Re: [Qemu-devel] [PATCH qemu v10 13/14] vfio: spapr: Add SPAPR IOMMU v2 support (DMA memory preregistering)

[Alex Williamson]

On Wed, 2015-07-08 at 14:30 +1000, David Gibson wrote:
> On Tue, Jul 07, 2015 at 09:05:02PM +1000, Alexey Kardashevskiy wrote:
> > On 07/07/2015 08:21 PM, Thomas Huth wrote:
> > >On Tue, 7 Jul 2015 20:05:25 +1000
> > >Alexey Kardashevskiy <address@hidden> wrote:
> > >
> > >>On 07/07/2015 05:23 PM, Thomas Huth wrote:
> > >>>On Mon,  6 Jul 2015 12:11:09 +1000
> > >>>Alexey Kardashevskiy <address@hidden> wrote:
> > >...
> > >>>>diff --git a/hw/vfio/common.c b/hw/vfio/common.c
> > >>>>index 8eacfd7..0c7ba8c 100644
> > >>>>--- a/hw/vfio/common.c
> > >>>>+++ b/hw/vfio/common.c
> > >>>>@@ -488,6 +488,76 @@ static void vfio_listener_release(VFIOContainer 
> > >>>>*container)
> > >>>>       
> > >>>> memory_listener_unregister(&container->iommu_data.type1.listener);
> > >>>>   }
> > >>>>
> > >>>>+static void vfio_ram_do_region(VFIOContainer *container,
> > >>>>+                              MemoryRegionSection *section, unsigned 
> > >>>>long req)
> > >>>>+{
> > >>>>+    int ret;
> > >>>>+    struct vfio_iommu_spapr_register_memory reg = { .argsz = 
> > >>>>sizeof(reg) };
> > >>>>+
> > >>>>+    if (!memory_region_is_ram(section->mr) ||
> > >>>>+        memory_region_is_skip_dump(section->mr)) {
> > >>>>+        return;
> > >>>>+    }
> > >>>>+
> > >>>>+    if (unlikely((section->offset_within_region & (getpagesize() - 
> > >>>>1)))) {
> > >>>>+        error_report("%s received unaligned region", __func__);
> > >>>>+        return;
> > >>>>+    }
> > >>>>+
> > >>>>+    reg.vaddr = (__u64) memory_region_get_ram_ptr(section->mr) +
> > >>>
> > >>>We're in usespace here ... I think it would be better to use uint64_t
> > >>>instead of the kernel-type __u64.
> > >>
> > >>We are calling a kernel here - @reg is a kernel-defined struct.
> > >
> > >If you grep for __u64 in the QEMU sources, you'll see that hardly
> > >anybody is using this type - even if calling ioctls. So for
> > >consistency, I'd really suggest to use uint64_t here.
> > 
> > I am not using it, I am packing data to a struct. So does vfio_dma_map()
> > already.
> 
> __u64 is just an alias typedef used by the kernel in uapi headers for
> 64-bit integers.  You should use uint64_t here.
> 
> > >>>>@@ -698,14 +768,18 @@ static int vfio_connect_container(VFIOGroup 
> > >>>>*group, AddressSpace *as)
> > >>>>
> > >>>>           container->iommu_data.type1.initialized = true;
> > >>>>
> > >>>>-    } else if (ioctl(fd, VFIO_CHECK_EXTENSION, VFIO_SPAPR_TCE_IOMMU)) {
> > >>>>+    } else if (ioctl(fd, VFIO_CHECK_EXTENSION, VFIO_SPAPR_TCE_IOMMU) ||
> > >>>>+               ioctl(fd, VFIO_CHECK_EXTENSION, 
> > >>>>VFIO_SPAPR_TCE_v2_IOMMU)) {
> > >>>>+        bool v2 = !!ioctl(fd, VFIO_CHECK_EXTENSION, 
> > >>>>VFIO_SPAPR_TCE_v2_IOMMU);
> > >>>
> > >>>That "!!" sounds somewhat wrong here. I think you either want to check
> > >>>for "ioctl() == 1" (because only in this case you can be sure that v2
> > >>>is supported), or you can simply omit the "!!" because you're 100% sure
> > >>>that the ioctl only returns 0 or 1 (and never a negative error code).
> > >>
> > >>
> > >>The host kernel does not return an error on these ioctls, it returns 0 or
> > >>1. And "!!" is shorter than "(bool)". VFIO_CHECK_EXTENSION for Type1 does
> > >>exactly the same already.
> > >
> > >Simply using nothing instead is even shorter than using "!!". The
> > >compiler is smart enough to convert from 0 and 1 to bool.
> > >"!!" is IMHO quite ugly and should only be used when it is really
> > >necessary.
> > 
> > 
> > imho it is not but either way I'd rather follow the existing style,
> > especially if I do literally the same thing (checking IOMMU version). Unless
> > the original author tells me to convert all the existing occurences of "!!"
> > to "!=0" (or something like this) before I post new ones.
> > 
> > Alex, should I get rid of "!!"s in the patch?
> 
> I think !! is the lesser evil here.  The trouble is that in C "bool"
> is not a first-class datatype, but just a typedef for some integer
> type.  Which means that, confusingly, (bool)2 != (bool)1.  So using
> the !! trick to force a value to be either 0 or 1 when assigning it to
> a bool variable is probably a good idea.

I agree that it shouldn't be necessary, but we do it elsewhere and it
doesn't bother me to do it here too.  Thanks,

Alex