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

[Matthew Rosato <mjrosato@linux.ibm.com>]

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);
>>> +}
>>> +
> 
> (...)
> 
>