Subject: Re: [PATCH kernel v8 14/31] vfio: powerpc/spapr: powerpc/powernv/ioda2: Rework IOMMU ownership control

From: Alexey Kardashevskiy <[email-masked]>
Date: Mon, 20 Apr 2015 16:55:32 +1000

On 04/20/2015 12:44 PM, David Gibson wrote:
> On Fri, Apr 17, 2015 at 08:09:29PM +1000, Alexey Kardashevskiy wrote:
>> On 04/16/2015 04:07 PM, David Gibson wrote:
>>> On Fri, Apr 10, 2015 at 04:30:56PM +1000, Alexey Kardashevskiy wrote:
>>>> At the moment the iommu_table struct has a set_bypass() which enables/
>>>> disables DMA bypass on IODA2 PHB. This is exposed to POWERPC IOMMU code
>>>> which calls this callback when external IOMMU users such as VFIO are
>>>> about to get over a PHB.
>>>>
>>>> The set_bypass() callback is not really an iommu_table function but
>>>> IOMMU/PE function. This introduces a iommu_table_group_ops struct and
>>>> adds a set_ownership() callback to it which is called when an external
>>>> user takes control over the IOMMU.
>>>
>>> Do you really need separate ops structures at both the single table
>>> and table group level?  The different tables in a group will all
>>> belong to the same basic iommu won't they?
>>
>>
>> IOMMU tables exist alone in VIO. Also, the platform code uses just a table
>> (or it is in bypass mode) and does not care about table groups. It looked
>> more clean for myself to keep them separated. Should I still merge
>> those?
>
> Ok, that sounds like a reasonable argument for keeping them separate,
> at least for now.
>
>>>> This renames set_bypass() to set_ownership() as it is not necessarily
>>>> just enabling bypassing, it can be something else/more so let's give it
>>>> more generic name. The bool parameter is inverted.
>>>>
>>>> The callback is implemented for IODA2 only. Other platforms (P5IOC2,
>>>> IODA1) will use the old iommu_take_ownership/iommu_release_ownership API.
>>>>
>>>> Signed-off-by: Alexey Kardashevskiy <[email-masked]>
>>>> ---
>>>>   arch/powerpc/include/asm/iommu.h          | 14 +++++++++++++-
>>>>   arch/powerpc/platforms/powernv/pci-ioda.c | 30 ++++++++++++++++++++++--------
>>>>   drivers/vfio/vfio_iommu_spapr_tce.c       | 25 +++++++++++++++++++++----
>>>>   3 files changed, 56 insertions(+), 13 deletions(-)
>>>>
>>>> diff --git a/arch/powerpc/include/asm/iommu.h b/arch/powerpc/include/asm/iommu.h
>>>> index b9e50d3..d1f8c6c 100644
>>>> --- a/arch/powerpc/include/asm/iommu.h
>>>> +++ b/arch/powerpc/include/asm/iommu.h
>>>> @@ -92,7 +92,6 @@ struct iommu_table {
>>>>   	unsigned long  it_page_shift;/* table iommu page size */
>>>>   	struct iommu_table_group *it_group;
>>>>   	struct iommu_table_ops *it_ops;
>>>> -	void (*set_bypass)(struct iommu_table *tbl, bool enable);
>>>>   };
>>>>
>>>>   /* Pure 2^n version of get_order */
>>>> @@ -127,11 +126,24 @@ extern struct iommu_table *iommu_init_table(struct iommu_table * tbl,
>>>>
>>>>   #define IOMMU_TABLE_GROUP_MAX_TABLES	1
>>>>
>>>> +struct iommu_table_group;
>>>> +
>>>> +struct iommu_table_group_ops {
>>>> +	/*
>>>> +	 * Switches ownership from the kernel itself to an external
>>>> +	 * user. While onwership is enabled, the kernel cannot use IOMMU
>>>> +	 * for itself.
>>>> +	 */
>>>> +	void (*set_ownership)(struct iommu_table_group *table_group,
>>>> +			bool enable);
>>>
>>> The meaning of "enable" in a function called "set_ownership" is
>>> entirely obscure.
>>
>> Suggest something better please :) I have nothing better...
>
> Well, given it's "set_ownershuip" you could have "owner" - that would
> want to be an enum with OWNER_KERNEL and OWNER_VFIO or something
> rather than a bool.

It is iommu_take_ownership() in upstream and it is assumed that the owner 
is anything but the platform code (for now and probably for ever - VFIO). I 
am not changing this now, just using same naming approach when adding a 
callback with a similar name.

> Or you could leave it a bool but call it "allow_bypass".

Commented below.

>
>>
>>
>>>
>>>> +};
>>>> +
>>>>   struct iommu_table_group {
>>>>   #ifdef CONFIG_IOMMU_API
>>>>   	struct iommu_group *group;
>>>>   #endif
>>>>   	struct iommu_table tables[IOMMU_TABLE_GROUP_MAX_TABLES];
>>>> +	struct iommu_table_group_ops *ops;
>>>>   };
>>>>
>>>>   #ifdef CONFIG_IOMMU_API
>>>> diff --git a/arch/powerpc/platforms/powernv/pci-ioda.c b/arch/powerpc/platforms/powernv/pci-ioda.c
>>>> index a964c50..9687731 100644
>>>> --- a/arch/powerpc/platforms/powernv/pci-ioda.c
>>>> +++ b/arch/powerpc/platforms/powernv/pci-ioda.c
>>>> @@ -1255,10 +1255,8 @@ static void pnv_pci_ioda_setup_dma_pe(struct pnv_phb *phb,
>>>>   		__free_pages(tce_mem, get_order(TCE32_TABLE_SIZE * segs));
>>>>   }
>>>>
>>>> -static void pnv_pci_ioda2_set_bypass(struct iommu_table *tbl, bool enable)
>>>> +static void pnv_pci_ioda2_set_bypass(struct pnv_ioda_pe *pe, bool enable)
>>>>   {
>>>> -	struct pnv_ioda_pe *pe = container_of(tbl->it_group, struct pnv_ioda_pe,
>>>> -					      table_group);
>>>>   	uint16_t window_id = (pe->pe_number << 1 ) + 1;
>>>>   	int64_t rc;
>>>>
>>>> @@ -1286,7 +1284,8 @@ static void pnv_pci_ioda2_set_bypass(struct iommu_table *tbl, bool enable)
>>>>   		 * host side.
>>>>   		 */
>>>>   		if (pe->pdev)
>>>> -			set_iommu_table_base(&pe->pdev->dev, tbl);
>>>> +			set_iommu_table_base(&pe->pdev->dev,
>>>> +					&pe->table_group.tables[0]);
>>>>   		else
>>>>   			pnv_ioda_setup_bus_dma(pe, pe->pbus, false);
>>>>   	}
>>>> @@ -1302,13 +1301,27 @@ static void pnv_pci_ioda2_setup_bypass_pe(struct pnv_phb *phb,
>>>>   	/* TVE #1 is selected by PCI address bit 59 */
>>>>   	pe->tce_bypass_base = 1ull << 59;
>>>>
>>>> -	/* Install set_bypass callback for VFIO */
>>>> -	pe->table_group.tables[0].set_bypass = pnv_pci_ioda2_set_bypass;
>>>> -
>>>>   	/* Enable bypass by default */
>>>> -	pnv_pci_ioda2_set_bypass(&pe->table_group.tables[0], true);
>>>> +	pnv_pci_ioda2_set_bypass(pe, true);
>>>>   }
>>>>
>>>> +static void pnv_ioda2_set_ownership(struct iommu_table_group *table_group,
>>>> +				     bool enable)
>>>> +{
>>>> +	struct pnv_ioda_pe *pe = container_of(table_group, struct pnv_ioda_pe,
>>>> +						table_group);
>>>> +	if (enable)
>>>> +		iommu_take_ownership(table_group);
>>>> +	else
>>>> +		iommu_release_ownership(table_group);
>>>> +
>>>> +	pnv_pci_ioda2_set_bypass(pe, !enable);
>>>> +}
>>>> +
>>>> +static struct iommu_table_group_ops pnv_pci_ioda2_ops = {
>>>> +	.set_ownership = pnv_ioda2_set_ownership,
>>>> +};
>>>> +
>>>>   static void pnv_pci_ioda2_setup_dma_pe(struct pnv_phb *phb,
>>>>   				       struct pnv_ioda_pe *pe)
>>>>   {
>>>> @@ -1376,6 +1389,7 @@ static void pnv_pci_ioda2_setup_dma_pe(struct pnv_phb *phb,
>>>>   	}
>>>>   	tbl->it_ops = &pnv_iommu_ops;
>>>>   	iommu_init_table(tbl, phb->hose->node);
>>>> +	pe->table_group.ops = &pnv_pci_ioda2_ops;
>>>>   	iommu_register_group(&pe->table_group, phb->hose->global_number,
>>>>   			pe->pe_number);
>>>>
>>>> diff --git a/drivers/vfio/vfio_iommu_spapr_tce.c b/drivers/vfio/vfio_iommu_spapr_tce.c
>>>> index 9f38351..d5d8c50 100644
>>>> --- a/drivers/vfio/vfio_iommu_spapr_tce.c
>>>> +++ b/drivers/vfio/vfio_iommu_spapr_tce.c
>>>> @@ -535,9 +535,22 @@ static int tce_iommu_attach_group(void *iommu_data,
>>>>   		goto unlock_exit;
>>>>   	}
>>>>
>>>> -	ret = iommu_take_ownership(table_group);
>>>> -	if (!ret)
>>>> -		container->grp = iommu_group;
>>>> +	if (!table_group->ops || !table_group->ops->set_ownership) {
>>>> +		ret = iommu_take_ownership(table_group);
>>>> +	} else {
>>>> +		/*
>>>> +		 * Disable iommu bypass, otherwise the user can DMA to all of
>>>> +		 * our physical memory via the bypass window instead of just
>>>> +		 * the pages that has been explicitly mapped into the iommu
>>>> +		 */
>>>> +		table_group->ops->set_ownership(table_group, true);
>>>
>>> And here to disable bypass you call it with enable=true, so it doesn't
>>> even have the same meaning as it used to.
>>
>>
>> I do not disable bypass per se (even if it what set_ownership(true) does) as
>> it is IODA business and VFIO has no idea about it. I do take control over
>> the group. I am not following you here - what used to have the same
>> meaning?
>
> Well, in set_bypass, the enable parameter was whether bypass was
> enabled.  Here you're setting enable to true, when you want to
> *disable* bypass (in the existing case).  If the "enable" parameter
> isn't about enabling bypass, it's meaning is even more confusing than
> I thought.

Its meaning is "take ownership over the group". In this patch 
set_ownership(true) means set_bypass(false).

But later (in 25/31) set_ownership(true) becomes unset(windows0) + 
free(table0) + set_bypass(false) = clear DMA setup for the group (i.e. 
invalidate both TVTs) so it is not just about bypass (which is TVT#1 but 
not TVT#0) anymore.

>>> Plus, you should fold the logic to call the callback if necessary into
>>> iommu_take_ownership().
>>
>>
>> I really want to keep VFIO stuff out of arch/powerpc/kernel/iommu.c as much
>> as possible as it is for platform DMA/IOMMU, not VFIO (which got SPAPR
>> driver for that). ops->set_ownership() is one of these things.
>
> What's VFIO specific about this fragment - it's just if you have the
> callback, call it, otherwise fall back to the default.
>
>> iommu_take_ownership()/iommu_release_ownership() are helpers for old-style
>> commercially-unsupported P5IOC2/IODA1, and this is kind of a hack while
>> ops->set_ownership() is an interface for VFIO to do dynamic windows thing.
>
> Can you put their logic into a set_ownership callback for IODA1 then?

And P5IOC2, and pseries. We know these callbacks will call the same 
iommu_take_ownership() and iommu_release_ownership() and this is not going 
to change. Too invasive for such a hack imho.

>> If it makes sense, I could fold the previous patch into this one and move
>> iommu_take_ownership()/iommu_release_ownership() to vfio_iommu_spapr_tce.c,
>> should I? Or leave things are they are now.
>
> That sounds like it might make sense.

This is what will go to v9, looks cleaner. Thanks.

>>
>>
>>>> +		ret = 0;
>>>> +	}
>>>> +
>>>> +	if (ret)
>>>> +		goto unlock_exit;
>>>> +
>>>> +	container->grp = iommu_group;
>>>>
>>>>   unlock_exit:
>>>>   	mutex_unlock(&container->lock);
>>>> @@ -572,7 +585,11 @@ static void tce_iommu_detach_group(void *iommu_data,
>>>>   	table_group = iommu_group_get_iommudata(iommu_group);
>>>>   	BUG_ON(!table_group);
>>>>
>>>> -	iommu_release_ownership(table_group);
>>>> +	/* Kernel owns the device now, we can restore bypass */
>>>> +	if (!table_group->ops || !table_group->ops->set_ownership)
>>>> +		iommu_release_ownership(table_group);
>>>> +	else
>>>> +		table_group->ops->set_ownership(table_group, false);
>>>
>>> Likewise fold this if into iommu_release_ownership().
>>>
>>>>   unlock_exit:
>>>>   	mutex_unlock(&container->lock);
>>>
>>
>>
>

-- 
Alexey