Re: [PATCH v6 12/19] vfio-user: IOMMU support for remote device

[Jag Raman]

> On Feb 22, 2022, at 5:40 AM, Stefan Hajnoczi <stefanha@redhat.com> wrote:
> 
> On Thu, Feb 17, 2022 at 02:48:59AM -0500, Jagannathan Raman wrote:
>> +struct RemoteIommuElem {
>> +    AddressSpace  as;
>> +    MemoryRegion  mr;
>> +};
>> +
>> +GHashTable *remote_iommu_elem_by_bdf;
> 
> A mutable global hash table requires synchronization when device
> emulation runs in multiple threads.
> 
> I suggest using pci_setup_iommu()'s iommu_opaque argument to avoid the
> global. If there is only 1 device per remote PCI bus, then there are no
> further synchronization concerns.

OK, will avoid the global. We would need to access the hash table
concurrently since there could be more than one device in the
same bus - so a mutex would be needed here.

> 
>> +
>> +#define INT2VOIDP(i) (void *)(uintptr_t)(i)
>> +
>> +static AddressSpace *remote_iommu_find_add_as(PCIBus *pci_bus,
>> +                                              void *opaque, int devfn)
>> +{
>> +    struct RemoteIommuElem *elem = NULL;
>> +    int pci_bdf = PCI_BUILD_BDF(pci_bus_num(pci_bus), devfn);
>> +
>> +    if (!remote_iommu_elem_by_bdf) {
>> +        return &address_space_memory;
>> +    }
> 
> When can this happen? remote_configure_iommu() allocates
> remote_iommu_elem_by_bdf so it should always be non-NULL.

I think we won’t hit this case. g_hash_table_new_full() would always succeed.

Thank you!
--
Jag