[PATCH] x86/sev: Unroll string mmio with CC_ATTR_GUEST_UNROLL_STRING_IO

[Joerg Roedel]

From: Joerg Roedel <jroedel@xxxxxxx>

The io specific memcpy/memset functions use string mmio accesses to do
their work. Under SEV the hypervisor can't emulate these instructions,
because they read/write directly from/to encrypted memory.

KVM will inject a page fault exception into the guest when it is asked
to emulate string mmio instructions for an SEV guest:

	BUG: unable to handle page fault for address: ffffc90000065068
	#PF: supervisor read access in kernel mode
	#PF: error_code(0x0000) - not-present page
	PGD 8000100000067 P4D 8000100000067 PUD 80001000fb067 PMD 80001000fc067 PTE 80000000fed40173
	Oops: 0000 [#1] PREEMPT SMP NOPTI
	CPU: 0 PID: 1 Comm: swapper/0 Not tainted 5.17.0-rc7 #3

As string mmio for an SEV guest can not be supported by the
hypervisor, unroll the instructions for CC_ATTR_GUEST_UNROLL_STRING_IO
enabled kernels.

Cc: stable@xxxxxxxxxxxxxxx #4.15+
Fixes: d8aa7eea78a1 ('x86/mm: Add Secure Encrypted Virtualization (SEV) support')
Signed-off-by: Joerg Roedel <jroedel@xxxxxxx>
---
 arch/x86/lib/iomem.c | 63 ++++++++++++++++++++++++++++++++++++++++----
 1 file changed, 58 insertions(+), 5 deletions(-)

diff --git a/arch/x86/lib/iomem.c b/arch/x86/lib/iomem.c
index df50451d94ef..1246dd558f8d 100644
--- a/arch/x86/lib/iomem.c
+++ b/arch/x86/lib/iomem.c
@@ -22,7 +22,7 @@ static __always_inline void rep_movs(void *to, const void *from, size_t n)
 		     : "memory");
 }
 
-void memcpy_fromio(void *to, const volatile void __iomem *from, size_t n)
+static void string_memcpy_fromio(void *to, const volatile void __iomem *from, size_t n)
 {
 	if (unlikely(!n))
 		return;
@@ -38,9 +38,8 @@ void memcpy_fromio(void *to, const volatile void __iomem *from, size_t n)
 	}
 	rep_movs(to, (const void *)from, n);
 }
-EXPORT_SYMBOL(memcpy_fromio);
 
-void memcpy_toio(volatile void __iomem *to, const void *from, size_t n)
+static void string_memcpy_toio(volatile void __iomem *to, const void *from, size_t n)
 {
 	if (unlikely(!n))
 		return;
@@ -56,9 +55,8 @@ void memcpy_toio(volatile void __iomem *to, const void *from, size_t n)
 	}
 	rep_movs((void *)to, (const void *) from, n);
 }
-EXPORT_SYMBOL(memcpy_toio);
 
-void memset_io(volatile void __iomem *a, int b, size_t c)
+static void string_memset_io(volatile void __iomem *a, int b, size_t c)
 {
 	/*
 	 * TODO: memset can mangle the IO patterns quite a bit.
@@ -66,4 +64,59 @@ void memset_io(volatile void __iomem *a, int b, size_t c)
 	 */
 	memset((void *)a, b, c);
 }
+
+static void unrolled_memcpy_fromio(void *to, const volatile void __iomem *from, size_t n)
+{
+	const volatile char __iomem *in = from;
+	char *out = to;
+	int i;
+
+	for (i = 0; i < n; ++i)
+		out[i] = in[i];
+}
+
+static void unrolled_memcpy_toio(volatile void __iomem *to, const void *from, size_t n)
+{
+	volatile char __iomem *out = to;
+	const char *in = from;
+	int i;
+
+	for (i = 0; i < n; ++i)
+		out[i] = in[i];
+}
+
+static void unrolled_memset_io(volatile void __iomem *a, int b, size_t c)
+{
+	volatile char __iomem *mem = a;
+	int i;
+
+	for (i = 0; i < c; ++i)
+		mem[i] = b;
+}
+
+void memcpy_fromio(void *to, const volatile void __iomem *from, size_t n)
+{
+	if (cc_platform_has(CC_ATTR_GUEST_UNROLL_STRING_IO))
+		unrolled_memcpy_fromio(to, from, n);
+	else
+		string_memcpy_fromio(to, from, n);
+}
+EXPORT_SYMBOL(memcpy_fromio);
+
+void memcpy_toio(volatile void __iomem *to, const void *from, size_t n)
+{
+	if (cc_platform_has(CC_ATTR_GUEST_UNROLL_STRING_IO))
+		unrolled_memcpy_toio(to, from, n);
+	else
+		string_memcpy_toio(to, from, n);
+}
+EXPORT_SYMBOL(memcpy_toio);
+
+void memset_io(volatile void __iomem *a, int b, size_t c)
+{
+	if (cc_platform_has(CC_ATTR_GUEST_UNROLL_STRING_IO))
+		unrolled_memset_io(a, b, c);
+	else
+		string_memset_io(a, b, c);
+}
 EXPORT_SYMBOL(memset_io);
-- 
2.35.1