools/xl/xl_virtio_disk.c            |  46 ++++++++++++++
> > >   12 files changed, 360 insertions(+), 15 deletions(-)
> > >   create mode 100644 tools/libxl/libxl_virtio_disk.c
> > >   create mode 100644 tools/xl/xl_virtio_disk.c
> > > 
> > > diff --git a/tools/libxl/Makefile b/tools/libxl/Makefile
> > > index 38cd43a..df94b13 100644
> > > --- a/tools/libxl/Makefile
> > > +++ b/tools/libxl/Makefile
> > > @@ -141,7 +141,9 @@ LIBXL_OBJS = flexarray.o libxl.o libxl_create.o
> > > libxl_dm.o libxl_pci.o \
> > >   			libxl_vtpm.o libxl_nic.o libxl_disk.o libxl_console.o
> > > \
> > >   			libxl_cpupool.o libxl_mem.o libxl_sched.o libxl_tmem.o
> > > \
> > >   			libxl_9pfs.o libxl_domain.o libxl_vdispl.o \
> > > -			libxl_pvcalls.o libxl_vsnd.o libxl_vkb.o
> > > $(LIBXL_OBJS-y)
> > > +			libxl_pvcalls.o libxl_vsnd.o libxl_vkb.o \
> > > +			libxl_virtio_disk.o $(LIBXL_OBJS-y)
> > > +
> > >   LIBXL_OBJS += libxl_genid.o
> > >   LIBXL_OBJS += _libxl_types.o libxl_flask.o _libxl_types_internal.o
> > >   diff --git a/tools/libxl/libxl_arm.c b/tools/libxl/libxl_arm.c
> > > index 4f748e3..469a8b0 100644
> > > --- a/tools/libxl/libxl_arm.c
> > > +++ b/tools/libxl/libxl_arm.c
> > > @@ -13,6 +13,12 @@
> > >   #define GUEST_VIRTIO_MMIO_SIZE  xen_mk_ullong(0x200)
> > >   #define GUEST_VIRTIO_MMIO_SPI   33
> > >   +#ifndef container_of
> > > +#define container_of(ptr, type, member) ({			\
> > > +        typeof( ((type *)0)->member ) *__mptr = (ptr);	\
> > > +        (type *)( (char *)__mptr - offsetof(type,member) );})
> > > +#endif
> > > +
> > >   static const char *gicv_to_string(libxl_gic_version gic_version)
> > >   {
> > >       switch (gic_version) {
> > > @@ -44,14 +50,32 @@ int libxl__arch_domain_prepare_config(libxl__gc *gc,
> > >           vuart_enabled = true;
> > >       }
> > >   -    /*
> > > -     * XXX: Handle properly virtio
> > > -     * A proper solution would be the toolstack to allocate the
> > > interrupts
> > > -     * used by each virtio backend and let the backend now which one is
> > > used
> > > -     */
> > >       if (libxl_defbool_val(d_config->b_info.arch_arm.virtio)) {
> > > -        nr_spis += (GUEST_VIRTIO_MMIO_SPI - 32) + 1;
> > > +        uint64_t virtio_base;
> > > +        libxl_device_virtio_disk *virtio_disk;
> > > +
> > > +        virtio_base = GUEST_VIRTIO_MMIO_BASE;
> > >           virtio_irq = GUEST_VIRTIO_MMIO_SPI;
> > > +
> > > +        if (!d_config->num_virtio_disks) {
> > > +            LOG(ERROR, "Virtio is enabled, but no Virtio devices
> > > present\n");
> > > +            return ERROR_FAIL;
> > > +        }
> > > +        virtio_disk = &d_config->virtio_disks[0];
> > > +
> > > +        for (i = 0; i < virtio_disk->num_disks; i++) {
> > > +            virtio_disk->disks[i].base = virtio_base;
> > > +            virtio_disk->disks[i].irq = virtio_irq;
> > > +
> > > +            LOG(DEBUG, "Allocate Virtio MMIO params: IRQ %u BASE
> > > 0x%"PRIx64,
> > > +                virtio_irq, virtio_base);
> > > +
> > > +            virtio_irq ++;
> > > +            virtio_base += GUEST_VIRTIO_MMIO_SIZE;
> > > +        }
> > > +        virtio_irq --;
> > > +
> > > +        nr_spis += (virtio_irq - 32) + 1;
> > It looks like it is an interrupt per device, which could lead to quite a
> > few of them being allocated.
> 
> Yes.
> 
> 
> > So, if we end up allocating
> > let's say 6 virtio interrupts for a domain, the chance of a clash with a
> > physical interrupt of a passthrough device is real.
> 
> Yes.
> 
> 
> > 
> > I am not entirely sure how to solve it, but these are a few ideas:
> > - choosing virtio interrupts that are less likely to conflict (maybe >
> >    1000)
> > - make the virtio irq (optionally) configurable so that a user could
> >    override the default irq and specify one that doesn't conflict
> > - implementing support for virq != pirq (even the xl interface doesn't
> >    allow to specify the virq number for passthrough devices, see "irqs")
> 
> Good ideas. The first requires minimum effort. Couldn't we choose virtio
> interrupt to allocate after making sure it is absent in guest "irqs"
> (d_config->b_info.irqs[i]), I mean to find some holes?

Yes, that might be possible too.

So far we have tried to stay away from dynamic irq allocation for guests
virtual devices, but we also didn't have to deal with potentially large
amount of them :-)