Re: [PATCH v6 2/3] drivers/vfio: EEH support for VFIO PCI device

[Alex Williamson <alex.williamson@redhat.com>]

On Fri, 2014-05-23 at 10:37 +1000, Gavin Shan wrote:
> On Fri, May 23, 2014 at 10:17:30AM +1000, Gavin Shan wrote:
> >On Thu, May 22, 2014 at 11:55:29AM +0200, Alexander Graf wrote:
> >>On 22.05.14 10:23, Gavin Shan wrote:
> 
> .../...
> 
> >>>diff --git a/include/uapi/linux/vfio.h b/include/uapi/linux/vfio.h
> >>>index cb9023d..ef55682 100644
> >>>--- a/include/uapi/linux/vfio.h
> >>>+++ b/include/uapi/linux/vfio.h
> >>>@@ -455,6 +455,59 @@ struct vfio_iommu_spapr_tce_info {
> >>>  #define VFIO_IOMMU_SPAPR_TCE_GET_INFO	_IO(VFIO_TYPE, VFIO_BASE + 12)
> >>>+/*
> >>>+ * EEH functionality can be enabled or disabled on one specific device.
> >>>+ * Also, the DMA or IO frozen state can be removed from the frozen PE
> >>>+ * if required.
> >>>+ */
> >>>+struct vfio_eeh_pe_set_option {
> >>>+	__u32 argsz;
> >>
> >>What is this argsz thing? Is this your way of maintaining backwards
> >>compatibility when we introduce new fields? A new field will change
> >>the ioctl number, so I don't think that makes a lot of sense :).
> >>
> >>Just make the ioctl have a u32 as incoming argument. No fancy
> >>structs, no complicated code.
> >>
> >>The same applies for a number of structs below.
> >>
> >
> >ok. Will do in next revision.
> >
> 
> Rechecked include/uapi/linux/vfio.h, the data struct for each ioctl command
> always has "argsz". I guess it was used as checker by Alex.W. Do you really
> want remove "argsz" ?


IIRC, this was actually a suggestion incorporated from David Gibson, but
using _IO with an argsz and flags field we can maintain compatibility
without bumping the ioctl number.  It really only makes sense if we have
a flags field so we can identify what additional information is being
provided.  Flags can be used as a bitmap of trailing structures or as
revision if we want a set of trailing structures that may change over
time.  Unless you can come up with a good argument against it that would
prevent us inventing a new ioctl as soon as we need a minor tweak, I'd
prefer to keep it.  As I noted in a previous comment, the one ioctl we
have for reset that doesn't take any options is likely going to be the
first ioctl that we need to entirely replace.  If we don't keep argsz,
it seems like we probably need a flags field and reserved structures.

> >>>+	__u32 option;
> >>>+};
> >>>+
> >>>+#define VFIO_EEH_PE_SET_OPTION		_IO(VFIO_TYPE, VFIO_BASE + 21)
> >>>+
> >>>+/*
> >>>+ * Each EEH PE should have unique address to be identified. The command
> >>>+ * helps to retrieve the address and the sharing mode of the PE.
> >>>+ */
> >>>+struct vfio_eeh_pe_get_addr {
> >>>+	__u32 argsz;
> >>>+	__u32 option;
> >>>+	__u32 info;
> >>
> >>Any particular reason you need the info field? Can't the return value
> >>of the ioctl hold this? Then you only have a single u32 argument left
> >>to the ioctl again.
> >>
> >
> >ok. Will do in next revision.
> >
> 
> If we eventually remove "argsz" and let ioctl() return value to hold
> information (or negative number for errors), we don't need any data
> struct because the 3rd parameter of ioctl() would be used as input
> and I only need one input parameter. Do you want see this ?
> 
> Hopefully, Alex.W saw this and hasn't objections :)

I'm not sure why we're pushing for the minimal data set to pass to an
ioctl.  Seems like a recipe for dead, useless ioctls.  Thanks,

Alex