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

From: David Gibson <[email-masked]>
Date: Tue, 21 Apr 2015 19:43:07 +1000

On Mon, Apr 20, 2015 at 04:55:32PM +1000, Alexey Kardashevskiy wrote:
> 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.

So "enabled" is actually that non kernel ownership is enabled.  That
is totally non-obvious.

> >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).

Ok.  So "take_ownership" isn't quite as clear as I'd like, but it's
not too bad because it's implied that it's the caller that's taking
the ownership.  *set* ownership makes no sense without saying who the
new owner is.  "enable" has no clear meaning in that context.

Calling it "kernel_owned" or "non_kernel_owned" would be ok if a bit
clunky.

> 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.

Right, I have no problem with a combined function for the operation
here.  It's purely a naming thing "set_ownership" and "enable" are
just not concepts that fit together sensibly.

-- 
David Gibson			| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you.  NOT _the_ _other_
				| _way_ _around_!
http://www.ozlabs.org/~dgibson