Re: [PATCH 2/4] powerpc: Prepare to support kernel handling of IOMMU map/unmap
On 06/16/2013 02:26 PM, Benjamin Herrenschmidt wrote:
+#if defined(CONFIG_SPARSEMEM_VMEMMAP) || defined(CONFIG_FLATMEM)
+int realmode_get_page(struct page *page)
+{
+if (PageCompound(page))
+return -EAGAIN;
+
+get_page(page);
+
+return 0;
+}
+EXPORT_SYMBOL_GPL(realmode_get_page);
+
+int realmode_put_page(struct page *page)
+{
+if (PageCompound(page))
+return -EAGAIN;
+
+if (!atomic_add_unless(page-_count, -1, 1))
+return -EAGAIN;
+
+return 0;
+}
+EXPORT_SYMBOL_GPL(realmode_put_page);
+#endif
Several worries here, mostly that if the generic code ever changes
(something gets added to get_page() that makes it no-longer safe for use
in real mode for example, or some other condition gets added to
put_page()), we go out of sync and potentially end up with very hard and
very subtle bugs.
It might be worth making sure that:
- This is reviewed by some generic VM people (and make sure they
understand why we need to do that)
- A comment is added to get_page() and put_page() to make sure that if
they are changed in any way, dbl check the impact on our
realmode_get_page() (or ping us to make sure things are still ok).
After changing get_page() to get_page_unless_zero(), the get_page API I use is:
get_page_unless_zero() - basically atomic_inc_not_zero()
atomic_add_unless() - just operated with the counter
PageCompound() - check if it is a huge page.
No usage of get_page or put_page.
If any of those changes, I would expect it to hit us immediately, no?
So it may only make sense to add a comment to PageCompound(). But the
comment says PageCompound is generally not used in hot code paths, and
our path is hot. Heh.
diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
index 6d53675..c70a654 100644
--- a/include/linux/page-flags.h
+++ b/include/linux/page-flags.h
@@ -329,7 +329,8 @@ static inline void set_page_writeback(struct page *page)
* System with lots of page flags available. This allows separate
* flags for PageHead() and PageTail() checks of compound pages so that bit
* tests can be used in performance sensitive paths. PageCompound is
- * generally not used in hot code paths.
+ * generally not used in hot code paths except arch/powerpc/mm/init_64.c
+ * which uses it to detect huge pages and avoid handling those in real mode.
*/
__PAGEFLAG(Head, head) CLEARPAGEFLAG(Head, head)
__PAGEFLAG(Tail, tail)
So?
--
Alexey
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Re: [PATCH 2/4] powerpc: Prepare to support kernel handling of IOMMU map/unmap
+#if defined(CONFIG_SPARSEMEM_VMEMMAP) || defined(CONFIG_FLATMEM)
+int realmode_get_page(struct page *page)
+{
+ if (PageCompound(page))
+ return -EAGAIN;
+
+ get_page(page);
+
+ return 0;
+}
+EXPORT_SYMBOL_GPL(realmode_get_page);
+
+int realmode_put_page(struct page *page)
+{
+ if (PageCompound(page))
+ return -EAGAIN;
+
+ if (!atomic_add_unless(page-_count, -1, 1))
+ return -EAGAIN;
+
+ return 0;
+}
+EXPORT_SYMBOL_GPL(realmode_put_page);
+#endif
Several worries here, mostly that if the generic code ever changes
(something gets added to get_page() that makes it no-longer safe for use
in real mode for example, or some other condition gets added to
put_page()), we go out of sync and potentially end up with very hard and
very subtle bugs.
It might be worth making sure that:
- This is reviewed by some generic VM people (and make sure they
understand why we need to do that)
- A comment is added to get_page() and put_page() to make sure that if
they are changed in any way, dbl check the impact on our
realmode_get_page() (or ping us to make sure things are still ok).
Cheers,
Ben.
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Re: [PATCH 2/4] powerpc: Prepare to support kernel handling of IOMMU map/unmap
On Sun, 2013-06-16 at 14:26 +1000, Benjamin Herrenschmidt wrote:
+int realmode_get_page(struct page *page)
+{
+ if (PageCompound(page))
+ return -EAGAIN;
+
+ get_page(page);
+
+ return 0;
+}
Shouldn't it be get_page_unless_zero ?
Cheers,
Ben.
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[PATCH 2/4] powerpc: Prepare to support kernel handling of IOMMU map/unmap
The current VFIO-on-POWER implementation supports only user mode
driven mapping, i.e. QEMU is sending requests to map/unmap pages.
However this approach is really slow, so we want to move that to KVM.
Since H_PUT_TCE can be extremely performance sensitive (especially with
network adapters where each packet needs to be mapped/unmapped) we chose
to implement that as a fast hypercall directly in real
mode (processor still in the guest context but MMU off).
To be able to do that, we need to provide some facilities to
access the struct page count within that real mode environment as things
like the sparsemem vmemmap mappings aren't accessible.
This adds an API to increment/decrement page counter as
get_user_pages API used for user mode mapping does not work
in the real mode.
CONFIG_SPARSEMEM_VMEMMAP and CONFIG_FLATMEM are supported.
Signed-off-by: Alexey Kardashevskiy a...@ozlabs.ru
Reviewed-by: Paul Mackerras pau...@samba.org
Cc: David Gibson da...@gibson.dropbear.id.au
Signed-off-by: Paul Mackerras pau...@samba.org
---
Changes:
2013-05-20:
* PageTail() is replaced by PageCompound() in order to have the same checks
for whether the page is huge in realmode_get_page() and realmode_put_page()
---
arch/powerpc/include/asm/pgtable-ppc64.h |4 ++
arch/powerpc/mm/init_64.c| 77 +-
2 files changed, 80 insertions(+), 1 deletion(-)
diff --git a/arch/powerpc/include/asm/pgtable-ppc64.h
b/arch/powerpc/include/asm/pgtable-ppc64.h
index e3d55f6f..7b46e5f 100644
--- a/arch/powerpc/include/asm/pgtable-ppc64.h
+++ b/arch/powerpc/include/asm/pgtable-ppc64.h
@@ -376,6 +376,10 @@ static inline pte_t *find_linux_pte_or_hugepte(pgd_t
*pgdir, unsigned long ea,
}
#endif /* !CONFIG_HUGETLB_PAGE */
+struct page *realmode_pfn_to_page(unsigned long pfn);
+int realmode_get_page(struct page *page);
+int realmode_put_page(struct page *page);
+
#endif /* __ASSEMBLY__ */
#endif /* _ASM_POWERPC_PGTABLE_PPC64_H_ */
diff --git a/arch/powerpc/mm/init_64.c b/arch/powerpc/mm/init_64.c
index a90b9c4..ce3d8d4 100644
--- a/arch/powerpc/mm/init_64.c
+++ b/arch/powerpc/mm/init_64.c
@@ -297,5 +297,80 @@ void vmemmap_free(unsigned long start, unsigned long end)
{
}
-#endif /* CONFIG_SPARSEMEM_VMEMMAP */
+/*
+ * We do not have access to the sparsemem vmemmap, so we fallback to
+ * walking the list of sparsemem blocks which we already maintain for
+ * the sake of crashdump. In the long run, we might want to maintain
+ * a tree if performance of that linear walk becomes a problem.
+ *
+ * Any of realmode_ functions can fail due to:
+ * 1) As real sparsemem blocks do not lay in RAM continously (they
+ * are in virtual address space which is not available in the real mode),
+ * the requested page struct can be split between blocks so get_page/put_page
+ * may fail.
+ * 2) When huge pages are used, the get_page/put_page API will fail
+ * in real mode as the linked addresses in the page struct are virtual
+ * too.
+ * When 1) or 2) takes place, the API returns an error code to cause
+ * an exit to kernel virtual mode where the operation will be completed.
+ */
+struct page *realmode_pfn_to_page(unsigned long pfn)
+{
+ struct vmemmap_backing *vmem_back;
+ struct page *page;
+ unsigned long page_size = 1 mmu_psize_defs[mmu_vmemmap_psize].shift;
+ unsigned long pg_va = (unsigned long) pfn_to_page(pfn);
+
+ for (vmem_back = vmemmap_list; vmem_back; vmem_back = vmem_back-list) {
+ if (pg_va vmem_back-virt_addr)
+ continue;
+ /* Check that page struct is not split between real pages */
+ if ((pg_va + sizeof(struct page))
+ (vmem_back-virt_addr + page_size))
+ return NULL;
+
+ page = (struct page *) (vmem_back-phys + pg_va -
+ vmem_back-virt_addr);
+ return page;
+ }
+
+ return NULL;
+}
+EXPORT_SYMBOL_GPL(realmode_pfn_to_page);
+
+#elif defined(CONFIG_FLATMEM)
+
+struct page *realmode_pfn_to_page(unsigned long pfn)
+{
+ struct page *page = pfn_to_page(pfn);
+ return page;
+}
+EXPORT_SYMBOL_GPL(realmode_pfn_to_page);
+
+#endif /* CONFIG_SPARSEMEM_VMEMMAP/CONFIG_FLATMEM */
+
+#if defined(CONFIG_SPARSEMEM_VMEMMAP) || defined(CONFIG_FLATMEM)
+int realmode_get_page(struct page *page)
+{
+ if (PageCompound(page))
+ return -EAGAIN;
+
+ get_page(page);
+
+ return 0;
+}
+EXPORT_SYMBOL_GPL(realmode_get_page);
+
+int realmode_put_page(struct page *page)
+{
+ if (PageCompound(page))
+ return -EAGAIN;
+
+ if (!atomic_add_unless(page-_count, -1, 1))
+ return -EAGAIN;
+
+ return 0;
+}
+EXPORT_SYMBOL_GPL(realmode_put_page);
+#endif
--
1.7.10.4
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[PATCH 2/4] powerpc: Prepare to support kernel handling of IOMMU map/unmap
The current VFIO-on-POWER implementation supports only user mode
driven mapping, i.e. QEMU is sending requests to map/unmap pages.
However this approach is really slow, so we want to move that to KVM.
Since H_PUT_TCE can be extremely performance sensitive (especially with
network adapters where each packet needs to be mapped/unmapped) we chose
to implement that as a fast hypercall directly in real
mode (processor still in the guest context but MMU off).
To be able to do that, we need to provide some facilities to
access the struct page count within that real mode environment as things
like the sparsemem vmemmap mappings aren't accessible.
This adds an API to increment/decrement page counter as
get_user_pages API used for user mode mapping does not work
in the real mode.
CONFIG_SPARSEMEM_VMEMMAP and CONFIG_FLATMEM are supported.
Signed-off-by: Alexey Kardashevskiy a...@ozlabs.ru
Reviewed-by: Paul Mackerras pau...@samba.org
Cc: David Gibson da...@gibson.dropbear.id.au
Signed-off-by: Paul Mackerras pau...@samba.org
---
Changes:
2013-05-20:
* PageTail() is replaced by PageCompound() in order to have the same checks
for whether the page is huge in realmode_get_page() and realmode_put_page()
---
arch/powerpc/include/asm/pgtable-ppc64.h |4 ++
arch/powerpc/mm/init_64.c| 77 +-
2 files changed, 80 insertions(+), 1 deletion(-)
diff --git a/arch/powerpc/include/asm/pgtable-ppc64.h
b/arch/powerpc/include/asm/pgtable-ppc64.h
index e3d55f6f..7b46e5f 100644
--- a/arch/powerpc/include/asm/pgtable-ppc64.h
+++ b/arch/powerpc/include/asm/pgtable-ppc64.h
@@ -376,6 +376,10 @@ static inline pte_t *find_linux_pte_or_hugepte(pgd_t
*pgdir, unsigned long ea,
}
#endif /* !CONFIG_HUGETLB_PAGE */
+struct page *realmode_pfn_to_page(unsigned long pfn);
+int realmode_get_page(struct page *page);
+int realmode_put_page(struct page *page);
+
#endif /* __ASSEMBLY__ */
#endif /* _ASM_POWERPC_PGTABLE_PPC64_H_ */
diff --git a/arch/powerpc/mm/init_64.c b/arch/powerpc/mm/init_64.c
index c2787bf..ba6cf9b 100644
--- a/arch/powerpc/mm/init_64.c
+++ b/arch/powerpc/mm/init_64.c
@@ -296,5 +296,80 @@ void vmemmap_free(unsigned long start, unsigned long end)
{
}
-#endif /* CONFIG_SPARSEMEM_VMEMMAP */
+/*
+ * We do not have access to the sparsemem vmemmap, so we fallback to
+ * walking the list of sparsemem blocks which we already maintain for
+ * the sake of crashdump. In the long run, we might want to maintain
+ * a tree if performance of that linear walk becomes a problem.
+ *
+ * Any of realmode_ functions can fail due to:
+ * 1) As real sparsemem blocks do not lay in RAM continously (they
+ * are in virtual address space which is not available in the real mode),
+ * the requested page struct can be split between blocks so get_page/put_page
+ * may fail.
+ * 2) When huge pages are used, the get_page/put_page API will fail
+ * in real mode as the linked addresses in the page struct are virtual
+ * too.
+ * When 1) or 2) takes place, the API returns an error code to cause
+ * an exit to kernel virtual mode where the operation will be completed.
+ */
+struct page *realmode_pfn_to_page(unsigned long pfn)
+{
+ struct vmemmap_backing *vmem_back;
+ struct page *page;
+ unsigned long page_size = 1 mmu_psize_defs[mmu_vmemmap_psize].shift;
+ unsigned long pg_va = (unsigned long) pfn_to_page(pfn);
+
+ for (vmem_back = vmemmap_list; vmem_back; vmem_back = vmem_back-list) {
+ if (pg_va vmem_back-virt_addr)
+ continue;
+ /* Check that page struct is not split between real pages */
+ if ((pg_va + sizeof(struct page))
+ (vmem_back-virt_addr + page_size))
+ return NULL;
+
+ page = (struct page *) (vmem_back-phys + pg_va -
+ vmem_back-virt_addr);
+ return page;
+ }
+
+ return NULL;
+}
+EXPORT_SYMBOL_GPL(realmode_pfn_to_page);
+
+#elif defined(CONFIG_FLATMEM)
+
+struct page *realmode_pfn_to_page(unsigned long pfn)
+{
+ struct page *page = pfn_to_page(pfn);
+ return page;
+}
+EXPORT_SYMBOL_GPL(realmode_pfn_to_page);
+
+#endif /* CONFIG_SPARSEMEM_VMEMMAP/CONFIG_FLATMEM */
+
+#if defined(CONFIG_SPARSEMEM_VMEMMAP) || defined(CONFIG_FLATMEM)
+int realmode_get_page(struct page *page)
+{
+ if (PageCompound(page))
+ return -EAGAIN;
+
+ get_page(page);
+
+ return 0;
+}
+EXPORT_SYMBOL_GPL(realmode_get_page);
+
+int realmode_put_page(struct page *page)
+{
+ if (PageCompound(page))
+ return -EAGAIN;
+
+ if (!atomic_add_unless(page-_count, -1, 1))
+ return -EAGAIN;
+
+ return 0;
+}
+EXPORT_SYMBOL_GPL(realmode_put_page);
+#endif
--
1.7.10.4
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