Re: [PATCH v1 09/10] powerpc/pseries/iommu: Make use of DDW even if it does not map the partition

From: Alexey Kardashevskiy
Date: Thu Sep 03 2020 - 21:00:51 EST

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In reply to: Leonardo Bras: "Re: [PATCH v1 09/10] powerpc/pseries/iommu: Make use of DDW even if it does not map the partition"
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On 02/09/2020 16:11, Leonardo Bras wrote:

On Mon, 2020-08-31 at 14:35 +1000, Alexey Kardashevskiy wrote:

On 29/08/2020 04:36, Leonardo Bras wrote:

On Mon, 2020-08-24 at 15:17 +1000, Alexey Kardashevskiy wrote:

On 18/08/2020 09:40, Leonardo Bras wrote:

As of today, if the biggest DDW that can be created can't map the whole
partition, it's creation is skipped and the default DMA window
"ibm,dma-window" is used instead.

DDW is 16x bigger than the default DMA window,

16x only under very specific circumstances which are
1. phyp
2. sriov
3. device class in hmc (or what that priority number is in the lpar config).

Yeah, missing details.

having the same amount of
pages, but increasing the page size to 64k.
Besides larger DMA window,

"Besides being larger"?

You are right there.

it performs better for allocations over 4k,

Better how?

I was thinking for allocations larger than (512 * 4k), since >2
hypercalls are needed here, and for 64k pages would still be just 1
hypercall up to (512 * 64k).
But yeah, not the usual case anyway.

Yup.

so it would be nice to use it instead.

I'd rather say something like:
===
So far we assumed we can map the guest RAM 1:1 to the bus which worked
with a small number of devices. SRIOV changes it as the user can
configure hundreds VFs and since phyp preallocates TCEs and does not
allow IOMMU pages bigger than 64K, it has to limit the number of TCEs
per a PE to limit waste of physical pages.
===

I mixed this in my commit message, it looks like this:

===
powerpc/pseries/iommu: Make use of DDW for indirect mapping

So far it's assumed possible to map the guest RAM 1:1 to the bus, which
works with a small number of devices. SRIOV changes it as the user can
configure hundreds VFs and since phyp preallocates TCEs and does not
allow IOMMU pages bigger than 64K, it has to limit the number of TCEs
per a PE to limit waste of physical pages.

As of today, if the assumed direct mapping is not possible, DDW
creation is skipped and the default DMA window "ibm,dma-window" is used
instead.

The default DMA window uses 4k pages instead of 64k pages, and since
the amount of pages is the same,

Is the amount really the same? I thought you can prioritize some VFs
over others (== allocate different number of TCEs). Does it really
matter if it is the same?

On a conversation with Travis Pizel, he explained how it's supposed to
work, and I understood this:

When a VF is created, it will be assigned a capacity, like 4%, 20%, and
so on. The number of 'TCE entries' that are available to that partition
are proportional to that capacity.

If we use the default DMA window, the IOMMU pagesize/entry will be 4k,
and if we use DDW, we will get 64k pagesize. As the number of entries
will be the same (for the same capacity), the total space that can be
addressed by the IOMMU will be 16 times bigger. This sometimes enable
direct mapping, but sometimes it's still not enough.

Good to know. This is still an implementation detail, QEMU does not allocate TCEs like this.

On Travis words :
"A low capacity VF, with less resources available, will certainly have
less DMA window capability than a high capacity VF. But, an 8GB DMA
window (with 64k pages) is still 16x larger than an 512MB window (with
4K pages).
A high capacity VF - for example, one that Leonardo has in his scenario
- will go from 8GB (using 4K pages) to 128GB (using 64K pages) - again,
16x larger - but it's obviously still possible to create a partition
that exceeds 128GB of memory in size."

Right except the default dma window is not 8GB, it is <=2GB.

making use of DDW instead of the
default DMA window for indirect mapping will expand in 16x the amount
of memory that can be mapped on DMA.

Stop saying "16x", it is not guaranteed by anything :)

The DDW created will be used for direct mapping by default. [...]
===

What do you think?

The DDW created will be used for direct mapping by default.
If it's not available, indirect mapping will be used instead.

For indirect mapping, it's necessary to update the iommu_table so
iommu_alloc() can use the DDW created. For this,
iommu_table_update_window() is called when everything else succeeds
at enable_ddw().

Removing the default DMA window for using DDW with indirect mapping
is only allowed if there is no current IOMMU memory allocated in
the iommu_table. enable_ddw() is aborted otherwise.

As there will never have both direct and indirect mappings at the same
time, the same property name can be used for the created DDW.

So renaming
define DIRECT64_PROPNAME "linux,direct64-ddr-window-info"
to
define DMA64_PROPNAME "linux,dma64-ddr-window-info"
looks the right thing to do.

I know I suggested this but this does not look so good anymore as I
suspect it breaks kexec (from older kernel to this one) so you either
need to check for both DT names or just keep the old one. Changing the
macro name is fine.

Yeah, having 'direct' in the name don't really makes sense if it's used
for indirect mapping. I will just add this new define instead of
replacing the old one, and check for both.
Is that ok?

No, having two of these does not seem right or useful. It is pseries
which does not use petitboot (relies on grub instead so until the target
kernel is started, there will be no ddw) so realistically we need this
property for kexec/kdump which uses the same kernel but different
initramdisk so for that purpose we need the same property name.

But I can see myself annoyed when I try petitboot in the hacked pseries
qemu and things may crash :) On this basis I'd suggest keeping the name
and adding a comment next to it that it is not always "direct" anymore.

Keeping the same name should bring more problems than solve.
If we have indirect mapping and kexec() to an older kernel, it will
think direct mapping is enabled, and trying to use a DMA address
without doing H_PUT_* first may cause a crash.

I tested with a new property name, and it doesn't crash.
As the property is not found, it does try to create a new DDW, which
fails and it falls back to using the default DMA window.
The device that need the IOMMU don't work well, but when iommu_map()
fails, it doesn't try to use the DMA address as valid.

Right, as discussed on slack.

To make sure the property differentiates both cases, a new u32 for flags
was added at the end of the property, where BIT(0) set means direct
mapping.

Signed-off-by: Leonardo Bras <leobras.c@xxxxxxxxx>
---
arch/powerpc/platforms/pseries/iommu.c | 108 +++++++++++++++++++------
1 file changed, 84 insertions(+), 24 deletions(-)

diff --git a/arch/powerpc/platforms/pseries/iommu.c b/arch/powerpc/platforms/pseries/iommu.c
index 3a1ef02ad9d5..9544e3c91ced 100644
--- a/arch/powerpc/platforms/pseries/iommu.c
+++ b/arch/powerpc/platforms/pseries/iommu.c
@@ -350,8 +350,11 @@ struct dynamic_dma_window_prop {
__be64 dma_base; /* address hi,lo */
__be32 tce_shift; /* ilog2(tce_page_size) */
__be32 window_shift; /* ilog2(tce_window_size) */
+ __be32 flags; /* DDW properties, see bellow */
};
+#define DDW_FLAGS_DIRECT 0x01

This is set if ((1<<window_shift) >= ddw_memory_hotplug_max()), you
could simply check window_shift and drop the flags.

Yeah, it's better this way, I will revert this.

+
struct direct_window {
struct device_node *device;
const struct dynamic_dma_window_prop *prop;
@@ -377,7 +380,7 @@ static LIST_HEAD(direct_window_list);
static DEFINE_SPINLOCK(direct_window_list_lock);
/* protects initializing window twice for same device */
static DEFINE_MUTEX(direct_window_init_mutex);
-#define DIRECT64_PROPNAME "linux,direct64-ddr-window-info"
+#define DMA64_PROPNAME "linux,dma64-ddr-window-info"
static int tce_clearrange_multi_pSeriesLP(unsigned long start_pfn,
unsigned long num_pfn, const void *arg)
@@ -836,7 +839,7 @@ static void remove_ddw(struct device_node *np, bool remove_prop)
if (ret)
return;
- win = of_find_property(np, DIRECT64_PROPNAME, NULL);
+ win = of_find_property(np, DMA64_PROPNAME, NULL);
if (!win)
return;
@@ -852,7 +855,7 @@ static void remove_ddw(struct device_node *np, bool remove_prop)
np, ret);
}
-static bool find_existing_ddw(struct device_node *pdn, u64 *dma_addr)
+static bool find_existing_ddw(struct device_node *pdn, u64 *dma_addr, bool *direct_mapping)
{
struct direct_window *window;
const struct dynamic_dma_window_prop *direct64;
@@ -864,6 +867,7 @@ static bool find_existing_ddw(struct device_node *pdn, u64 *dma_addr)
if (window->device == pdn) {
direct64 = window->prop;
*dma_addr = be64_to_cpu(direct64->dma_base);
+ *direct_mapping = be32_to_cpu(direct64->flags) & DDW_FLAGS_DIRECT;
found = true;
break;
}
@@ -901,8 +905,8 @@ static int find_existing_ddw_windows(void)
if (!firmware_has_feature(FW_FEATURE_LPAR))
return 0;
- for_each_node_with_property(pdn, DIRECT64_PROPNAME) {
- direct64 = of_get_property(pdn, DIRECT64_PROPNAME, &len);
+ for_each_node_with_property(pdn, DMA64_PROPNAME) {
+ direct64 = of_get_property(pdn, DMA64_PROPNAME, &len);
if (!direct64)
continue;
@@ -1124,7 +1128,8 @@ static void reset_dma_window(struct pci_dev *dev, struct device_node *par_dn)
}
static int ddw_property_create(struct property **ddw_win, const char *propname,
- u32 liobn, u64 dma_addr, u32 page_shift, u32 window_shift)
+ u32 liobn, u64 dma_addr, u32 page_shift,
+ u32 window_shift, bool direct_mapping)
{
struct dynamic_dma_window_prop *ddwprop;
struct property *win64;
@@ -1144,6 +1149,36 @@ static int ddw_property_create(struct property **ddw_win, const char *propname,
ddwprop->dma_base = cpu_to_be64(dma_addr);
ddwprop->tce_shift = cpu_to_be32(page_shift);
ddwprop->window_shift = cpu_to_be32(window_shift);
+ if (direct_mapping)
+ ddwprop->flags = cpu_to_be32(DDW_FLAGS_DIRECT);
+
+ return 0;
+}
+
+static int iommu_table_update_window(struct iommu_table **tbl, int nid, unsigned long liobn,
+ unsigned long win_addr, unsigned long page_shift,
+ unsigned long window_size)

Rather strange helper imho. I'd extract the most of
iommu_table_setparms_lpar() into iommu_table_setparms() (except
of_parse_dma_window) and call new helper from where you call
iommu_table_update_window; and do
iommu_pseries_alloc_table/iommu_tce_table_put there.

I don't see how to extract iommu_table_setparms_lpar() into
iommu_table_setparms(), they look to be used for different machine
types.

Do mean you extracting most of iommu_table_setparms_lpar() (and maybe
iommu_table_setparms() ) into a new helper, which is called in both
functions and use it instead of iommu_table_update_window() ?

Yes, this.

I will do that then, seems better. :)

+{
+ struct iommu_table *new_tbl, *old_tbl;
+
+ new_tbl = iommu_pseries_alloc_table(nid);
+ if (!new_tbl)
+ return -ENOMEM;
+
+ old_tbl = *tbl;
+ new_tbl->it_index = liobn;
+ new_tbl->it_offset = win_addr >> page_shift;
+ new_tbl->it_page_shift = page_shift;
+ new_tbl->it_size = window_size >> page_shift;
+ new_tbl->it_base = old_tbl->it_base;

Should not be used in pseries.

The point here is to migrate the values from the older tbl to the

The actual window/table is new (on the hypervisor side), you are not
migrating a single TCE, you deleted one whole window and created another
whole window, calling it "migration" is confusing, especially when PAPR
actually defines TCE migration.

Ok, I understand it's confusing now. I will avoid using this term from
now on.

newer. I Would like to understand why this is bad, if it will still be
'unused' as the older tbl.

Having explicit values is more readable imho.

Ok, I understand why it should be improved.!

Alexey, thank you for reviewing, and for helping me with my questions!

Thanks for doing this all!

Best regards,

+ new_tbl->it_busno = old_tbl->it_busno;
+ new_tbl->it_blocksize = old_tbl->it_blocksize;

16 for pseries and does not change (may be even make it a macro).

+ new_tbl->it_type = old_tbl->it_type;

TCE_PCI.

Same as above.

+ new_tbl->it_ops = old_tbl->it_ops;
+
+ iommu_init_table(new_tbl, nid, old_tbl->it_reserved_start, old_tbl->it_reserved_end);
+ iommu_tce_table_put(old_tbl);
+ *tbl = new_tbl;
return 0;
}
@@ -1171,12 +1206,16 @@ static bool enable_ddw(struct pci_dev *dev, struct device_node *pdn)
struct direct_window *window;
struct property *win64 = NULL;
struct failed_ddw_pdn *fpdn;
- bool default_win_removed = false;
+ bool default_win_removed = false, maps_whole_partition = false;

s/maps_whole_partition/direct_mapping/

Sure, I will get it replaced.

+ struct pci_dn *pci = PCI_DN(pdn);
+ struct iommu_table *tbl = pci->table_group->tables[0];
mutex_lock(&direct_window_init_mutex);
- if (find_existing_ddw(pdn, &dev->dev.archdata.dma_offset))
- goto out_unlock;
+ if (find_existing_ddw(pdn, &dev->dev.archdata.dma_offset, &maps_whole_partition)) {
+ mutex_unlock(&direct_window_init_mutex);
+ return maps_whole_partition;
+ }
/*
* If we already went through this for a previous function of
@@ -1258,16 +1297,24 @@ static bool enable_ddw(struct pci_dev *dev, struct device_node *pdn)
query.page_size);
goto out_failed;
}
+
/* verify the window * number of ptes will map the partition */
- /* check largest block * page size > max memory hotplug addr */
max_addr = ddw_memory_hotplug_max();
if (query.largest_available_block < (max_addr >> page_shift)) {
- dev_dbg(&dev->dev, "can't map partition max 0x%llx with %llu "
- "%llu-sized pages\n", max_addr, query.largest_available_block,
- 1ULL << page_shift);
- goto out_failed;
+ dev_dbg(&dev->dev, "can't map partition max 0x%llx with %llu %llu-sized pages\n",
+ max_addr, query.largest_available_block,
+ 1ULL << page_shift);
+
+ len = order_base_2(query.largest_available_block << page_shift);
+ } else {
+ maps_whole_partition = true;
+ len = order_base_2(max_addr);
}
- len = order_base_2(max_addr);
+
+ /* DDW + IOMMU on single window may fail if there is any allocation */
+ if (default_win_removed && !maps_whole_partition &&
+ iommu_table_in_use(tbl))
+ goto out_failed;
ret = create_ddw(dev, ddw_avail, &create, page_shift, len);
if (ret != 0)
@@ -1277,8 +1324,8 @@ static bool enable_ddw(struct pci_dev *dev, struct device_node *pdn)
create.liobn, dn);
win_addr = ((u64)create.addr_hi << 32) | create.addr_lo;
- ret = ddw_property_create(&win64, DIRECT64_PROPNAME, create.liobn, win_addr,
- page_shift, len);
+ ret = ddw_property_create(&win64, DMA64_PROPNAME, create.liobn, win_addr,
+ page_shift, len, maps_whole_partition);
if (ret) {
dev_info(&dev->dev,
"couldn't allocate property, property name, or value\n");
@@ -1297,12 +1344,25 @@ static bool enable_ddw(struct pci_dev *dev, struct device_node *pdn)
if (!window)
goto out_prop_del;
- ret = walk_system_ram_range(0, memblock_end_of_DRAM() >> PAGE_SHIFT,
- win64->value, tce_setrange_multi_pSeriesLP_walk);
- if (ret) {
- dev_info(&dev->dev, "failed to map direct window for %pOF: %d\n",
- dn, ret);
- goto out_free_window;
+ if (maps_whole_partition) {
+ /* DDW maps the whole partition, so enable direct DMA mapping */
+ ret = walk_system_ram_range(0, memblock_end_of_DRAM() >> PAGE_SHIFT,
+ win64->value, tce_setrange_multi_pSeriesLP_walk);
+ if (ret) {
+ dev_info(&dev->dev, "failed to map direct window for %pOF: %d\n",
+ dn, ret);
+ goto out_free_window;
+ }
+ } else {
+ /* New table for using DDW instead of the default DMA window */
+ if (iommu_table_update_window(&tbl, pci->phb->node, create.liobn,
+ win_addr, page_shift, 1UL << len))
+ goto out_free_window;
+
+ set_iommu_table_base(&dev->dev, tbl);
+ WARN_ON(dev->dev.archdata.dma_offset >= SZ_4G);

What is this check for exactly? Why 4G, not >= 0, for example?

I am not really sure, you suggested adding it here:
http://patchwork.ozlabs.org/project/linuxppc-dev/patch/20200716071658.467820-6-leobras.c@xxxxxxxxx/#2488874

Ah right I did suggest this :) My bad. I think I suggested it before
suggesting to keep the reserved area boundaries checked/adjusted to the
window boundaries, may as well drop this. Thanks,

I can remove it if it's ok.

+ goto out_unlock;
+
}
dev->dev.archdata.dma_offset = win_addr;
@@ -1340,7 +1400,7 @@ static bool enable_ddw(struct pci_dev *dev, struct device_node *pdn)
out_unlock:
mutex_unlock(&direct_window_init_mutex);
- return win64;
+ return win64 && maps_whole_partition;
}
static void pci_dma_dev_setup_pSeriesLP(struct pci_dev *dev)

--
Alexey

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In reply to: Leonardo Bras: "Re: [PATCH v1 09/10] powerpc/pseries/iommu: Make use of DDW even if it does not map the partition"
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