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

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

On 9/15/20 7:28 AM, Cornelia Huck wrote:
> On Mon, 14 Sep 2020 18:29:29 -0400
> Matthew Rosato <mjrosato@linux.ibm.com> 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(-)
>>
> 
> (...)
> 
>> @@ -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)
> 
> Not sure I like the name. It sounds like the function checks whether
> "sync dma" is available. Maybe s390_update_dma_avail()?
> 

Sounds fine to me.

>> +{
>> +    struct vfio_iommu_type1_info *info;
>> +    uint32_t argsz;
>> +    bool rval = false;
>> +    int ret;
>> +
>> +    if (avail == NULL) {
>> +        return false;
>> +    }
>> +
>> +    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);
> 
> Adding vfio specific things into the generic s390 pci emulation code
> looks a bit ugly... I'd prefer to factor that out into a separate file,
> especially if you plan to add more vfio-specific things in the future.
> 

Fair.   hw/s390x/s390-pci-vfio.* ?

>> +
>> +out:
>> +    g_free(info);
>> +    return rval;
>> +}
>> +
> 
> (...)
> 
>> diff --git a/hw/s390x/s390-pci-inst.c b/hw/s390x/s390-pci-inst.c
>> index 2f7a7d7..6af9af4 100644
>> --- a/hw/s390x/s390-pci-inst.c
>> +++ b/hw/s390x/s390-pci-inst.c
>> @@ -32,6 +32,9 @@
>>           }                                                          \
>>       } while (0)
>>   
>> +#define INC_DMA_AVAIL(iommu) if (iommu->dma_limit) iommu->dma_limit->avail++;
>> +#define DEC_DMA_AVAIL(iommu) if (iommu->dma_limit) iommu->dma_limit->avail--;
> 
> Hm... maybe lowercase inline functions might be better here?
> 

OK

>> +
>>   static void s390_set_status_code(CPUS390XState *env,
>>                                    uint8_t r, uint64_t status_code)
>>   {
> 
> (...)
> 
>> @@ -620,6 +629,7 @@ int rpcit_service_call(S390CPU *cpu, uint8_t r1, uint8_t r2, uintptr_t ra)
>>       S390PCIIOMMU *iommu;
>>       S390IOTLBEntry entry;
>>       hwaddr start, end;
>> +    uint32_t dma_avail;
>>   
>>       if (env->psw.mask & PSW_MASK_PSTATE) {
>>           s390_program_interrupt(env, PGM_PRIVILEGED, ra);
>> @@ -675,8 +685,9 @@ int rpcit_service_call(S390CPU *cpu, uint8_t r1, uint8_t r2, uintptr_t ra)
>>           }
>>   
>>           start += entry.len;
>> -        while (entry.iova < start && entry.iova < end) {
>> -            s390_pci_update_iotlb(iommu, &entry);
>> +        dma_avail = 1; /* Assume non-zero dma_avail to start */
>> +        while (entry.iova < start && entry.iova < end && dma_avail > 0) {
>> +            dma_avail = s390_pci_update_iotlb(iommu, &entry);
>>               entry.iova += PAGE_SIZE;
>>               entry.translated_addr += PAGE_SIZE;
>>           }
>> @@ -689,7 +700,13 @@ err:
>>           s390_pci_generate_error_event(error, pbdev->fh, pbdev->fid, start, 0);
>>       } else {
>>           pbdev->fmb.counter[ZPCI_FMB_CNT_RPCIT]++;
>> -        setcc(cpu, ZPCI_PCI_LS_OK);
>> +        if (dma_avail > 0) {
> 
> When I compile this (with a headers update), the compiler moans here
> about an uninitialized variable.
> 

D'oh.  Obviously dma_avail needs to be initialized outside of the 
if/else -- I'll double-check the logic here and fix.

>> +            setcc(cpu, ZPCI_PCI_LS_OK);
>> +        } else {
>> +            /* vfio DMA mappings are exhausted, trigger a RPCIT */
>> +            setcc(cpu, ZPCI_PCI_LS_ERR);
>> +            s390_set_status_code(env, r1, ZPCI_RPCIT_ST_INSUFF_RES);
>> +        }
>>       }
>>       return 0;
>>   }
> 
> (...)
>