Re: [RFC PATCH v2 08/32] x86: Use PAGE_KERNEL protection for ioremap of memory page

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On 3/7/2017 8:59 AM, Borislav Petkov wrote:
On Thu, Mar 02, 2017 at 10:13:32AM -0500, Brijesh Singh wrote:
From: Tom Lendacky <thomas.lendacky@xxxxxxx>

In order for memory pages to be properly mapped when SEV is active, we
need to use the PAGE_KERNEL protection attribute as the base protection.
This will insure that memory mapping of, e.g. ACPI tables, receives the
proper mapping attributes.

Signed-off-by: Tom Lendacky <thomas.lendacky@xxxxxxx>
---

diff --git a/arch/x86/mm/ioremap.c b/arch/x86/mm/ioremap.c
index c400ab5..481c999 100644
--- a/arch/x86/mm/ioremap.c
+++ b/arch/x86/mm/ioremap.c
@@ -151,7 +151,15 @@ static void __iomem *__ioremap_caller(resource_size_t phys_addr,
                pcm = new_pcm;
        }

+       /*
+        * If the page being mapped is in memory and SEV is active then
+        * make sure the memory encryption attribute is enabled in the
+        * resulting mapping.
+        */
        prot = PAGE_KERNEL_IO;
+       if (sev_active() && page_is_mem(pfn))

Hmm, a resource tree walk per ioremap call. This could get expensive for
ioremap-heavy workloads.

__ioremap_caller() gets called here during boot 55 times so not a whole
lot but I wouldn't be surprised if there were some nasty use cases which
ioremap a lot.

...

diff --git a/kernel/resource.c b/kernel/resource.c
index 9b5f044..db56ba3 100644
--- a/kernel/resource.c
+++ b/kernel/resource.c
@@ -518,6 +518,46 @@ int __weak page_is_ram(unsigned long pfn)
 }
 EXPORT_SYMBOL_GPL(page_is_ram);

+/*
+ * This function returns true if the target memory is marked as
+ * IORESOURCE_MEM and IORESOUCE_BUSY and described as other than
+ * IORES_DESC_NONE (e.g. IORES_DESC_ACPI_TABLES).
+ */
+static int walk_mem_range(unsigned long start_pfn, unsigned long nr_pages)
+{
+	struct resource res;
+	unsigned long pfn, end_pfn;
+	u64 orig_end;
+	int ret = -1;
+
+	res.start = (u64) start_pfn << PAGE_SHIFT;
+	res.end = ((u64)(start_pfn + nr_pages) << PAGE_SHIFT) - 1;
+	res.flags = IORESOURCE_MEM | IORESOURCE_BUSY;
+	orig_end = res.end;
+	while ((res.start < res.end) &&
+		(find_next_iomem_res(&res, IORES_DESC_NONE, true) >= 0)) {
+		pfn = (res.start + PAGE_SIZE - 1) >> PAGE_SHIFT;
+		end_pfn = (res.end + 1) >> PAGE_SHIFT;
+		if (end_pfn > pfn)
+			ret = (res.desc != IORES_DESC_NONE) ? 1 : 0;
+		if (ret)
+			break;
+		res.start = res.end + 1;
+		res.end = orig_end;
+	}
+	return ret;
+}

So the relevant difference between this one and walk_system_ram_range()
is this:

-			ret = (*func)(pfn, end_pfn - pfn, arg);
+			ret = (res.desc != IORES_DESC_NONE) ? 1 : 0;

so it seems to me you can have your own *func() pointer which does that
IORES_DESC_NONE comparison. And then you can define your own workhorse
__walk_memory_range() which gets called by both walk_mem_range() and
walk_system_ram_range() instead of almost duplicating them.

And looking at walk_system_ram_res(), that one looks similar too except
the pfn computation. But AFAICT the pfn/end_pfn things are computed from
res.start and res.end so it looks to me like all those three functions
are crying for unification...

I'll take a look at what it takes to consolidate these with a pre-patch. Then I'll add the new support.

Thanks,
Tom





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