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[Qemu-devel] [PATCH] target-arm: Handle "extended small page" descriptor
From: |
Peter Maydell |
Subject: |
[Qemu-devel] [PATCH] target-arm: Handle "extended small page" descriptors correctly |
Date: |
Thu, 28 May 2015 21:14:45 +0100 |
The old ARMv5-style page table format includes a kind of second level
descriptor named the "extended small page" format, whose primary purpose
is to allow specification of the TEX memory attribute bits on a 4K page.
This exists on ARMv6 and also (as an implementation extension) on XScale
CPUs; it's UNPREDICTABLE on v5.
We were mishandling this in two ways:
(1) we weren't implementing it for v6 (probably never noticed because
Linux will use the new-style v6 page table format there)
(2) we were not correctly setting the page_size, which is 4K, not 1K
The latter bug went unnoticed for years because the only thing which
the page_size affects is which TLB entries get flushed when the guest
does a TLB invalidate on an address in the page, and prior to commit
2f0d8631b7 we were doing a full TLB flush very frequently due to Linux's
habit of writing the SCTLR pointlessly a lot.
(We can assume that after commit 2f0d8631b7 the bug went unnoticed
for a year because nobody's actually using the Zaurus/XScale emulation...)
Report the correct page size for these descriptors, and permit them
on ARMv6 CPUs. This fixes a problem where a kernel image for Zaurus
can boot the kernel OK but gets random segfaults when it tries to
run userspace programs.
Signed-off-by: Peter Maydell <address@hidden>
---
This took me something like three weeks to track down, on and off...
target-arm/helper.c | 13 +++++++++----
1 file changed, 9 insertions(+), 4 deletions(-)
diff --git a/target-arm/helper.c b/target-arm/helper.c
index f8f8d76..238da3c 100644
--- a/target-arm/helper.c
+++ b/target-arm/helper.c
@@ -5227,20 +5227,25 @@ static int get_phys_addr_v5(CPUARMState *env, uint32_t
address, int access_type,
ap = (desc >> (4 + ((address >> 9) & 6))) & 3;
*page_size = 0x1000;
break;
- case 3: /* 1k page. */
+ case 3: /* 1k page, or ARMv6/XScale "extended small (4k) page" */
if (type == 1) {
- if (arm_feature(env, ARM_FEATURE_XSCALE)) {
+ /* ARMv6/XScale extended small page format */
+ if (arm_feature(env, ARM_FEATURE_XSCALE)
+ || arm_feature(env, ARM_FEATURE_V6)) {
phys_addr = (desc & 0xfffff000) | (address & 0xfff);
+ *page_size = 0x1000;
} else {
- /* Page translation fault. */
+ /* UNPREDICTABLE in ARMv5; we choose to take a
+ * page translation fault.
+ */
code = 7;
goto do_fault;
}
} else {
phys_addr = (desc & 0xfffffc00) | (address & 0x3ff);
+ *page_size = 0x400;
}
ap = (desc >> 4) & 3;
- *page_size = 0x400;
break;
default:
/* Never happens, but compiler isn't smart enough to tell. */
--
1.9.1
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