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Date:   Wed, 23 Dec 2020 07:14:36 -0800
From:   isaacm@...eaurora.org
To:     Robin Murphy <robin.murphy@....com>
Cc:     kernel-team@...roid.com, will@...nel.org, pdaly@...eaurora.org,
        linux-arm-msm@...r.kernel.org, linux-kernel@...r.kernel.org,
        dri-devel@...ts.freedesktop.org, iommu@...ts.linux-foundation.org,
        freedreno@...ts.freedesktop.org,
        linux-arm-kernel@...ts.infradead.org, pratikp@...eaurora.org
Subject: Re: [PATCH v2 1/7] iommu/io-pgtable: Introduce dynamic io-pgtable fmt
 registration

On 2020-12-23 05:44, Robin Murphy wrote:
> On 2020-12-22 19:54, isaacm@...eaurora.org wrote:
>> On 2020-12-22 11:27, Robin Murphy wrote:
>>> On 2020-12-22 00:44, Isaac J. Manjarres wrote:
>>>> The io-pgtable code constructs an array of init functions for each
>>>> page table format at compile time. This is not ideal, as this
>>>> increases the footprint of the io-pgtable code, as well as prevents
>>>> io-pgtable formats from being built as kernel modules.
>>>> 
>>>> In preparation for modularizing the io-pgtable formats, switch to a
>>>> dynamic registration scheme, where each io-pgtable format can 
>>>> register
>>>> their init functions with the io-pgtable code at boot or module
>>>> insertion time.
>>>> 
>>>> Signed-off-by: Isaac J. Manjarres <isaacm@...eaurora.org>
>>>> ---
>>>>   drivers/iommu/io-pgtable-arm-v7s.c | 34 +++++++++++++-
>>>>   drivers/iommu/io-pgtable-arm.c     | 90 
>>>> ++++++++++++++++++++++++++----------
>>>>   drivers/iommu/io-pgtable.c         | 94 
>>>> ++++++++++++++++++++++++++++++++------
>>>>   include/linux/io-pgtable.h         | 51 +++++++++++++--------
>>>>   4 files changed, 209 insertions(+), 60 deletions(-)
>>>> 
>>>> diff --git a/drivers/iommu/io-pgtable-arm-v7s.c 
>>>> b/drivers/iommu/io-pgtable-arm-v7s.c
>>>> index 1d92ac9..89aad2f 100644
>>>> --- a/drivers/iommu/io-pgtable-arm-v7s.c
>>>> +++ b/drivers/iommu/io-pgtable-arm-v7s.c
>>>> @@ -28,6 +28,7 @@
>>>>   #include <linux/iommu.h>
>>>>   #include <linux/kernel.h>
>>>>   #include <linux/kmemleak.h>
>>>> +#include <linux/module.h>
>>>>   #include <linux/sizes.h>
>>>>   #include <linux/slab.h>
>>>>   #include <linux/spinlock.h>
>>>> @@ -835,7 +836,8 @@ static struct io_pgtable 
>>>> *arm_v7s_alloc_pgtable(struct io_pgtable_cfg *cfg,
>>>>       return NULL;
>>>>   }
>>>>   -struct io_pgtable_init_fns io_pgtable_arm_v7s_init_fns = {
>>>> +static struct io_pgtable_init_fns io_pgtable_arm_v7s_init_fns = {
>>>> +    .fmt    = ARM_V7S,
>>>>       .alloc    = arm_v7s_alloc_pgtable,
>>>>       .free    = arm_v7s_free_pgtable,
>>>>   };
>>>> @@ -982,5 +984,33 @@ static int __init arm_v7s_do_selftests(void)
>>>>       pr_info("self test ok\n");
>>>>       return 0;
>>>>   }
>>>> -subsys_initcall(arm_v7s_do_selftests);
>>>> +#else
>>>> +static int arm_v7s_do_selftests(void)
>>>> +{
>>>> +    return 0;
>>>> +}
>>>>   #endif
>>>> +
>>>> +static int __init arm_v7s_init(void)
>>>> +{
>>>> +    int ret;
>>>> +
>>>> +    ret = io_pgtable_ops_register(&io_pgtable_arm_v7s_init_fns);
>>>> +    if (ret < 0) {
>>>> +        pr_err("Failed to register ARM V7S format\n");
>>> 
>>> Super-nit: I think "v7s" should probably be lowercase there. Also
>>> general consistency WRT to showing the error code and whether or not
>>> to abbreviate "format" would be nice.
>>> 
>> Ok, I can fix this accordingly.
>> 
>>>> +        return ret;
>>>> +    }
>>>> +
>>>> +    ret = arm_v7s_do_selftests();
>>>> +    if (ret < 0)
>>>> +        io_pgtable_ops_unregister(&io_pgtable_arm_v7s_init_fns);
>>>> +
>>>> +    return ret;
>>>> +}
>>>> +core_initcall(arm_v7s_init);
>>>> +
>>>> +static void __exit arm_v7s_exit(void)
>>>> +{
>>>> +    io_pgtable_ops_unregister(&io_pgtable_arm_v7s_init_fns);
>>>> +}
>>>> +module_exit(arm_v7s_exit);
>>>> diff --git a/drivers/iommu/io-pgtable-arm.c 
>>>> b/drivers/iommu/io-pgtable-arm.c
>>>> index 87def58..ff0ea2f 100644
>>>> --- a/drivers/iommu/io-pgtable-arm.c
>>>> +++ b/drivers/iommu/io-pgtable-arm.c
>>>> @@ -13,6 +13,7 @@
>>>>   #include <linux/bitops.h>
>>>>   #include <linux/io-pgtable.h>
>>>>   #include <linux/kernel.h>
>>>> +#include <linux/module.h>
>>>>   #include <linux/sizes.h>
>>>>   #include <linux/slab.h>
>>>>   #include <linux/types.h>
>>>> @@ -1043,29 +1044,32 @@ arm_mali_lpae_alloc_pgtable(struct 
>>>> io_pgtable_cfg *cfg, void *cookie)
>>>>       return NULL;
>>>>   }
>>>>   -struct io_pgtable_init_fns io_pgtable_arm_64_lpae_s1_init_fns = {
>>>> -    .alloc    = arm_64_lpae_alloc_pgtable_s1,
>>>> -    .free    = arm_lpae_free_pgtable,
>>>> -};
>>>> -
>>>> -struct io_pgtable_init_fns io_pgtable_arm_64_lpae_s2_init_fns = {
>>>> -    .alloc    = arm_64_lpae_alloc_pgtable_s2,
>>>> -    .free    = arm_lpae_free_pgtable,
>>>> -};
>>>> -
>>>> -struct io_pgtable_init_fns io_pgtable_arm_32_lpae_s1_init_fns = {
>>>> -    .alloc    = arm_32_lpae_alloc_pgtable_s1,
>>>> -    .free    = arm_lpae_free_pgtable,
>>>> -};
>>>> -
>>>> -struct io_pgtable_init_fns io_pgtable_arm_32_lpae_s2_init_fns = {
>>>> -    .alloc    = arm_32_lpae_alloc_pgtable_s2,
>>>> -    .free    = arm_lpae_free_pgtable,
>>>> -};
>>>> -
>>>> -struct io_pgtable_init_fns io_pgtable_arm_mali_lpae_init_fns = {
>>>> -    .alloc    = arm_mali_lpae_alloc_pgtable,
>>>> -    .free    = arm_lpae_free_pgtable,
>>>> +static struct io_pgtable_init_fns io_pgtable_arm_lpae_init_fns[] = 
>>>> {
>>>> +    {
>>>> +        .fmt    = ARM_32_LPAE_S1,
>>>> +        .alloc    = arm_32_lpae_alloc_pgtable_s1,
>>>> +        .free    = arm_lpae_free_pgtable,
>>>> +    },
>>>> +    {
>>>> +        .fmt    = ARM_32_LPAE_S2,
>>>> +        .alloc    = arm_32_lpae_alloc_pgtable_s2,
>>>> +        .free    = arm_lpae_free_pgtable,
>>>> +    },
>>>> +    {
>>>> +        .fmt    = ARM_64_LPAE_S1,
>>>> +        .alloc    = arm_64_lpae_alloc_pgtable_s1,
>>>> +        .free    = arm_lpae_free_pgtable,
>>>> +    },
>>>> +    {
>>>> +        .fmt    = ARM_64_LPAE_S2,
>>>> +        .alloc    = arm_64_lpae_alloc_pgtable_s2,
>>>> +        .free    = arm_lpae_free_pgtable,
>>>> +    },
>>>> +    {
>>>> +        .fmt    = ARM_MALI_LPAE,
>>>> +        .alloc    = arm_mali_lpae_alloc_pgtable,
>>>> +        .free    = arm_lpae_free_pgtable,
>>>> +    },
>>>>   };
>>>>     #ifdef CONFIG_IOMMU_IO_PGTABLE_LPAE_SELFTEST
>>>> @@ -1250,5 +1254,43 @@ static int __init arm_lpae_do_selftests(void)
>>>>       pr_info("selftest: completed with %d PASS %d FAIL\n", pass, 
>>>> fail);
>>>>       return fail ? -EFAULT : 0;
>>>>   }
>>>> -subsys_initcall(arm_lpae_do_selftests);
>>>> +#else
>>>> +static int __init arm_lpae_do_selftests(void)
>>>> +{
>>>> +    return 0;
>>>> +}
>>>>   #endif
>>>> +
>>>> +static int __init arm_lpae_init(void)
>>>> +{
>>>> +    int ret, i;
>>>> +
>>>> +    for (i = 0; i < ARRAY_SIZE(io_pgtable_arm_lpae_init_fns); i++) 
>>>> {
>>>> +        ret = 
>>>> io_pgtable_ops_register(&io_pgtable_arm_lpae_init_fns[i]);
>>>> +        if (ret < 0) {
>>>> +            pr_err("Failed to register ARM LPAE fmt: %d\n");
>>>> +            goto err_io_pgtable_register;
>>>> +        }
>>>> +    }
>>>> +
>>>> +    ret = arm_lpae_do_selftests();
>>>> +    if (ret < 0)
>>>> +        goto err_io_pgtable_register;
>>>> +
>>>> +    return 0;
>>>> +
>>>> +err_io_pgtable_register:
>>>> +    for (i = i - 1; i >= 0; i--)
>>> 
>>> Personally I find "while (i--)" a bit clearer for this kind of
>>> unwinding, but maybe post-decrement isn't to everyone's taste.
>>> 
>> I'm not particularly attached to the current approach, so a while loop
>> is okay with me :).
>> 
>>>> +        
>>>> io_pgtable_ops_unregister(&io_pgtable_arm_lpae_init_fns[i]);
>>>> +    return ret;
>>>> +}
>>>> +core_initcall(arm_lpae_init);
>>>> +
>>>> +static void __exit arm_lpae_exit(void)
>>>> +{
>>>> +    int i;
>>>> +
>>>> +    for (i = 0; i < ARRAY_SIZE(io_pgtable_arm_lpae_init_fns); i++)
>>>> +        
>>>> io_pgtable_ops_unregister(&io_pgtable_arm_lpae_init_fns[i]);
>>>> +}
>>>> +module_exit(arm_lpae_exit);
>>>> diff --git a/drivers/iommu/io-pgtable.c b/drivers/iommu/io-pgtable.c
>>>> index 94394c8..2c6eb2e 100644
>>>> --- a/drivers/iommu/io-pgtable.c
>>>> +++ b/drivers/iommu/io-pgtable.c
>>>> @@ -10,33 +10,45 @@
>>>>   #include <linux/bug.h>
>>>>   #include <linux/io-pgtable.h>
>>>>   #include <linux/kernel.h>
>>>> +#include <linux/rwlock.h>
>>>> +#include <linux/slab.h>
>>>>   #include <linux/types.h>
>>>>   -static const struct io_pgtable_init_fns *
>>>> -io_pgtable_init_table[IO_PGTABLE_NUM_FMTS] = {
>>>> -#ifdef CONFIG_IOMMU_IO_PGTABLE_LPAE
>>>> -    [ARM_32_LPAE_S1] = &io_pgtable_arm_32_lpae_s1_init_fns,
>>>> -    [ARM_32_LPAE_S2] = &io_pgtable_arm_32_lpae_s2_init_fns,
>>>> -    [ARM_64_LPAE_S1] = &io_pgtable_arm_64_lpae_s1_init_fns,
>>>> -    [ARM_64_LPAE_S2] = &io_pgtable_arm_64_lpae_s2_init_fns,
>>>> -    [ARM_MALI_LPAE] = &io_pgtable_arm_mali_lpae_init_fns,
>>>> -#endif
>>>> -#ifdef CONFIG_IOMMU_IO_PGTABLE_ARMV7S
>>>> -    [ARM_V7S] = &io_pgtable_arm_v7s_init_fns,
>>>> -#endif
>>>> +struct io_pgtable_init_fns_node {
>>>> +    struct io_pgtable_init_fns *fns;
>>>> +    struct list_head list;
>>>>   };
>>>>   +static LIST_HEAD(io_pgtable_init_fns_list);
>>>> +static DEFINE_RWLOCK(io_pgtable_init_fns_list_lock);
>>>> +
>>>> +static struct io_pgtable_init_fns *io_pgtable_get_init_fns(enum 
>>>> io_pgtable_fmt fmt)
>>>> +{
>>>> +    struct io_pgtable_init_fns_node *iter;
>>>> +    struct io_pgtable_init_fns *fns = NULL;
>>>> +
>>>> +    read_lock(&io_pgtable_init_fns_list_lock);
>>>> +    list_for_each_entry(iter, &io_pgtable_init_fns_list, list)
>>>> +        if (iter->fns->fmt == fmt) {
>>>> +            fns = iter->fns;
>>>> +            break;
>>>> +        }
>>>> +    read_unlock(&io_pgtable_init_fns_list_lock);
>>>> +
>>>> +    return fns;
>>>> +}
>>> 
>>> I think it would be a lot easier to stick with a simple array indexed
>>> by enum - that way you can just set/clear/test entries without 
>>> needing
>>> to worry about locking. Basically just remove the const and the
>>> initialisers from the existing one ;)
>>> 
>>> (and if you think you're concerned about memory, consider that just
>>> the list head plus lock is already half the size of the table)
>>> 
>>> Other than that, I think this all looks pretty promising - I'd 
>>> suggest
>>> sending a non-RFC after rc1 so that it gets everyone's proper
>>> attention.
>>> 
>>> Thanks,
>>> Robin.
>>> 
>> 
>> Thanks for all of the feedback! With respect to the comment about 
>> keeping the current array,
>> don't we need some sort of lock to protect access to the data 
>> structure (e.g. concurrent access
>> if adding two ops for the same format)? Or do you mean that we should 
>> use atomic operations to
>> handle this cleanly?
> 
> I don't think any races are realistically possible - built-in
> initcalls should run sequentially, while module init should be
> serialised by the module loader lock, so registrations should never be
> able to race against each other. And if there could be a race between
> registering a format and somebody trying to use it, then something's
> fundamentally wrong anyway - sure we could use READ_ONCE/WRITE_ONCE
> for accessing the table, but that would be basically be admitting that
> we've failed at the primary purpose of making sure formats are loaded
> *before* the users that depend on them.
> 
> With a directly-indexed table there's no contention between looking up
> one format and registering another unrelated format, unlike with a
> list.
> 
> Robin.
> 

Got it. I'll stick with the array as suggested. Thanks again for the 
feedback.
I'll be sending the version with your suggestions when 5.11-rc1 comes 
out as
a non-RFC.

--Isaac
>> 
>> Thanks,
>> Isaac
>>>> +
>>>>   struct io_pgtable_ops *alloc_io_pgtable_ops(enum io_pgtable_fmt 
>>>> fmt,
>>>>                           struct io_pgtable_cfg *cfg,
>>>>                           void *cookie)
>>>>   {
>>>>       struct io_pgtable *iop;
>>>> -    const struct io_pgtable_init_fns *fns;
>>>> +    struct io_pgtable_init_fns *fns;
>>>>         if (fmt >= IO_PGTABLE_NUM_FMTS)
>>>>           return NULL;
>>>>   -    fns = io_pgtable_init_table[fmt];
>>>> +    fns = io_pgtable_get_init_fns(fmt);
>>>>       if (!fns)
>>>>           return NULL;
>>>>   @@ -59,12 +71,64 @@ EXPORT_SYMBOL_GPL(alloc_io_pgtable_ops);
>>>>   void free_io_pgtable_ops(struct io_pgtable_ops *ops)
>>>>   {
>>>>       struct io_pgtable *iop;
>>>> +    struct io_pgtable_init_fns *fns;
>>>>         if (!ops)
>>>>           return;
>>>>         iop = io_pgtable_ops_to_pgtable(ops);
>>>>       io_pgtable_tlb_flush_all(iop);
>>>> -    io_pgtable_init_table[iop->fmt]->free(iop);
>>>> +    fns = io_pgtable_get_init_fns(iop->fmt);
>>>> +    if (fns)
>>>> +        fns->free(iop);
>>>>   }
>>>>   EXPORT_SYMBOL_GPL(free_io_pgtable_ops);
>>>> +
>>>> +int io_pgtable_ops_register(struct io_pgtable_init_fns *init_fns)
>>>> +{
>>>> +    struct io_pgtable_init_fns_node *iter, *fns_node;
>>>> +    int ret = 0;
>>>> +
>>>> +    if (!init_fns || init_fns->fmt >= IO_PGTABLE_NUM_FMTS ||
>>>> +        !init_fns->alloc || !init_fns->free)
>>>> +        return -EINVAL;
>>>> +
>>>> +    fns_node = kzalloc(sizeof(*fns_node), GFP_KERNEL);
>>>> +    if (!fns_node)
>>>> +        return -ENOMEM;
>>>> +
>>>> +    write_lock(&io_pgtable_init_fns_list_lock);
>>>> +    list_for_each_entry(iter, &io_pgtable_init_fns_list, list)
>>>> +        if (iter->fns->fmt == init_fns->fmt) {
>>>> +            ret = -EEXIST;
>>>> +            kfree(fns_node);
>>>> +            break;
>>>> +        }
>>>> +
>>>> +    if (!ret) {
>>>> +        fns_node->fns = init_fns;
>>>> +        INIT_LIST_HEAD(&fns_node->list);
>>>> +        list_add_tail(&fns_node->list, &io_pgtable_init_fns_list);
>>>> +    }
>>>> +    write_unlock(&io_pgtable_init_fns_list_lock);
>>>> +    return ret;
>>>> +}
>>>> +EXPORT_SYMBOL_GPL(io_pgtable_ops_register);
>>>> +
>>>> +void io_pgtable_ops_unregister(struct io_pgtable_init_fns 
>>>> *init_fns)
>>>> +{
>>>> +    struct io_pgtable_init_fns_node *iter, *tmp;
>>>> +
>>>> +    if (!init_fns)
>>>> +        return;
>>>> +
>>>> +    write_lock(&io_pgtable_init_fns_list_lock);
>>>> +    list_for_each_entry_safe(iter, tmp, &io_pgtable_init_fns_list, 
>>>> list)
>>>> +        if (iter->fns == init_fns) {
>>>> +            list_del(&iter->list);
>>>> +            kfree(iter);
>>>> +            break;
>>>> +        }
>>>> +    write_unlock(&io_pgtable_init_fns_list_lock);
>>>> +}
>>>> +EXPORT_SYMBOL_GPL(io_pgtable_ops_unregister);
>>>> diff --git a/include/linux/io-pgtable.h b/include/linux/io-pgtable.h
>>>> index ea727eb..45b367ce 100644
>>>> --- a/include/linux/io-pgtable.h
>>>> +++ b/include/linux/io-pgtable.h
>>>> @@ -163,6 +163,38 @@ struct io_pgtable_ops {
>>>>   };
>>>>     /**
>>>> + * struct io_pgtable_init_fns - Alloc/free a set of page tables for 
>>>> a
>>>> + *                              particular format.
>>>> + *
>>>> + * @fmt:   The page table format.
>>>> + * @alloc: Allocate a set of page tables described by cfg.
>>>> + * @free:  Free the page tables associated with iop.
>>>> + */
>>>> +struct io_pgtable_init_fns {
>>>> +    enum io_pgtable_fmt fmt;
>>>> +    struct io_pgtable *(*alloc)(struct io_pgtable_cfg *cfg, void 
>>>> *cookie);
>>>> +    void (*free)(struct io_pgtable *iop);
>>>> +};
>>>> +
>>>> +/**
>>>> + * io_pgtable_ops_register() - Register the page table routines for 
>>>> a page table
>>>> + *                             format.
>>>> + *
>>>> + * @init_fns: The functions for allocating and freeing the page 
>>>> tables of
>>>> + *            a particular format.
>>>> + */
>>>> +int io_pgtable_ops_register(struct io_pgtable_init_fns *init_fns);
>>>> +
>>>> +/**
>>>> + * io_pgtable_ops_unregister() - Unregister the page table routines 
>>>> for a page
>>>> + *                               table format.
>>>> + *
>>>> + * @init_fns: The functions for allocating and freeing the page 
>>>> tables of
>>>> + *            a particular format.
>>>> + */
>>>> +void io_pgtable_ops_unregister(struct io_pgtable_init_fns 
>>>> *init_fns);
>>>> +
>>>> +/**
>>>>    * alloc_io_pgtable_ops() - Allocate a page table allocator for 
>>>> use by an IOMMU.
>>>>    *
>>>>    * @fmt:    The page table format.
>>>> @@ -233,23 +265,4 @@ io_pgtable_tlb_add_page(struct io_pgtable *iop,
>>>>           iop->cfg.tlb->tlb_add_page(gather, iova, granule, 
>>>> iop->cookie);
>>>>   }
>>>>   -/**
>>>> - * struct io_pgtable_init_fns - Alloc/free a set of page tables for 
>>>> a
>>>> - *                              particular format.
>>>> - *
>>>> - * @alloc: Allocate a set of page tables described by cfg.
>>>> - * @free:  Free the page tables associated with iop.
>>>> - */
>>>> -struct io_pgtable_init_fns {
>>>> -    struct io_pgtable *(*alloc)(struct io_pgtable_cfg *cfg, void 
>>>> *cookie);
>>>> -    void (*free)(struct io_pgtable *iop);
>>>> -};
>>>> -
>>>> -extern struct io_pgtable_init_fns 
>>>> io_pgtable_arm_32_lpae_s1_init_fns;
>>>> -extern struct io_pgtable_init_fns 
>>>> io_pgtable_arm_32_lpae_s2_init_fns;
>>>> -extern struct io_pgtable_init_fns 
>>>> io_pgtable_arm_64_lpae_s1_init_fns;
>>>> -extern struct io_pgtable_init_fns 
>>>> io_pgtable_arm_64_lpae_s2_init_fns;
>>>> -extern struct io_pgtable_init_fns io_pgtable_arm_v7s_init_fns;
>>>> -extern struct io_pgtable_init_fns 
>>>> io_pgtable_arm_mali_lpae_init_fns;
>>>> -
>>>>   #endif /* __IO_PGTABLE_H */
>>>> 
>>> 
>>> _______________________________________________
>>> linux-arm-kernel mailing list
>>> linux-arm-kernel@...ts.infradead.org
>>> http://lists.infradead.org/mailman/listinfo/linux-arm-kernel
>> _______________________________________________
>> iommu mailing list
>> iommu@...ts.linux-foundation.org
>> https://lists.linuxfoundation.org/mailman/listinfo/iommu

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