[PATCH v2 5/5] s390/pci: use lock-free I/O translation updates

Niklas Schnelle
2022-11-09T14:29:45Z

I/O translation tables on s390 use 8 byte page table entries and tables
which are allocated lazily but only freed when the entire I/O
translation table is torn down. Also each IOVA can at any time only
translate to one physical address Furthermore I/O table accesses by the
IOMMU hardware are cache coherent. With a bit of care we can thus use
atomic updates to manipulate the translation table without having to use
a global lock at all. This is done analogous to the existing I/O
translation table handling code used on Intel and AMD x86 systems.

Signed-off-by: Niklas Schnelle <schnelle@linux.ibm.com>
---
arch/s390/include/asm/pci.h | 1 -
arch/s390/pci/pci_dma.c | 74 ++++++++++++++++++++++---------------
drivers/iommu/s390-iommu.c | 37 +++++++------------
3 files changed, 58 insertions(+), 54 deletions(-)

diff --git a/arch/s390/include/asm/pci.h b/arch/s390/include/asm/pci.h
index e4c3e4e04d30..b248694e0024 100644
--- a/arch/s390/include/asm/pci.h
+++ b/arch/s390/include/asm/pci.h
@@ -157,7 +157,6 @@ struct zpci_dev {

/* DMA stuff */
unsigned long *dma_table;
- spinlock_t dma_table_lock;
int tlb_refresh;

spinlock_t iommu_bitmap_lock;
diff --git a/arch/s390/pci/pci_dma.c b/arch/s390/pci/pci_dma.c
index dee825ee7305..ea478d11fbd1 100644
--- a/arch/s390/pci/pci_dma.c
+++ b/arch/s390/pci/pci_dma.c
@@ -63,37 +63,55 @@ static void dma_free_page_table(void *table)
kmem_cache_free(dma_page_table_cache, table);
}

-static unsigned long *dma_get_seg_table_origin(unsigned long *entry)
+static unsigned long *dma_get_seg_table_origin(unsigned long *rtep)
{
+ unsigned long old_rte, rte;
unsigned long *sto;

- if (reg_entry_isvalid(*entry))
- sto = get_rt_sto(*entry);
- else {
+ rte = READ_ONCE(*rtep);
+ if (reg_entry_isvalid(rte)) {
+ sto = get_rt_sto(rte);
+ } else {
sto = dma_alloc_cpu_table();
if (!sto)
return NULL;

- set_rt_sto(entry, virt_to_phys(sto));
- validate_rt_entry(entry);
- entry_clr_protected(entry);
+ set_rt_sto(&rte, virt_to_phys(sto));
+ validate_rt_entry(&rte);
+ entry_clr_protected(&rte);
+
+ old_rte = cmpxchg(rtep, ZPCI_TABLE_INVALID, rte);
+ if (old_rte != ZPCI_TABLE_INVALID) {
+ /* Somone else was faster, use theirs */
+ dma_free_cpu_table(sto);
+ sto = get_rt_sto(old_rte);
+ }
}
return sto;
}

-static unsigned long *dma_get_page_table_origin(unsigned long *entry)
+static unsigned long *dma_get_page_table_origin(unsigned long *step)
{
+ unsigned long old_ste, ste;
unsigned long *pto;

- if (reg_entry_isvalid(*entry))
- pto = get_st_pto(*entry);
- else {
+ ste = READ_ONCE(*step);
+ if (reg_entry_isvalid(ste)) {
+ pto = get_st_pto(ste);
+ } else {
pto = dma_alloc_page_table();
if (!pto)
return NULL;
- set_st_pto(entry, virt_to_phys(pto));
- validate_st_entry(entry);
- entry_clr_protected(entry);
+ set_st_pto(&ste, virt_to_phys(pto));
+ validate_st_entry(&ste);
+ entry_clr_protected(&ste);
+
+ old_ste = cmpxchg(step, ZPCI_TABLE_INVALID, ste);
+ if (old_ste != ZPCI_TABLE_INVALID) {
+ /* Somone else was faster, use theirs */
+ dma_free_page_table(pto);
+ pto = get_st_pto(old_ste);
+ }
}
return pto;
}
@@ -117,19 +135,24 @@ unsigned long *dma_walk_cpu_trans(unsigned long *rto, dma_addr_t dma_addr)
return &pto[px];
}

-void dma_update_cpu_trans(unsigned long *entry, phys_addr_t page_addr, int flags)
+void dma_update_cpu_trans(unsigned long *ptep, phys_addr_t page_addr, int flags)
{
+ unsigned long pte;
+
+ pte = READ_ONCE(*ptep);
if (flags & ZPCI_PTE_INVALID) {
- invalidate_pt_entry(entry);
+ invalidate_pt_entry(&pte);
} else {
- set_pt_pfaa(entry, page_addr);
- validate_pt_entry(entry);
+ set_pt_pfaa(&pte, page_addr);
+ validate_pt_entry(&pte);
}

if (flags & ZPCI_TABLE_PROTECTED)
- entry_set_protected(entry);
+ entry_set_protected(&pte);
else
- entry_clr_protected(entry);
+ entry_clr_protected(&pte);
+
+ xchg(ptep, pte);
}

static int __dma_update_trans(struct zpci_dev *zdev, phys_addr_t pa,
@@ -137,18 +160,14 @@ static int __dma_update_trans(struct zpci_dev *zdev, phys_addr_t pa,
{
unsigned int nr_pages = PAGE_ALIGN(size) >> PAGE_SHIFT;
phys_addr_t page_addr = (pa & PAGE_MASK);
- unsigned long irq_flags;
unsigned long *entry;
int i, rc = 0;

if (!nr_pages)
return -EINVAL;

- spin_lock_irqsave(&zdev->dma_table_lock, irq_flags);
- if (!zdev->dma_table) {
- rc = -EINVAL;
- goto out_unlock;
- }
+ if (!zdev->dma_table)
+ return -EINVAL;

for (i = 0; i < nr_pages; i++) {
entry = dma_walk_cpu_trans(zdev->dma_table, dma_addr);
@@ -173,8 +192,6 @@ static int __dma_update_trans(struct zpci_dev *zdev, phys_addr_t pa,
dma_update_cpu_trans(entry, page_addr, flags);
}
}
-out_unlock:
- spin_unlock_irqrestore(&zdev->dma_table_lock, irq_flags);
return rc;
}

@@ -558,7 +575,6 @@ int zpci_dma_init_device(struct zpci_dev *zdev)
WARN_ON(zdev->s390_domain);

spin_lock_init(&zdev->iommu_bitmap_lock);
- spin_lock_init(&zdev->dma_table_lock);

zdev->dma_table = dma_alloc_cpu_table();
if (!zdev->dma_table) {
diff --git a/drivers/iommu/s390-iommu.c b/drivers/iommu/s390-iommu.c
index 2b9a3e3bc606..ed33c6cce083 100644
--- a/drivers/iommu/s390-iommu.c
+++ b/drivers/iommu/s390-iommu.c
@@ -20,7 +20,6 @@ struct s390_domain {
struct iommu_domain domain;
struct list_head devices;
unsigned long *dma_table;
- spinlock_t dma_table_lock;
spinlock_t list_lock;
struct rcu_head rcu;
};
@@ -62,7 +61,6 @@ static struct iommu_domain *s390_domain_alloc(unsigned domain_type)
s390_domain->domain.geometry.aperture_start = 0;
s390_domain->domain.geometry.aperture_end = ZPCI_TABLE_SIZE_RT - 1;

- spin_lock_init(&s390_domain->dma_table_lock);
spin_lock_init(&s390_domain->list_lock);
INIT_LIST_HEAD_RCU(&s390_domain->devices);

@@ -265,14 +263,10 @@ static int s390_iommu_validate_trans(struct s390_domain *s390_domain,
unsigned long nr_pages, int flags)
{
phys_addr_t page_addr = pa & PAGE_MASK;
- unsigned long irq_flags, i;
unsigned long *entry;
+ unsigned long i;
int rc;

- if (!nr_pages)
- return 0;
-
- spin_lock_irqsave(&s390_domain->dma_table_lock, irq_flags);
for (i = 0; i < nr_pages; i++) {
entry = dma_walk_cpu_trans(s390_domain->dma_table, dma_addr);
if (unlikely(!entry)) {
@@ -283,7 +277,6 @@ static int s390_iommu_validate_trans(struct s390_domain *s390_domain,
page_addr += PAGE_SIZE;
dma_addr += PAGE_SIZE;
}
- spin_unlock_irqrestore(&s390_domain->dma_table_lock, irq_flags);

return 0;

@@ -296,7 +289,6 @@ static int s390_iommu_validate_trans(struct s390_domain *s390_domain,
break;
dma_update_cpu_trans(entry, 0, ZPCI_PTE_INVALID);
}
- spin_unlock_irqrestore(&s390_domain->dma_table_lock, irq_flags);

return rc;
}
@@ -304,14 +296,10 @@ static int s390_iommu_validate_trans(struct s390_domain *s390_domain,
static int s390_iommu_invalidate_trans(struct s390_domain *s390_domain,
dma_addr_t dma_addr, unsigned long nr_pages)
{
- unsigned long irq_flags, i;
unsigned long *entry;
+ unsigned long i;
int rc = 0;

- if (!nr_pages)
- return 0;
-
- spin_lock_irqsave(&s390_domain->dma_table_lock, irq_flags);
for (i = 0; i < nr_pages; i++) {
entry = dma_walk_cpu_trans(s390_domain->dma_table, dma_addr);
if (unlikely(!entry)) {
@@ -321,7 +309,6 @@ static int s390_iommu_invalidate_trans(struct s390_domain *s390_domain,
dma_update_cpu_trans(entry, 0, ZPCI_PTE_INVALID);
dma_addr += PAGE_SIZE;
}
- spin_unlock_irqrestore(&s390_domain->dma_table_lock, irq_flags);

return rc;
}
@@ -363,7 +350,8 @@ static phys_addr_t s390_iommu_iova_to_phys(struct iommu_domain *domain,
dma_addr_t iova)
{
struct s390_domain *s390_domain = to_s390_domain(domain);
- unsigned long *sto, *pto, *rto, flags;
+ unsigned long *rto, *sto, *pto;
+ unsigned long ste, pte, rte;
unsigned int rtx, sx, px;
phys_addr_t phys = 0;

@@ -376,16 +364,17 @@ static phys_addr_t s390_iommu_iova_to_phys(struct iommu_domain *domain,
px = calc_px(iova);
rto = s390_domain->dma_table;

- spin_lock_irqsave(&s390_domain->dma_table_lock, flags);
- if (rto && reg_entry_isvalid(rto[rtx])) {
- sto = get_rt_sto(rto[rtx]);
- if (sto && reg_entry_isvalid(sto[sx])) {
- pto = get_st_pto(sto[sx]);
- if (pto && pt_entry_isvalid(pto[px]))
- phys = pto[px] & ZPCI_PTE_ADDR_MASK;
+ rte = READ_ONCE(rto[rtx]);
+ if (reg_entry_isvalid(rte)) {
+ sto = get_rt_sto(rte);
+ ste = READ_ONCE(sto[sx]);
+ if (reg_entry_isvalid(ste)) {
+ pto = get_st_pto(ste);
+ pte = READ_ONCE(pto[px]);
+ if (pt_entry_isvalid(pte))
+ phys = pte & ZPCI_PTE_ADDR_MASK;
}
}
- spin_unlock_irqrestore(&s390_domain->dma_table_lock, flags);

return phys;
}
--
2.34.1