From: Alex Williamson <alex.williamson@redhat.com>
To: Jacob Pan <jacob.jun.pan@linux.intel.com>
Cc: iommu@lists.linux-foundation.org,
LKML <linux-kernel@vger.kernel.org>,
"Lu Baolu" <baolu.lu@linux.intel.com>,
Joerg Roedel <joro@8bytes.org>,
David Woodhouse <dwmw2@infradead.org>,
"Yi Liu" <yi.l.liu@intel.com>,
"Tian, Kevin" <kevin.tian@intel.com>,
Raj Ashok <ashok.raj@intel.com>,
"Christoph Hellwig" <hch@infradead.org>,
Jean-Philippe Brucker <jean-philippe@linaro.com>,
Jonathan Cameron <jic23@kernel.org>,
Eric Auger <eric.auger@redhat.com>
Subject: Re: [PATCH 3/3] iommu/uapi: Add helper function for size lookup
Date: Mon, 3 Feb 2020 11:27:08 -0700 [thread overview]
Message-ID: <20200203112708.14174ce2@w520.home> (raw)
In-Reply-To: <20200131155125.53475a72@jacob-builder>
On Fri, 31 Jan 2020 15:51:25 -0800
Jacob Pan <jacob.jun.pan@linux.intel.com> wrote:
> Hi Alex,
> Sorry I missed this part in the previous reply. Comments below.
>
> On Wed, 29 Jan 2020 15:19:51 -0700
> Alex Williamson <alex.williamson@redhat.com> wrote:
>
> > Also, is the 12-bytes of padding in struct iommu_gpasid_bind_data
> > excessive with this new versioning scheme? Per rule #2 I'm not sure
> > if we're allowed to repurpose those padding bytes,
> We can still use the padding bytes as long as there is a new flag bit
> to indicate the validity of the new filed within the padding.
> I should have made it clear in rule #2 when mentioning the flags bits.
> Should define what extension constitutes.
> How about this?
> "
> * 2. Data structures are open to extension but closed to modification.
> * Extension should leverage the padding bytes first where a new
> * flag bit is required to indicate the validity of each new member.
> * The above rule for padding bytes also applies to adding new union
> * members.
> * After padding bytes are exhausted, new fields must be added at the
> * end of each data structure with 64bit alignment. Flag bits can be
> * added without size change but existing ones cannot be altered.
> *
> "
> So if we add new field by doing re-purpose of padding bytes, size
> lookup result will remain the same. New code would recognize the new
> flag, old code stays the same.
>
> VFIO layer checks for UAPI compatibility and size to copy, version
> sanity check and flag usage are done in the IOMMU code.
>
> > but if we add
> > fields to the end of the structure as the scheme suggests, we're
> > stuck with not being able to expand the union for new fields.
> Good point, it does sound contradictory. I hope the rewritten rule #2
> address that.
> Adding data after the union should be extremely rare. Do you see any
> issues with the example below?
>
> offsetofend() can still find the right size.
> e.g.
> V1
> struct iommu_gpasid_bind_data {
> __u32 version;
> #define IOMMU_PASID_FORMAT_INTEL_VTD 1
> __u32 format;
> #define IOMMU_SVA_GPASID_VAL (1 << 0) /* guest PASID valid */
> __u64 flags;
> __u64 gpgd;
> __u64 hpasid;
> __u64 gpasid;
> __u32 addr_width;
> __u8 padding[12];
> /* Vendor specific data */
> union {
> struct iommu_gpasid_bind_data_vtd vtd;
> };
> };
>
> const static int
> iommu_uapi_data_size[NR_IOMMU_UAPI_TYPE][IOMMU_UAPI_VERSION] = { /*
> IOMMU_UAPI_BIND_GPASID */ {offsetofend(struct iommu_gpasid_bind_data,
> vtd)}, ...
> };
>
> V2, Add new_member at the end (forget padding for now).
> struct iommu_gpasid_bind_data {
> __u32 version;
> #define IOMMU_PASID_FORMAT_INTEL_VTD 1
> __u32 format;
> #define IOMMU_SVA_GPASID_VAL (1 << 0) /* guest PASID valid */
> #define IOMMU_NEW_MEMBER_VAL (1 << 1) /* new member added */
> __u64 flags;
> __u64 gpgd;
> __u64 hpasid;
> __u64 gpasid;
> __u32 addr_width;
> __u8 padding[12];
> /* Vendor specific data */
> union {
> struct iommu_gpasid_bind_data_vtd vtd;
> };
> __u64 new_member;
> };
> const static int
> iommu_uapi_data_size[NR_IOMMU_UAPI_TYPE][IOMMU_UAPI_VERSION] = { /*
> IOMMU_UAPI_BIND_GPASID */
> {offsetofend(struct iommu_gpasid_bind_data,
> vtd), offsetofend(struct iommu_gpasid_bind_data,new_member)},
>
> };
>
> V3, Add smmu to the union,larger than vtd
>
> struct iommu_gpasid_bind_data {
> __u32 version;
> #define IOMMU_PASID_FORMAT_INTEL_VTD 1
> #define IOMMU_PASID_FORMAT_INTEL_SMMU 2
> __u32 format;
> #define IOMMU_SVA_GPASID_VAL (1 << 0) /* guest PASID valid */
> #define IOMMU_NEW_MEMBER_VAL (1 << 1) /* new member added */
> #define IOMMU_SVA_SMMU_SUPP (1 << 2) /* SMMU data supported */
> __u64 flags;
> __u64 gpgd;
> __u64 hpasid;
> __u64 gpasid;
> __u32 addr_width;
> __u8 padding[12];
> /* Vendor specific data */
> union {
> struct iommu_gpasid_bind_data_vtd vtd;
> struct iommu_gpasid_bind_data_smmu smmu;
> };
> __u64 new_member;
> };
> const static int
> iommu_uapi_data_size[NR_IOMMU_UAPI_TYPE][IOMMU_UAPI_VERSION] = {
> /* IOMMU_UAPI_BIND_GPASID */
> {offsetofend(struct iommu_gpasid_bind_data,vtd),
> offsetofend(struct iommu_gpasid_bind_data, new_member),
> offsetofend(struct iommu_gpasid_bind_data, new_member)},
> ...
> };
>
How are you not breaking rule #3, "Versions are backward compatible"
with this? If the kernel is at version 3 and userspace is at version 2
then new_member exists at different offsets of the structure. The
kernels iommu_uapi_data_size for V2 changed between version 2 and 3.
Thanks,
Alex
next prev parent reply other threads:[~2020-02-03 18:27 UTC|newest]
Thread overview: 17+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-01-29 6:02 [PATCH 0/3] IOMMU user API enhancement Jacob Pan
2020-01-29 6:02 ` [PATCH 1/3] iommu/uapi: Define uapi version and capabilities Jacob Pan
2020-02-06 10:14 ` Auger Eric
2020-02-06 18:22 ` Jacob Pan
2020-01-29 6:02 ` [PATCH 2/3] iommu/uapi: Use unified UAPI version Jacob Pan
2020-01-29 6:02 ` [PATCH 3/3] iommu/uapi: Add helper function for size lookup Jacob Pan
2020-01-29 21:40 ` Alex Williamson
2020-01-29 22:19 ` Alex Williamson
2020-01-31 19:51 ` Jacob Pan
2020-01-31 23:51 ` Jacob Pan
2020-02-03 18:27 ` Alex Williamson [this message]
2020-02-03 20:41 ` Jacob Pan
2020-02-03 21:12 ` Alex Williamson
2020-02-03 22:41 ` Jacob Pan
2020-02-06 10:14 ` Auger Eric
2020-02-07 8:47 ` Jean-Philippe Brucker
2020-01-31 17:56 ` Jacob Pan
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