Patchwork [04/10] powerpc: Prepare to support kernel handling of IOMMU map/unmap

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Submitter Alexey Kardashevskiy
Date July 16, 2013, 12:53 a.m.
Message ID <1373936045-22653-5-git-send-email-aik@ozlabs.ru>
Download mbox | patch
Permalink /patch/259318/
State New
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Comments

Alexey Kardashevskiy - July 16, 2013, 12:53 a.m.
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.

Cc: linux-mm@kvack.org
Reviewed-by: Paul Mackerras <paulus@samba.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>
Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>

---

Changes:
2013/07/10:
* adjusted comment (removed sentence about virtual mode)
* get_page_unless_zero replaced with atomic_inc_not_zero to minimize
effect of a possible get_page_unless_zero() rework (if it ever happens).

2013/06/27:
* realmode_get_page() fixed to use get_page_unless_zero(). If failed,
the call will be passed from real to virtual mode and safely handled.
* added comment to PageCompound() in include/linux/page-flags.h.

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()

Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>
---
 arch/powerpc/include/asm/pgtable-ppc64.h |  4 ++
 arch/powerpc/mm/init_64.c                | 76 +++++++++++++++++++++++++++++++-
 include/linux/page-flags.h               |  4 +-
 3 files changed, 82 insertions(+), 2 deletions(-)
Alexey Kardashevskiy - July 23, 2013, 2:22 a.m.
Ping, anyone, please?

Ben needs ack from any of MM people before proceeding with this patch. Thanks!


On 07/16/2013 10:53 AM, Alexey Kardashevskiy wrote:
> 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.
> 
> Cc: linux-mm@kvack.org
> Reviewed-by: Paul Mackerras <paulus@samba.org>
> Signed-off-by: Paul Mackerras <paulus@samba.org>
> Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>
> 
> ---
> 
> Changes:
> 2013/07/10:
> * adjusted comment (removed sentence about virtual mode)
> * get_page_unless_zero replaced with atomic_inc_not_zero to minimize
> effect of a possible get_page_unless_zero() rework (if it ever happens).
> 
> 2013/06/27:
> * realmode_get_page() fixed to use get_page_unless_zero(). If failed,
> the call will be passed from real to virtual mode and safely handled.
> * added comment to PageCompound() in include/linux/page-flags.h.
> 
> 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()
> 
> Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>
> ---
>  arch/powerpc/include/asm/pgtable-ppc64.h |  4 ++
>  arch/powerpc/mm/init_64.c                | 76 +++++++++++++++++++++++++++++++-
>  include/linux/page-flags.h               |  4 +-
>  3 files changed, 82 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/powerpc/include/asm/pgtable-ppc64.h b/arch/powerpc/include/asm/pgtable-ppc64.h
> index 46db094..aa7b169 100644
> --- a/arch/powerpc/include/asm/pgtable-ppc64.h
> +++ b/arch/powerpc/include/asm/pgtable-ppc64.h
> @@ -394,6 +394,10 @@ static inline void mark_hpte_slot_valid(unsigned char *hpte_slot_array,
>  	hpte_slot_array[index] = hidx << 4 | 0x1 << 3;
>  }
>  
> +struct page *realmode_pfn_to_page(unsigned long pfn);
> +int realmode_get_page(struct page *page);
> +int realmode_put_page(struct page *page);
> +
>  static inline char *get_hpte_slot_array(pmd_t *pmdp)
>  {
>  	/*
> diff --git a/arch/powerpc/mm/init_64.c b/arch/powerpc/mm/init_64.c
> index d0cd9e4..dcbb806 100644
> --- a/arch/powerpc/mm/init_64.c
> +++ b/arch/powerpc/mm/init_64.c
> @@ -300,5 +300,79 @@ 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_XXXX 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.
> + */
> +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;
> +
> +	if (!atomic_inc_not_zero(&page->_count))
> +		return -EAGAIN;
> +
> +	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
> diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
> index 6d53675..98ada58 100644
> --- a/include/linux/page-flags.h
> +++ b/include/linux/page-flags.h
> @@ -329,7 +329,9 @@ 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
> + * and arch/powerpc/kvm/book3s_64_vio_hv.c which use it to detect huge pages
> + * and avoid handling those in real mode.
>   */
>  __PAGEFLAG(Head, head) CLEARPAGEFLAG(Head, head)
>  __PAGEFLAG(Tail, tail)
>
Andrew Morton - July 24, 2013, 10:43 p.m.
On Tue, 23 Jul 2013 12:22:59 +1000 Alexey Kardashevskiy <aik@ozlabs.ru> wrote:

> Ping, anyone, please?

ew, you top-posted.

> Ben needs ack from any of MM people before proceeding with this patch. Thanks!

For what?  The three lines of comment in page-flags.h?   ack :)

Manipulating page->_count directly is considered poor form.  Don't
blame us if we break your code ;)

Actually, the manipulation in realmode_get_page() duplicates the
existing get_page_unless_zero() and the one in realmode_put_page()
could perhaps be placed in mm.h with a suitable name and some
documentation.  That would improve your form and might protect the code
from getting broken later on.

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Benjamin Herrenschmidt - July 24, 2013, 11:13 p.m.
On Wed, 2013-07-24 at 15:43 -0700, Andrew Morton wrote:
> For what?  The three lines of comment in page-flags.h?   ack :)
> 
> Manipulating page->_count directly is considered poor form.  Don't
> blame us if we break your code ;)
> 
> Actually, the manipulation in realmode_get_page() duplicates the
> existing get_page_unless_zero() and the one in realmode_put_page()
> could perhaps be placed in mm.h with a suitable name and some
> documentation.  That would improve your form and might protect the code
> from getting broken later on.

Yes, this stuff makes me really nervous :-) If it didn't provide an order
of magnitude performance improvement in KVM I would avoid it but heh...

Alexey, I like having that stuff in generic code.

However the meaning of the words "real mode" can be ambiguous accross
architectures, it might be best to then name it "mmu_off_put_page" to
make things a bit clearer, along with a comment explaining that this is
called in a context where none of the virtual mappings are accessible
(vmalloc, vmemmap, IOs, ...), and that in the case of sparsemem vmemmap
the caller must have taken care of getting the physical address of the
struct page and of ensuring it isn't split accross two vmemmap blocks.

Cheers,
Ben.


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Patch

diff --git a/arch/powerpc/include/asm/pgtable-ppc64.h b/arch/powerpc/include/asm/pgtable-ppc64.h
index 46db094..aa7b169 100644
--- a/arch/powerpc/include/asm/pgtable-ppc64.h
+++ b/arch/powerpc/include/asm/pgtable-ppc64.h
@@ -394,6 +394,10 @@  static inline void mark_hpte_slot_valid(unsigned char *hpte_slot_array,
 	hpte_slot_array[index] = hidx << 4 | 0x1 << 3;
 }
 
+struct page *realmode_pfn_to_page(unsigned long pfn);
+int realmode_get_page(struct page *page);
+int realmode_put_page(struct page *page);
+
 static inline char *get_hpte_slot_array(pmd_t *pmdp)
 {
 	/*
diff --git a/arch/powerpc/mm/init_64.c b/arch/powerpc/mm/init_64.c
index d0cd9e4..dcbb806 100644
--- a/arch/powerpc/mm/init_64.c
+++ b/arch/powerpc/mm/init_64.c
@@ -300,5 +300,79 @@  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_XXXX 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.
+ */
+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;
+
+	if (!atomic_inc_not_zero(&page->_count))
+		return -EAGAIN;
+
+	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
diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
index 6d53675..98ada58 100644
--- a/include/linux/page-flags.h
+++ b/include/linux/page-flags.h
@@ -329,7 +329,9 @@  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
+ * and arch/powerpc/kvm/book3s_64_vio_hv.c which use it to detect huge pages
+ * and avoid handling those in real mode.
  */
 __PAGEFLAG(Head, head) CLEARPAGEFLAG(Head, head)
 __PAGEFLAG(Tail, tail)