Re: [PATCH v2 2/3] s390x/pci: Honor DMA limits set by vfio

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

On 9/15/20 8:54 AM, Thomas Huth wrote:
> On 15/09/2020 00.29, Matthew Rosato wrote:
>> When an s390 guest is using lazy unmapping, it can result in a very
>> large number of oustanding DMA requests, far beyond the default
>> limit configured for vfio.  Let's track DMA usage similar to vfio
>> in the host, and trigger the guest to flush their DMA mappings
>> before vfio runs out.
>>
>> Signed-off-by: Matthew Rosato <mjrosato@linux.ibm.com>
>> ---
>>   hw/s390x/s390-pci-bus.c  | 99 +++++++++++++++++++++++++++++++++++++++++++++---
>>   hw/s390x/s390-pci-bus.h  |  9 +++++
>>   hw/s390x/s390-pci-inst.c | 29 +++++++++++---
>>   hw/s390x/s390-pci-inst.h |  3 ++
>>   4 files changed, 129 insertions(+), 11 deletions(-)
>>
>> diff --git a/hw/s390x/s390-pci-bus.c b/hw/s390x/s390-pci-bus.c
>> index 92146a2..23474cd 100644
>> --- a/hw/s390x/s390-pci-bus.c
>> +++ b/hw/s390x/s390-pci-bus.c
>> @@ -11,6 +11,8 @@
>>    * directory.
>>    */
>>   
>> +#include <sys/ioctl.h>
>> +
>>   #include "qemu/osdep.h"
>>   #include "qapi/error.h"
>>   #include "qapi/visitor.h"
>> @@ -24,6 +26,9 @@
>>   #include "qemu/error-report.h"
>>   #include "qemu/module.h"
>>   
>> +#include "hw/vfio/pci.h"
>> +#include "hw/vfio/vfio-common.h"
>> +
>>   #ifndef DEBUG_S390PCI_BUS
>>   #define DEBUG_S390PCI_BUS  0
>>   #endif
>> @@ -737,6 +742,82 @@ static void s390_pci_iommu_free(S390pciState *s, PCIBus *bus, int32_t devfn)
>>       object_unref(OBJECT(iommu));
>>   }
>>   
>> +static bool s390_sync_dma_avail(int fd, unsigned int *avail)
>> +{
>> +    struct vfio_iommu_type1_info *info;
> 
> You could use g_autofree to get rid of the g_free() at the end.
> 

OK

>> +    uint32_t argsz;
>> +    bool rval = false;
>> +    int ret;
>> +
>> +    if (avail == NULL) {
>> +        return false;
>> +    }
> 
> Since this is a "static" local function, and calling it with avail ==
> NULL does not make too much sense, I think I'd rather turn this into an
> assert() instead. >

Sure, sounds good.


>> +    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);
>> +        info->argsz = argsz;
>> +        ret = ioctl(fd, VFIO_IOMMU_GET_INFO, info);
>> +    }
>> +
>> +    if (ret) {
>> +        goto out;
>> +    }
>> +
>> +    /* If the capability exists, update with the current value */
>> +    rval = vfio_get_info_dma_avail(info, avail);
>> +
>> +out:
>> +    g_free(info);
>> +    return rval;
>> +}
>> +
>> +static S390PCIDMACount *s390_start_dma_count(S390pciState *s, VFIODevice *vdev)
>> +{
>> +    int id = vdev->group->container->fd;
>> +    S390PCIDMACount *cnt;
>> +    uint32_t avail;
>> +
>> +    if (!s390_sync_dma_avail(id, &avail)) {
>> +        return NULL;
>> +    }
>> +
>> +    QTAILQ_FOREACH(cnt, &s->zpci_dma_limit, link) {
>> +        if (cnt->id  == id) {
>> +            cnt->users++;
>> +            return cnt;
>> +        }
>> +    }
>> +
>> +    cnt = g_new0(S390PCIDMACount, 1);
>> +    cnt->id = id;
>> +    cnt->users = 1;
>> +    cnt->avail = avail;
>> +    QTAILQ_INSERT_TAIL(&s->zpci_dma_limit, cnt, link);
>> +    return cnt;
>> +}
>> +
>> +static void s390_end_dma_count(S390pciState *s, S390PCIDMACount *cnt)
>> +{
>> +    if (cnt == NULL) {
>> +        return;
>> +    }
> 
> Either use assert() or drop this completely (since you're checking it at
> the caller site already).
> 

Fair - I'll assert() here.  Thanks!