Re: [PATCH v3 4/5] s390x/pci: Add routine to get the vfio dma available count

[Matthew Rosato]

On 9/16/20 6:27 AM, Cornelia Huck wrote:
> On Wed, 16 Sep 2020 09:21:39 +0200
> Philippe Mathieu-Daudé <philmd@redhat.com> wrote:
>
> > On 9/15/20 9:14 PM, Matthew Rosato wrote:
> > > Create new files for separating out vfio-specific work for s390
> > > pci. Add the first such routine, which issues VFIO_IOMMU_GET_INFO
> > > ioctl to collect the current dma available count.
> > >
> > > Signed-off-by: Matthew Rosato <mjrosato@linux.ibm.com>
> > > ---
> > >   hw/s390x/meson.build     |  1 +
> > >   hw/s390x/s390-pci-vfio.c | 54 ++++++++++++++++++++++++++++++++++++++++++++++++
> > >   hw/s390x/s390-pci-vfio.h | 17 +++++++++++++++
> > >   3 files changed, 72 insertions(+)
> > >   create mode 100644 hw/s390x/s390-pci-vfio.c
> > >   create mode 100644 hw/s390x/s390-pci-vfio.h
>
> (...)
>
> > > diff --git a/hw/s390x/s390-pci-vfio.c b/hw/s390x/s390-pci-vfio.c
> > > new file mode 100644
> > > index 0000000..75e3ac1
> > > --- /dev/null
> > > +++ b/hw/s390x/s390-pci-vfio.c
> > > @@ -0,0 +1,54 @@
> > > +/*
> > > + * s390 vfio-pci interfaces
> > > + *
> > > + * Copyright 2020 IBM Corp.
> > > + * Author(s): Matthew Rosato <mjrosato@linux.ibm.com>
> > > + *
> > > + * This work is licensed under the terms of the GNU GPL, version 2 or (at
> > > + * your option) any later version. See the COPYING file in the top-level
> > > + * directory.
> > > + */
> > > +
> > > +#include <sys/ioctl.h>
> > > +
> > > +#include "qemu/osdep.h"
> > > +#include "s390-pci-vfio.h"
> > > +#include "hw/vfio/vfio-common.h"
> > > +
> > > +/*
> > > + * Get the current DMA available count from vfio.  Returns true if vfio is
> > > + * limiting DMA requests, false otherwise.  The current available count read
> > > + * from vfio is returned in avail.
> > > + */
> > > +bool s390_pci_update_dma_avail(int fd, unsigned int *avail)
> > > +{
> > > +    g_autofree struct vfio_iommu_type1_info *info;
> > > +    uint32_t argsz;
> > > +    int ret;
> > > +
> > > +    assert(avail);
> > > +
> > > +    argsz = sizeof(struct vfio_iommu_type1_info);
> > > +    info = g_malloc0(argsz);
> > > +    info->argsz = argsz;
> > > +    /*
> > > +     * If the specified argsz is not large enough to contain all
> > > +     * capabilities it will be updated upon return.  In this case
> > > +     * use the updated value to get the entire capability chain.
> > > +     */
> > > +    ret = ioctl(fd, VFIO_IOMMU_GET_INFO, info);
> > > +    if (argsz != info->argsz) {
> > > +        argsz = info->argsz;
> > > +        info = g_realloc(info, argsz);
> >
> > Do we need to bzero [sizeof(struct vfio_iommu_type1_info)..argsz[?
>
> If we do, I think we need to do the equivalent in
> vfio_get_region_info() as well?

I agree that it would need to be in both places or neither -- I would 
expect the re-driven ioctl to overwrite the prior contents of info 
(unless we get a bad ret, but in this case we don't care what is in info)?

Perhaps the fundamental difference between this code and 
vfio_get_region_info is that the latter checks for only a growing argsz 
and retries, whereas this code checks for != so it's technically 
possible for a smaller argsz to trigger the retry here, and we wouldn't 
know for sure that all bytes from the first ioctl call were overwritten.

What if I adjust this code to look like vfio_get_region_info:

retry:
        info->argsz = argsz;

        if (ioctl(fd, VFIO_IOMMU_GET_INFO, info)) {
                // no need to g_free() bc of g_autofree
                return false;   
        }

        if (info->argsz > argsz) {
                argsz = info->argsz;
                info = g_realloc(info, argsz);
                goto retry;
        }

        /* If the capability exists, update with the current value */
        return vfio_get_info_dma_avail(info, avail);

Now we would only trigger when we are told by the host that the buffer 
must be larger.

> (Also, shouldn't we check ret before looking at info->argsz?)

Yes, you are correct.  The above proposal would fix that issue too.

> > > +        info->argsz = argsz;
> > > +        ret = ioctl(fd, VFIO_IOMMU_GET_INFO, info);
> > > +    }
> > > +
> > > +    if (ret) {
> > > +        return false;
> > > +    }
> > > +
> > > +    /* If the capability exists, update with the current value */
> > > +    return vfio_get_info_dma_avail(info, avail);
> > > +}
> > > +
>
> (...)